Abrupt Climate Change May Have Rocked the Cradle of Civilization

Abrupt Climate Change May Have Rocked the Cradle of Civilization

New research reveals that some of the earliest civilizations in the Middle East and the Fertile Crescent may have been affected by abrupt climate change. These findings show that while socio-economic factors were traditionally considered to shape ancient human societies in this region, the influence of abrupt climate change should not be underestimated.

A team of international scientists led by researchers from the University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science found that during the first half of the last interglacial period known as the Holocene epoch, which began about 12,000 years ago and continues today, the Middle East most likely experienced wetter conditions in comparison with the last 6,000 years, when the conditions were drier and dustier.

Artist’s reconstruction of the Sumerian city of Ur. ( Kings Academy )

"Evidence for wet early Holocene was previously found in the Eastern Mediterranean Sea region, North and East African lakes and cave deposits from Southwest Asia, and attributed to higher solar insolation during this period," said Ali Pourmand, assistant professor of marine geosciences at the UM Rosenstiel School, who supervised the project. "Our study, however, is the first of its kind from the interior of West Asia and unique in its resolution and multi-proxy approach."

The Fertile Crescent, a region in west Asia that extends from Iran and the Arabian Peninsula to the eastern Mediterranean Sea and northern Egypt is one of the most climatically dynamic regions in the world and is widely considered the birthplace of early human civilizations.

"The high-resolution nature of this record afforded us the rare opportunity to examine the influence of abrupt climate change on early human societies. We see that transitions in several major civilizations across this region, as evidenced by the available historical and archeological records, coincided with episodes of high atmospheric dust; higher fluxes of dust are attributed to drier conditions across the region over the last 5,000 years," said Arash Sharifi, Ph.D. candidate at the department of marine geosciences and the lead author of the study.

  • Civilizations out of Nowhere
  • The Rise and Fall of Sumer and Akkad
  • Scholars rethink the beginnings of civilizations following discoveries in Burnt City of Iran

Setting up the core in multi sensor core logger (MSCL) at the paleoceanography lab at the Rosenstiel School, to make a high-resolution image and measure the physical properties such as density and magnetic susceptibility. (Diana Udel, UM Rosenstiel School Communications Office)

The researchers investigated climate variability and changes in paleoenvironmental conditions during the last 13,000 years based on a high-resolution (sub-decadal to centennial) peat record from Neor Lake in Northwest Iran. Abrupt climate changes occur in the span of years to decades.

Featured image: Illustration of Mesopotamia. ( Jeff Brown Graphics )

Source: University of Miami Rosenstiel School of Marine & Atmospheric Science. "Abrupt climate change may have rocked the cradle of civilization: Effects of climate on human societies." ScienceDaily. ScienceDaily, 23 July 2015.

    Collapse of Earliest Known Empire Is Linked to Long, Harsh Drought

    UNDER the renowned Sargon and his successors, the Akkadians of Mesopotamia forged the world's first empire more than 4,300 years ago. They seized control of cities along the Euphrates River and on the fruitful plains to the north, all in what is now Iraq, Syria and parts of southern Turkey. Then, after only a century of prosperity, the Akkadian empire collapsed abruptly, for reasons that have been lost to history.

    The traditional explanation is one of divine retribution. Angered by the hubris of Naram-Sin, Sargon's grandson and most dynamic successor, the gods supposedly unleashed the barbaric Gutians to descend out of the highlands and overwhelm Akkadian towns. More recent and conventional explanations have put the blame on overpopulation, provincial revolt, nomadic incursions or managerial incompetence, though many scholars despaired of ever identifying the root cause of the collapse.

    A team of archeologists, geologists and soil scientists has now found evidence that seems to solve the mystery. The Akkadian empire, they suggest, was beset by a 300-year drought and literally dried up. A microscopic analysis of soil moisture at the ruins of Akkadian cities in the northern farmlands disclosed that the onset of the drought was swift and the consequences severe, beginning about 2200 B.C.

    "This is the first time an abrupt climate change has been directly linked to the collapse of a thriving civilization," said Dr. Harvey Weiss, a Yale University archeologist and leader of the American-French research team.

    Such a devastating drought would explain the abandonment at that time of Akkadian cities across the northern plain, a puzzling phenomenon observed in archeological excavations. It would also account for the sudden migrations of people to the south, as recorded in texts on clay tablets. These migrations doubled the populations of southern cities, overtaxed food and water supplies, and led to fighting and the fall of the Sargon dynasty.

    The new findings thus call attention to the role of chance -- call it fate, an act of God or simply an unpredictable natural disaster -- in the development of human cultures and the rise and fall of civilizations.

    Among the drought's refugees were a herding people known as Amorites, characterized by scribes in the city of Ur as "a ravaging people with the instincts of a beast, a people who know not grain" -- the ultimate put-down in an economy based on grain agriculture. An 110-mile wall, called the "Repeller of the Amorites," was erected to hold them off. But when the drought finally ended in about 1900 B.C., leadership in the region had passed from Akkad to Ur and then to the Amorites, whose power was centered at the rising city of Babylon. Hammurabi, the great ruler of Babylon in 1800 B.C., was a descendant of Amorites.

    The correlation between drastic climate change and the Akkadian downfall also appears to complete the picture of widespread environmental crisis disrupting societies throughout the Middle East in the same centuries. Earlier studies had noted the effects of severe drought, including abandoned towns, migrations and nomad incursions, in Greece, Egypt, Palestine and the Indus Valley. Until now, the connection between chronic drought and unstable social conditions had not been extended to Mesopotamia, the land between the two rivers, the Euphrates and the Tigris, often called "the cradle of civilization."

    As to what caused such a persistent dry spell, the scientists said they had no clear ideas, though they suggested that changing wind patterns and ocean currents could have been factors. A tremendous volcanic eruption that occurred in Turkey near the beginning of the drought, the scientists said, almost certainly could not have triggered such a long climate change. Archeology's Sophistication

    "This is a research frontier for climatologists," Dr. Weiss said in an interview.

    Dr. Weiss proposed the new theory for the Akkadian collapse at a recent meeting of the Society of American Archeology in St. Louis and then in a report in the current issue of the journal Science. His principal collaborators in the research were Dr. Marie-Agnes Courty, an archeologist and soil scientist at the National Center for Scientific Research in Paris, and Dr. Francois Guichard, a geologist at the same institution.

    Other archeologists said the theory was plausible and appeared to provide the first logical explanation for the Akkadian downfall. Although he had not studied the report, Dr. Robert Biggs, a specialist in Mesopotamian archeology at the University of Chicago, said this was a good example of "archeology's growing sophistication in seeking reasons for serious political changes in the past."

    In an article accompanying the report in Science, Dr. Robert McC. Adams, secretary of the Smithsonian Institution and an anthropologist specializing in Mesopotamia, cautioned that Dr. Weiss and his colleagues had not thoroughly established the link between climate and the empire's fall. He questioned whether such widespread and persistent drought could be inferred from local soil conditions at a few sites.

    "It will demand of other people in the field to either refute it or replicate it with their own work," Dr. Adams said of the theory. "And the only way to get people to pick up that challenge is for Weiss to stick his neck out. I applaud it."

    Dr. Weiss said the conclusions were based on tests of soils mainly at the sites of three Akkadian cities within a 30-mile radius, places now known as Tell Leilan, Tell Mozan and Tell Brak in present-day Syria. Evidence of similar climate change was found in adjacent regions, and the archeologist said further tests of the theory would be conducted with the resumption of field work this week. Land of Rainy Winters

    The most revealing evidence has come from Tell Leilan, where Dr. Weiss has been excavating for 14 years and finding successive layers of ruins going back some 8,000 years. For several millennia, this was a small village established by some of the world's first farmers. Around 2600 B.C., it suddenly expanded sixfold to become the city of Shekhna, with 10,000 to 20,000 inhabitants. They lived in the middle of a land of rainy winters, dry summers and a long growing season for wheat and barley, much as it is today.

    All the more reason the kings of Akkad, or Agade, a city-state whose location has never been exactly determined but is assumed to have been near ancient Kish and Babylon, reached out and conquered places like Tell Leilan about 2300 B.C. The region became the breadbasket for the Akkadian empire, which stretched 800 miles from the Persian Gulf to the headwaters of the Euphrates in Turkey.

    Ceramics and other artifacts established the Akkadian presence there in Tell Leilan and other northern towns. And for years archeologists puzzled over the 300-year gap in human occupation of Tell Leilan and neighboring towns, beginning in 2200 B.C. It occurred to Dr. Weiss that since no irrigation works had been uncovered there, the region must have relied on rain-fed agriculture, as is the case there today, in contrast to the irrigated farming in southern Mesopotamia. A severe drought, therefore, could be disastrous to life in the north.

    This idea was tested by Dr. Courty, using microscopic techniques she pioneered in a scientific specialty, soil micromorphology. By examining in detail the arrangement and nature of sediments at archeological sites, it is possible to reconstruct ancient environmental conditions and human activity.

    One of the first discoveries was a half-inch layer of volcanic ash covering the rooftops of buildings at Tell Leilan in 2200 B.C. All ash falls leave distinctive chemical signatures. An analysis by Dr. Guichard traced the likely source of this potassium-rich ash to volcanoes a few hundred miles away in present-day Turkey. Migration From North

    Since the abandonment of Tell Leilan occurred at the same time and the climate suddenly became more arid, volcanic fallout was first suspected as the culprit. Ash and gases from volcanic eruptions can remain suspended in the atmosphere for years, creating sun-blocking hazes and reducing temperatures. But from their knowledge of recent volcanoes, scientists doubted that the eruptions could have perturbed the climate over such a large area for 300 years.

    And there seemed no doubt about the drought lasting that long, Dr. Courty said. In the surrounding countryside at Tell Leilan and elsewhere, she examined a layer of soil nearly two feet thick and lying just above the volcanic ash. This layer contained large amounts of fine wind-blown sand and dust, in contrast to the richer soil in earlier periods. Another telltale sign was the absence of earthworm holes and insect tracks, which are usually present in soils from moister environments.

    This was strong evidence, the researchers reported, of a "marked aridity induced by intensification of wind circulation and an apparent increase" of dust storms in the northern plains of Mesopotamia.

    It was during the 300-year desertification that archives of the southern cities reported the migration of barbarians from the north and a sharp decline in agricultural production, and showed an increasing number of names of people from the northern tribes, mainly the Amorites.

    According to the evidence of the sediments, rain in more abundance returned to northern Mesopotamia in 1900 B.C. and with it the tracks of earthworms and the rebuilding of the deserted cities. Over the ruins of Shekhna, buried in the sands of the drought, rose a new city named Shubat Enlil, which means "dwelling place of Enlil," the paramount Mesopotamian god. The builders were Amorites.

    In earlier excavations at Tell Leilan, Dr. Weiss discovered an archive of clay tablets showing that this was the lost capital of a northern Amorite kingdom often mentioned in the cuneiform writing of the period. This was the archive of Shamshi-Adad, the Amorite king who reigned from 1813 to 1781 B.C., containing the king's correspondence with neighboring rulers who concluded the ransoming of spies.

    By then, the Akkadian kingdom of Sargon and Naram-Sin -- the world's first empire -- was long lost in the dust, apparently also the first empire to collapse as a result of catastrophic climate change.

    "Since this is probably the first abrupt climate change in recorded history that caused major social upheaval," Dr. Weiss said, "it raises some interesting questions about how volatile climate conditions can be and how well civilizations can adapt to abrupt crop failures." FROM THE AKKADIANS TO BABYLON

    Recent glacial and interglacial periods

    With glacial ice restricted to high latitudes and altitudes, Earth 125,000 years ago was in an interglacial period similar to the one occurring today. During the past 125,000 years, however, the Earth system went through an entire glacial-interglacial cycle, only the most recent of many taking place over the last million years. The most recent period of cooling and glaciation began approximately 120,000 years ago. Significant ice sheets developed and persisted over much of Canada and northern Eurasia.

    After the initial development of glacial conditions, the Earth system alternated between two modes, one of cold temperatures and growing glaciers and the other of relatively warm temperatures (although much cooler than today) and retreating glaciers. These Dansgaard-Oeschger (DO) cycles, recorded in both ice cores and marine sediments, occurred approximately every 1,500 years. A lower-frequency cycle, called the Bond cycle, is superimposed on the pattern of DO cycles Bond cycles occurred every 1,400–2,200 years. Each Bond cycle is characterized by unusually cold conditions that take place during the cold phase of a DO cycle, the subsequent Heinrich event (which is a brief dry and cold phase), and the rapid warming phase that follows each Heinrich event. During each Heinrich event, massive fleets of icebergs were released into the North Atlantic, carrying rocks picked up by the glaciers far out to sea. Heinrich events are marked in marine sediments by conspicuous layers of iceberg-transported rock fragments.

    Many of the transitions in the DO and Bond cycles were rapid and abrupt, and they are being studied intensely by paleoclimatologists and Earth system scientists to understand the driving mechanisms of such dramatic climatic variations. These cycles now appear to result from interactions between the atmosphere, oceans, ice sheets, and continental rivers that influence thermohaline circulation (the pattern of ocean currents driven by differences in water density, salinity, and temperature, rather than wind). Thermohaline circulation, in turn, controls ocean heat transport, such as the Gulf Stream.

    Anthropology of Food

    Global Warming / Global Cooling

    , Nicola Twilley, New York Times Magazine, 27 July 2014 [Summary: "A refrigeration boom is changing the way Chinese people eat--and threatening the planet in the process. Cooking is already responsible for 15 percent of all electricity consumption worldwide, and leaks of chemical refrigerants are a major source of greenhouse-gas pollution. Of all the shifts in the lifestyle that threaten the planet, perhaps none is as important as the changing way that Chinese people eat" (from p. 3)]

    • Earth-Now [This app from NASA gives readers a peek into the constantly fluctuating elements of the earth&rsquos atmosphere. Using 3D models constructed from satellite images, the app visually displays the causes and effects of climate change via surface air temperature, carbon dioxide and carbon monoxide levels, and sea level height anomalies among others. Fascinating for students, teachers, and anyone interested in climate change, Earth-Now is compatible with Apple devices running iOS 5.1+ and Android devices running 4.0+. [CNH -- Scout Report]

    [Three reporters from the online magazine, InsideClimate News, won a Pulitzer Prize in 2012 for their work uncovering a giant, and largely unpublicized, Canadian oil spill. Since then, the magazine has continued to publish hard-hitting journalism on a range of climate-related topics. Coverage of Exxon's own research into global warming in the 1970s - and its subsequent public campaign to discredit and block further investigation - is a case in point. In this multi-part series, published in late September of 2015, InsideClimate News reporters examine primary sources, including internal company files, to expose Exxon's outright war on the science of global warming. Readers may also scout the site by eight other categories, including All Stories, Carbon Copy, Tar Sands, Clean Economy, Today's Climate, Gas Drilling, ICN Books, and Big Oil, Bad Air. [CNH, The Scout Report, October 9, 2015 -- Volume 21, Number 39 ]

      Session III: Food, Ethics and the Environment -- Oceans, Climate and Animals


    'Evidence for wet early Holocene was previously found in the Eastern Mediterranean Sea region, North and East African lakes and cave deposits from Southwest Asia,' said Ali Pourmand, assistant professor of marine geosciences at UM.

    This study, however, fills 'a large gap in the existing terrestrial paleoclimate records from the interior of West Asia,' the study authors wrote, in the journal Quaternary Science Reviews.

    The vertical orange bands shows periods that were dry and dusty. The transition between ruling dynasties (grey arrows) in Iran and North Mesopotamia coincides with the change in climate

    The finding is based on an analysis of ancient peat deposits found deep within Neor Lake in Northwest Iran, which revealed unusual weather patterns in the region


    It marks the moment when the civilised world entered the Dark Ages and some of the most spectacular societies in the world disappeared.

    Now one historian claims he has unraveled what may have lead to the downfall of Ancient Egypt and other Bronze Age civilisations collapse.

    He claims they were hit by a 'perfect storm' of disasters 3,200 years ago that left the Ancient Egyptians, the Babylonians, Minoans and Mycenians unable to cope.

    As each of these great societies were interconnected, the collapse of one also affected the others, creating a domino-like effect, claims Professor Eric Cline, director of the Capitol Archaeological Institute at George Washington University.

    He says that a series of droughts, famines, climate change, earthquakes, invasions and internal rebellions between 1225BC and 1177 BC happened in quick succession.

    Speaking to Haaretz, he said: 'Normally if a culture if faced with just one of these tragedies, it can survive it, but what if they all happened at once or in quick succession.

    'I think that the Late Bronze Age civilisations were simply unable to weather the "perfect storm" and came crashing down.'

    The Fertile Crescent is a region in west Asia that extends from Iran and the Arabian Peninsula to the eastern Mediterranean Sea and northern Egypt.

    It is one of the most climatically dynamic regions in the world and is widely considered the birthplace of early human civilisations.

    'We see that transitions in several major civilisations across this region,' said Arash Sharifi, Ph.D. candidate at the department of marine geosciences and lead author of the study.

    '[This is] evidenced by the available historical and archaeological records, coincided with episodes of high atmospheric dust.

    'Higher fluxes of dust are attributed to drier conditions across the region over the last 5,000 years.'

    Comparing our record of palaeoclimate variability with historical, geological and archaeological archives from this region.

    The team compared the peat data matched previous evidence from marine sediments of the Arabian Sea that suggested climate change influenced the end of the Akkadian empire.

    Last year, tree ring samples found in an ancient Egyptian coffin which reveal the Akkadian civilisation came to its knees following changes to its food resources and infrastructure.

    Researchers at Cornell University said was just enough change in the climate to upset food resources and other infrastructure.

    The said this is likely what led to the collapse of the Akkadian Empire and affected the Old Kingdom of Egypt and a number of other civilisations.

    'We're in exactly the same situation as the Akkadians: If something suddenly undid the standard food production model in large areas of the U.S. it would be a disaster,' said Professor Stuart Manning of Cornell University.

    Neor Lake, where the peat deposits were collected, is in a hilly area near the Iranian Province of Ardabil

    Abrupt Climate Change: Inevitable Surprises (2002)

    Researchers first became intrigued by abrupt climate change when they discovered striking evidence of large, abrupt, and widespread changes preserved in paleoclimatic archives. Interpretation of such proxy records of climate&mdashfor example, using tree rings to judge occurrence of droughts or gas bubbles in ice cores to study the atmosphere at the time the bubbles were trapped&mdashis a well-established science that has grown much in recent years. This chapter summarizes techniques for studying paleoclimate and highlights research results. The chapter concludes with examples of modern climate change and techniques for observing it. Modern climate records include abrupt changes that are smaller and briefer than in paleoclimate records but show that abrupt climate change is not restricted to the distant past.


    Paleoclimatic interpretation relies ultimately on the use of the present or recent instrumental records as the key to the past. To accomplish this, modern values observed for a given characteristic of the climate system are compared with some record from the past, such as tree-ring thickness or the isotopic composition of water frozen in ice cores (see Plates 1 and 2). Detailed understanding of these records&mdashhow the thickness of tree rings

    changes in recent wet and dry periods&mdashlets scientists draw inferences about the past, and these records come to be considered &ldquoproxies,&rdquo or indicators of the past environment.

    The assumption of constancy of the relation between climate and its proxy might require little more to support it than constancy of physical law (for example, the assumption that in the past heat flowed from warm to cold rocks in the same way as today). Other assumptions might involve greater uncertainty (for example, the assumption that under different climatic conditions, marine organisms grew most vigorously during the same season and at the same water depth as in the modern environment). Testing of the underlying assumption that the present is the key to the past relies largely on the consistency of results from a wide array of proxies, particularly those depending on few assumptions. The use of multiple indicators increases the reliability of many paleoclimate reconstructions.

    The following pages provide a brief synopsis of paleoclimate proxies (Table 2.1) and age indicators. The description is not exhaustive and is intended only to orient the reader to some of the current paleoclimatic tools available. For more detailed reviews of methods involved in paleoclimatic interpretation see Broecker (1995), Bradley (1999), or Cronin (1999).

    Physical paleoclimatic indicators often rely on the fewest assumptions and so can be interpreted most directly. For example, old air extracted from bubbles in ice cores and old water from pore spaces in seabed sediments or continental rocks provide direct indications of past compositions of atmosphere, oceans, and groundwater (see Plate 1). Anomalously cold buried rocks or ice have not finished warming from the ice age and thus provide evidence that conditions were colder in the past. Conditions are also judged from the concentrations of noble gases found dissolved in old groundwaters. Some such records are subject to substantial loss of information through diffusion of the components being analyzed, which limits the ability to interpret older events. Physical indicators include the characteristics of sediments and land features. For example, the presence of sand dunes can indicate past arid conditions, and glacially polished bedrock is an indication of prior glacial conditions.

    Isotopic indicators are widely used in paleoclimate science. The subtle differences in behavior between chemically similar atoms having different weights (isotopes) prove to be sensitive indicators of paleoenvironmental conditions. One common application is paleothermometry. The physical and chemical discrimination of atoms of differing isotopic mass increases with decreasing temperature. For example, carbonate shells grow-

    TABLE 2.1 Paeloclimatic Proxies

    Climate Variable Recorded

    Source strength of wind-blown materials

    Abundance of pollen, dust, sea salt

    Thickness of annual layers

    Ocean sediments and corals

    Alkenone (U37 K ' ) thermometry

    Shell isotopes after correction for temperature and ice volume

    Isotopic composition of pore waters

    Shell isotopes after correction for temperature and salinity

    Corrosiveness/chemistry of ambient waters

    Atmospheric temperature and soil moisture

    Washed- or blown-in materials including pollen and spores

    Macrofossils such as leaves, needles, beetles, midge flies, etc.

    Water balance (precipitation minus evaporation

    Temperature and/or moisture availability

    Ring width or density of trees stressed by cold or drought

    Variations in the isotopic ratio of water related to temperature

    Cellulose isotopic ratios

    Growth rate of formations

    Isotopic ratios of water related to temperature or precipitation rate

    Oxygen isotopic composition

    Climate Variable Recorded

    Terrestrial sediment types/ nature of erosion

    Soil formation rate/moisture availability

    Isotopic and noble gas composition of water

    NOTE: Past climate conditions can be measured only through &ldquoproxies,&rdquo characteristics that give insights about past conditions. For example, gas bubbles trapped in ice can be analyzed to understand the atmosphere at the time the bubbles were trapped. This table lists examples of paleoclimatic proxies, what the proxy measures, and from where the proxy data originated.

    ing in water typically favor isotopically heavy oxygen and become isotopically heavier at lower temperatures. Isotopic ratios also are used to estimate the concentration of a chemical. When a chemical is common in the environment, a &ldquofavored&rdquo isotope will be used shortage of a chemical leads to greater use of a less favored isotope. Marine photosynthesis increasingly favors the light isotope of carbon as carbon dioxide becomes more abundant, and this allows estimation of changes in carbon dioxide concentration from the isotopic composition of organic matter in oceanic sediments. Similarly, the growth of ice sheets removes isotopically light water (ordinary water) from the ocean, increasing the use of isotopically heavy oxygen from water in carbonate shells, which then provide information on the size of ice sheets over time. Stable isotopic values in organic matter also provide important information on photosynthetic pathways and so can afford insight into the photosynthesizing organisms that were dominant at a given location in the past.

    Many chemical proxies of environmental change act like isotopic ratios in the measurement of availability of a species. For example, if decreased rainfall increases the concentration of magnesium or strontium ions in lake water, they will become more common in calcium-carbonate shells that grow in that water. However, warming can also allow increased incorporation of substitute ions in shells. Such nonuniqueness can usually be resolved through use of multiple indicators. Other chemical indicators are allied to biological processes. For example, some species of marine diatoms incorporate stiffer molecules in their cell walls to offset the softening effects of higher temperature, and these molecules are resistant to changes after the diatoms die. The fraction of stiffer molecules in sediments yields an estimate of past temperatures. This analytic technique, known as alkenone paleothermometry, is increasingly used to learn about paleotemperatures in the marine environment.

    Biological indicators of environmental conditions typically involve the presence or absence of indicator species or assemblages of species. For example, the existence of an old rooted tree stump shows that the climate was warm and wet enough for trees, and the type of wood indicates how warm and wet the climate was if that tree stump is in a region where trees do not grow today, the climate change is clear. In ocean and lake sediments, the microfossil species present can indicate the temperature, salinity, and nutrient concentration of the water column when they were deposited. Pollen and macrofossils preserved in sediments are important records of variability in the terrestrial environment (see Plate 3). The presence of specific organic compounds called biomarkers in sediments can reveal what species were present, how abundant they were, and other information.

    The complicated nature of paleoclimatic interpretation can be seen when proxies are viewed in a practical example. During ice ages, the oceans were colder, but the water in them was also isotopically heavier because light water was removed and used in growing ice sheets. Shells that grew in water during ice age intervals contain heavier isotopes owing to cooling and changes in the isotopic composition of ocean waters. The change in ocean isotopic composition can be estimated independently from the composition of pore waters in sediments, whereas the change in temperature can be estimated from both the abundance of cold- or warm-loving shells in sediment and the abundance of stiff diatom cell-wall molecules in sediments. Concentrations of non-carbonate ions substituted into calcium carbonate shells provide further information. Because there is redundancy in the available data, reliable results can be obtained.

    Any paleoclimatic record requires age estimates, and many techniques are used to obtain them. Annual layers in trees, in sediments of some lakes and shallow marine basins, in corals, and in some ice cores allow high-resolution dating for tens of thousands of years, or longer in exceptional cases. Various radiometric techniques are also used. Dates for the last 50,000 years are most commonly obtained by using radiocarbon ( 14 C). Changes in production of radiocarbon by cosmic rays have occurred over time, but their effects are now calibrated by using annual-layer counts or other radiometric techniques, such as the use of radioactive intermediates generated during the decay of uranium and thorium and also through the potassium-argon system. Other techniques rely on measurement of accumulated damage to mineral grains, rocks, or chemicals this permits dating on the basis of cosmogenic exposure ages, thermoluminescence, obsidian hydration, fission tracks, amino-acid racemization, and so on. Numerous techniques allow correlation of samples and assignment of ages from well-dated to initially less well-dated records. Such techniques include the identification of chemically &ldquofingerprinted&rdquo fallout from particular volcanic eruptions, of changes in the composition of atmospheric gases trapped in ice cores, and of changes in cosmogenic isotope production or rock magnetization linked to changes in the earth&rsquos magnetic field.


    Sedimentary records reveal numerous large, widespread abrupt climate changes over the last 100,000 years and beyond. The best known of them is the Younger Dryas cold interval. The Younger Dryas was a nearly global event that began about 12,800 years ago when there was an interruption in the gradual warming trend that followed the last ice age. The Younger Dryas event ended abruptly about 11,600 years ago (Figures 2.1 and 2.2). Because the Younger Dryas can be tracked quite clearly in geologic records and has received extensive study, a rather detailed summary of the evidence is given here, followed by briefer reviews of other abrupt climate changes. We then target Holocene 1 abrupt climate events as examples of substantial changes that have taken place when physical conditions on the earth were more similar to today. Understanding the causes of both types of abrupt

    The Holocene is the most recent 11,000 years since the last major glacial epoch or &ldquoice age.&rdquo

    climate change is essential for assessing the importance of their role in our climate future.

    Ice Core Evidence of the Younger Dryas

    The Younger Dryas cold reversal is especially prominent in ice-core records from Greenland, but it is also observed in ice cores from other locations. The ice-core records provide a unique perspective that demonstrates the synchronous nature of the large, widespread changes observed.

    Annual-layer counting in Greenland ice cores allows determination of the age, duration, and rapidity of change of the Younger Dryas event with dating errors of about one percent (Alley et al., 1993 Meese et al., 1997). Annual-layer thicknesses corrected for the effects of ice flow give the history of snow accumulation rate in Greenland (Alley et al., 1993). Concentrations of wind-blown materials&mdashsuch as dust (which in central Greenland has characteristics showing its origin in central Asia [Biscaye et al., 1997]) and sea salt&mdashreveal changes in atmospheric concentrations of these particles (Mayewski et al., 1997) after correction for variations in dilution caused by changing snow accumulation rate (Alley et al., 1995a). Gases trapped in bubbles reveal past atmospheric composition. Methane is of special interest because it probably records the global area of wetlands. Furthermore, differences between methane concentrations observed in Greenland ice cores and those from Antarctica allow inference of changes in the wetland areas in the tropics and high latitudes (Chappellaz et al., 1997 Brook et al., 1999).

    The combination of the isotopic record of water making up the Greenland ice (see Plate 2 Figure 1.2) (Johnsen et al., 1997 Grootes and Stuiver, 1997) and the physical temperature of the ice (Cuffey et al., 1994, 1995 Johnsen et al., 1995) yields estimates of past temperatures in central Greenland, which can be checked by using two additional thermometers based on the thermal fractionation of gas isotopes after abrupt temperature changes (Severinghaus et al., 1998). Ice-core records from Greenland thus provide high-resolution reconstructions of local environmental conditions in Greenland (temperature and snow accumulation rate), conditions well beyond Greenland (wind-blown materials including sea salt and Asian dust), and even some global conditions (wetland area inferred from methane), all on a common time scale (Figures 2.1, 2.2, and 2.3).

    A review of available Greenland ice-core data is given by Alley (2000). The data were collected by two international teams of investigators from multiple laboratories. The duplication shows the high reliability of the

    FIGURE 2.1 The Younger Dryas (YD) climate event, as recorded in an ice core from central Greenland and a sediment core from offshore Venezuela. The upper-most curve is the gray-scale (light or dark appearance) of the Cariaco Basin core, and probably records changes in windiness and rainfall (Hughen et al., 1998). The other curves are from the GISP2, Greenland ice core. The rate of snow accumulation and the temperature in central Greenland were calculated by Cuffey and Clow (1997), using the layer-thickness data from Alley et al. (1993) and the ice-isotopic ratios from Grootes and Stuiver (1997), respectively. The independent Severinghaus et al. (1998) temperature estimate is shown by the circle near the end of the Younger Dryas. Methane data are from Brook et al. (1996) (squares) and Severinghaus et al. (1998) (x), and probably record changes in global wetland area. Changes in the d 15 N values as measured by Severinghaus et al. (1998) record the temperature difference between the surface of the Greenland ice sheet and the depth at which bubbles were trapped abrupt warmings caused the short-lived spikes in this value near the end of the Younger Dryas and near 14.7 thousand years. Highs in sea-salt sodium indicate windy conditions from beyond Greenland, and even larger changes in calcium from continental dust indicate windy and dry or low-vegetation conditions in the Asian source regions (Mayewski et al., 1997 Biscaye et al., 1997). Calcium and sodium concentrations measured in the ice have been converted to concentrations in the air over Greenland, and are displayed by dividing by the estimated average atmospheric concentrations over Greenland in the millennium before the Little Ice Age, following Alley et al. (1997). Most of the ice-core data, and many related data sets, are available on The Greenland Summit Ice Cores CD-ROM, 1997, National Snow and Ice Data Center, University of Colorado at Boulder, and the World Data Center-A for Paleoclimatology, National Geophysical Data Center, Boulder, Colorado, www.ngdc.noaa.gov/paleo/icecore/greenland/summit/index.html. Figure is modified from Alley (2000).

    data from the cores over the most recent 110,000 years, and the multiparameter analyses give an exceptionally clear view of the climate system. Briefly, the data indicate that cooling into the Younger Dryas occurred in a few prominent decade(s)-long steps, whereas warming at the end of it occurred primarily in one especially large step (Figure 1.2) of about 8°C in about 10 years and was accompanied by a doubling of snow accumulation in 3 years most of the accumulation-rate change occurred in 1 year. (This matches well the change in wind-driven upwelling in the Cariaco Basin, offshore Venezuela, which occurred in 10 years or less [Hughen et al., 1996].)

    Ice core evidence also shows that wind-blown materials were more abundant in the atmosphere over Greenland by a factor of 3 (sea-salt, submicrometer dust) to 7 (dust measuring several micrometers) in the Younger Dryas atmosphere than after the event (Alley et al., 1995b Mayewski et al., 1997) (Figure 2.1). Taylor et al. (1997) found that most of the change in most indicators occurred in one step over about 5 years at the end of the Younger Dryas, although additional steps of similar length but much smaller magnitude preceded and followed the main step, spanning a total of about 50 years. Variability in at least some indicators was enhanced near this and other transitions in the ice cores (Taylor et al., 1993), complicating identification of when transitions occurred and emphasizing the need for improved statistical and analytical tools in dealing with abrupt climate change. Beginning immediately after the main warming in Greenland (by less than or equal to 30 years), methane rose by 50 percent over about a century this increase included tropical and high-latitude sources (Chappellaz et al., 1997 Severinghaus et al., 1998 Brook et al., 1999).

    FIGURE 2.2 The accumulation rate of ice in Greenland was low during the Younger Dryas, and both the start and end of the period show as abrupt changes. Modified from Alley et al. (1993).

    Ice cores from other sites, including Baffin Island, Canada (Fisher et al., 1995), Huascaran, Peru (Thompson et al., 1995), and Sajama, Bolivia (Thompson et al., 1998), show evidence of a late-glacial reversal that is probably the Younger Dryas, although the age control for these cores is not as accurate as for cores from the large ice sheets. The Byrd Station, Antarctica, ice core and possibly other southern cores (Bender et al., 1994 Blunier and Brook, 2001) indicate a broadly antiphased behavior between the high southern latitudes and much of the rest of the world, with southern warmth during the Younger Dryas interval (see Plate 2). The record from Taylor Dome, Antarctica, a near-coastal site, appears to show a slight cooling during the Younger Dryas, although details of the synchronization with other ice cores remain under discussion (Steig et al., 1998). The Southern Hemisphere records are not comparable with those from central Greenland in time resolution further coring is planned.

    The ice-core records demonstrate that much of the earth was affected simultaneously by the Younger Dryas, typically with cold, dry, windy con-

    FIGURE 2.3 Climate data from the GISP2 core, central Greenland, showing changes about 8,200 years ago probably caused by outburst flooding from around the melting ice sheet in Hudson Bay (Barber et al., 1999) and affecting widespread regions of the globe. The event punctuated generally warm conditions not too different from recently, so warmth is not a guarantee of climate stability. Accumulation and temperature reflect conditions in Greenland, chloride is wind-blown sea-salt from beyond Greenland, and calcium is continental dust probably from Asia (Biscaye et al., 1997). Forest-fire smoke likely is from North America, and methane probably records global wetland area. Data are shown as approximately 50-year running means. Accumulation from Alley et al. (1993) and Spinelli (1996), chloride and calcium from O&rsquoBrien et al. (1995), and fire data shown as a 50-year histogram of frequency of fallout from fires (Taylor et al., 1996), expressed as ratios to their average values during the approximately 2,000 years just prior to the Little Ice Age. Temperature is calculated as a deviation from the average over the same 2,000 years, from oxygen-isotopic data of ice (Stuiver et al., 1995), assuming a calibration of 0.33 per mil per degree C (Cuffey et al., 1995). Methane concentrations from the GISP2 core (heavier line Brook et al., 1996) and the GRIP core (Blunier et al., 1995) are shown in parts per billion by volume (ppb). Note that some scales increase upward and others downward, as indicated, so that all curves vary together at the major events. Modified from Alley et al. (1997).

    ditions. However, those records do not provide much spatial detail, nor do they sample the whole earth. For those, one must consider a global array of data sources of various types, as described in the following subsections.

    Terrestrial Pollen Evidence of the Younger Dryas

    The Younger Dryas was first discovered by studying the biological records found in terrestrial sediments. These records clearly reveal the global reach of the event. Owing to dating uncertainties, including those associated with the conversion of radiocarbon measurements to calendar years, the phasing of events between different locations is not known exactly. The ice cores show that much of the world must have changed nearly simultaneously to yield the observed changes in methane, Asian dust, and Greenland conditions, but we cannot say with confidence whether all events were simultaneous or some were sequential. A summary of much of the relevant terrestrial pollen information follows, organized by region.


    As the Northern Hemisphere was recovering from the last ice age about 15,000 years ago, the climate warmed dramatically and trees started to colonize the landscape. Evidence of the warming was first found in Scandinavia by geologists who noticed tree fossils in organic sediment. They named the warming interval the Allerød for the locale where it was first observed. Overlying the Allerød layer were leaves and fruits of Dryas octopetala, an arctic-alpine herb, in sandy or silty (minerogenic) layers above the peaty tree remains this suggested that the climate had reverted several times to very cold conditions. Two such reversals to frigid conditions were named the Older and Younger Dryas (Jansen, 1938). Considerable evidence of this sequence in hundreds of pollen diagrams throughout Europe (Iversen, l954 Watts, l980) brought attention to the strongest effects of the event, which occurred in coastal Europe. During the Younger Dryas, pollen of tundra plants, such as Artemisia (wormwood) and Chenopodiaceae, abruptly replaced birch and even conifer pollen (e.g., Lowe et al., 1995 Walker, 1995 Renssen and Isarin, 1998 Birks and Ammann, 2000). In Norway, mean July temperature was about 7-9°C lower than today and about 2-4°C lower than the preceding warm Allerød interval (Birks and Ammann, 2000). It is now apparent that regional climate changes were also large in southern Europe (Lowe and Watson, l993 Beaulieu et

    al., l994). For example, mean July temperatures in northern Spain might have been as much as 8°C lower than today (Beaulieu et al., l994).

    North America

    For many years, the Younger Dryas was thought to be a solely European event (Mercer, l969 Davis et al., 1983). It was the high-resolution reexamination of pollen stratigraphy, the identification of plant macrofossils, and the new technique of accelerator mass spectrometry 14 C dating of these macrofossils that enabled documentation of the event in the southern New England region of the United States (Peteet et al., l990, 1993) and in the eastern maritime provinces of Canada (Mott, 1994 Mayle et al., l993). The climate signal in southern New England was a 3-4°C July cooling in eastern Canada, a cooling of 6-7°C is estimated (from pollen). Midge fly fossils in lake sediments from the White Mountains of New Hampshire indicate about 5°C Younger Dryas cooling of maximum summertime lake temperatures, a somewhat smaller change than suggested for a coastal transect from Maine to New Brunswick (Cwynar and Spear, 2001). In the central Appalachians, a warm, wet interval coincident with the Younger Dryas event suggests a sharp climatic gradient that might have forced the northward movement of storm-track moisture (Kneller and Peteet, l999). Later North American studies have identified the Younger Dryas event in other regions, such as the US Midwest (Shane and Anderson, l993), coastal British Columbia (Mathewes, l993) and coastal Alaska (Peteet and Mann, l994). The documentation of the Younger Dryas event over much of North America demonstrated that it was not limited to the circum-Atlantic region (Peteet et al., l997).

    Central America and the Caribbean

    Marine evidence of the Younger Dryas event is recorded as an interval of increased upwelling or decreased riverine runoff from adjacent South American land in a core from the Cariaco Basin in the Caribbean (Hughen et al., 1996, 2000a,b Peterson et al., 2000) (Figure 2.4). Terrestrial evidence is primarily from three sites (Leyden, 1995). Evidence indicates a temperature decline of 1.5-2.5°C during deglaciation, probably correlated with the Younger Dryas, registered at high and low elevations about 13,100-12,300 years ago as far south as Costa Rica, and just before 12,000 years ago in Guatemala (Hooghiemstra et al., l992 Leyden et al., l994). The

    FIGURE 2.4 Global extent of terrestrial (pollen) and ice core (isotopic) evidence where the Younger Dryas cooling (11,500 &ndash 13,000 BP) has been found. While northern hemispheric evidence is consistently strong for cooling, southern hemispheric sites contain controversial evidence and in some cases lack of evidence for a cooling during the YD interval. Possible upwelling in the Cariaco Basin during this time is also indicated, attributed to trade wind increase. Strong cooling ranges from 13-4° C controversial means some sites show cooling and some do not (after Peteet, 1995).

    decrease was not observed on the western Panamanian slope (Piperno et al., l990 Bush et al., l992).

    South America

    In Colombia, the El Abra stadial (a Younger Dryas equivalent) was a cold interval about 13,000-11,700 years ago characterized by low temperature and low precipitation (van der Hammen and Hooghiemstra, 1995). The upper forest line during the stadial was 600-800 m lower than today, and average temperatures were about 4-6°C lower than today. This evi-

    dence comes from about 14 areas, mostly at high elevations (2000-4000 m) in the Eastern and Central Cordillera and in the Sierra Nevada de Santa Marta some data were collected from the tropical lowlands.

    Late-glacial records from Ecuador do not exhibit evidence of a climatic reversal (Hansen and Sutera, l995). Several sites in Peru give indications of a late-glacial climatic reversal although sediments from Laguna Junin indicate that the cooling occurred between 14,000-13,000 years ago, before what is normally observed for the Younger Dryas event (Hansen and Sutera, 1995). Further radiocarbon dating accompanied by high-resolution sampling is necessary. As noted above, ice cores from Peru and Bolivia show a strong late-glacial reversal (Thompson et al., 1995, 1998) that is probably correlative with the Younger Dryas, but dating is not yet unequivocal.

    For several decades, southern South America has been a controversial region with respect to a possible Younger Dryas signal (Heusser, 1990 Markgraf, l991 Denton et al., 1999). Two recent studies continue the debate from different regions of southern Chile. A study in the Lake District (Moreno et al., 2001) describes three sites at which conditions approached modern climate by about 15,000 years ago followed by cooling in two steps and then by warming around 11,200 years ago in a pattern similar to that in Europe and Greenland. The rough synchronism between Northern and Southern Hemispheres argues for a common forcing or rapid transmission of a climate signal between hemispheres. In contrast, a study farther south of four lakes shows no Younger Dryas signal (Bennett et al., 2000).

    New Zealand

    Late-glacial pollen evidence from New Zealand shows no substantial reversal of the trend toward warmer conditions after deglaciation (McGlone et al., 1997 Singer et al., 1998). However, a later study (Newnham and Lowe, 2000) found an interval of cooling that began about 600 years before the Younger Dryas and lasted for about a millennium also, as noted below, one New Zealand glacier advanced near the start of the Younger Dryas interval (Denton and Hendy, 1994 cf. Denton et al., 1999).


    Data from Central Africa suggest that arid conditions characterized the Younger Dryas in both highlands and lowlands (Bonnefille et al., 1995). The research focused on a high-resolution record from Burundi and com-

    pared data from 25 additional sites with limited sampling resolution and 14 C dating. Similarly, evidence of dry conditions during the Younger Dryas is summarized by Gasse (2000) for equatorial regions, subequatorial West Africa, and the Sahel. In South Africa, however, no strong terrestrial evidence of changes in temperature or moisture during the Younger Dryas was observed (Scott et al., 1995).

    Glacial-Geological Evidence of the Younger Dryas

    Glaciers are highly responsive to rapid climate change. Notable Younger Dryas advances of Norwegian and Finnish outlet glaciers and those in the Scottish mountains have been documented (Mangerud, 1991 Sissons, 1967). In the Americas, potential glacial evidence of the Younger Dryas event was observed near the Crowfoot glacier in Canada (Osborne et al., 1995 Lowell, 2000), the Titcomb Lakes moraine in the Wind River range in Wyoming (Gosse et al., 1995), and the Reschreiter glacier in Ecuador. More recent research suggests that the Younger Dryas in Peru was marked by retreating ice fronts, probably driven by a reduction in precipitation (Rodbell and Seltzer, 2000). In New Zealand, the Franz Joseph glacier began advancing early in the Younger Dryas (Denton and Hendy, 1994).

    Marine Evidence of Younger Dryas Oscillation

    The first evidence of Younger Dryas cooling in marine sediment cores was the observation of a return to increased abundance of the polar planktonic foraminiferal species Neogloboquadrina pachyderma in the North Atlantic (Ruddiman and McIntyre, 1981). This change suggested that reduction in formation of North Atlantic deep water was responsible for the Younger Dryas cooling observed on land (Oeschger et al., 1984 Broecker et al., 1985 Boyle and Keigwin, 1987). Later work documented North Atlantic ice-rafting events that correlate with rapid climate oscillations in Greenland, not only during the glacial period but also throughout the Holocene (Bond and Lotti, 1995). Deep-water corals from Orphan Knoll in the North Atlantic show large changes in intermediate-water circulation during the Younger Dryas (Smith et al., 1997). Cadmium:calcium ratios in shells from the North Atlantic subtropical gyre indicate increased nutrient concentrations during the Younger Dryas and the glacial period, and suggest millennial-scale oscillations affecting climate (Marchitto et al., l998). Sediment color and other data from the Cariaco Basin in the Caribbean

    indicate enhanced nutrient upwelling and thus higher productivity caused by increased trade wind strength during the Younger Dryas (Hughen et al., 1996), or decreased riverine runoff from adjacent land masses (Peterson et al., 2000).

    In the last decade, substantial paleooceanographic oscillations correlated with the Younger Dryas have been documented from as far away as the North Pacific. In the Santa Barbara Basin (Kennett and Ingram, 1995) and the Gulf of California (Keigwin and Jones, 1990), sediments that are normally anoxic became oxic during the Younger Dryas. Evidence of rapid climate variability in the northwestern Pacific over the last 95,000 years has been observed (Kotilainen and Shackleton, 1995). Even the eastern equatorial Pacific has yielded a Younger Dryas event determined from &delta 18 O and &delta 13 C records (Koutavas and Lynch-Steiglitz, 1999).

    In the North Arabian Sea and Indian Ocean, high-frequency climate variability linked to events in the Northern Hemisphere has also been demonstrated (Schulz et al., 1998). Off the coast of Africa at Ocean Drilling Program Site 658, an arid period corresponding to the Younger Dryas punctuated a longer humid period (deMenocal et al., 2000a). Between 20°N and 20°S, Younger Dryas cooling is observed on the basis of alkenone paleothermometry (Bard et al., 1997). In a sediment record that links land to ocean, Maslin and Burns (2000) documented evidence of a dry Younger Dryas in the tropical Atlantic Amazon Fan. As reviewed by Boyle (2000), work including that by Boyle and Keigwin (1987) and Bond et al. (1997) showed that changes in proxies from bottom-dwelling foraminiferal shells indicate reduction in deep export of waters that sank in the North Atlantic during the Younger Dryas. Alley and Clark (1999) reviewed evidence from several marine cores that show warmth during the Younger Dyras in the southern Atlantic and Indian Oceans, opposite to most global anomalies but consistent with the warmth indicated in most Antarctic ice cores at that time (Steig et al., 1998 Bender et al., 1999 Blunier and Brook, 2001).

    Overall, the available data indicate that the Younger Dryas was a strong event with a global footprint. Available data are not sufficient to identify the climate anomaly everywhere, and further understanding almost certainly will require more data. Different paleoclimatic recorders respond to different aspects of the climate system with different time resolution, so it is not surprising that the picture is not perfectly clear. Broadly, however, the Younger Dryas was a cold, dry, and windy time in much of the world although with locally wetter regions probably linked to storm-track shifts. The far southern Atlantic and many regions downwind in the southern

    Indian Ocean and Antarctica were warm during the Younger Dryas. Changes probably were largest around the North Atlantic and probably included reduced export of North Atlantic deep water. Changes into and especially out of the event were very rapid.


    The 110,000-year-long ice-core records from central Greenland (Johnsen et al., 1997 Grootes and Stuiver, 1997) confirmed that the Younger Dryas was one in a long string of large, abrupt, widespread climate changes (Figure 2.5). To a first approximation, the Younger Dryas pattern of change (size, rate, extent) occurred more than 24 times during that interval additional evidence from marine sediments indicates similar changes over longer times in earlier ice-age cycles (McManus et al., 1998).

    Such climate oscillations have a characteristic form consisting of gradual cooling followed by more abrupt cooling, a cold interval, and finally an abrupt warming. Events were most commonly spaced about 1,500 years apart, although spacing of 3,000 or 4,500 years is also observed (Mayewski et al., 1997 Yiou et al., 1997 Alley et al., 2001). The name Dansgaard/ Oeschger oscillation is often applied to such changes on the basis of early work by Dansgaard et al. (1984) and Oeschger et al. (1984). The terminology can be inconsistent the warm times associated with these during the ice age originally were termed Dansgaard/Oeschger events, but evidence of cyclic behavior suggests that oscillation is more appropriate.

    The sequence of Dansgaard/Oeschger oscillations is observed in various records, such as the histories of surface-water temperatures near Bermuda (which were cold when Greenland was cold) (Sachs and Lehman, 1999) oxygenation patterns of the bottom waters in the Santa Barbara basin (which were oxygenated when Greenland was cold) (Behl and Kennett, 1996) wind-blown dust supply to the Arabian Sea (which was dusty when Greenland was cold) (Schulz et al., 1998) and temperature records from the Byrd ice core, West Antarctica (which was warm when Greenland was cold) (Blunier and Brook, 2001). Methane decreased with almost all the Greenland coolings and rose with the warmings, although it changed more slowly than temperature (Chappellaz et al., 1997 Brook et al, 1999 Dällenbach et al., 2000). The colder phases of Dansgaard/Oeschger oscillations in the North Atlantic were marked by increased ice rafting of debris into colder, fresher surface water and by reduction in the strength of

    FIGURE 2.5 History of temperature in central Greenland over the last 100,000 years, as calculated by Cuffey and Clow (1997) from the data of Grootes and Stuiver (1997). The large Younger Dryas temperature oscillation (labeled YD), and the smaller temperature change of the event about 8,200 years ago (labeled 8ka) are just the most recent in a long sequence of such abrupt temperature jumps. Changes in materials from beyond Greenland trapped in the ice cores, including dust and methane, demonstrate that just as for the YD and 8ka events, the earlier events affected large areas of the earth nearly simultaneously.

    North Atlantic deep water formation (e.g., Lehman and Keigwin, 1992 Oppo and Lehman, 1995 Bond et al., 1993 Bond and Lotti, 1995). The geographic pattern of climate anomalies associated with the cold phases of the Dansgaard/Oeschger oscillations is thus quite similar to that of the Younger Dryas event.

    The millennial Dansgaard/Oeschger oscillations are bundled into multimillennial Bond cycles, although with variable spacing (Bond et al., 1993). Each Dansgaard/Oeschger oscillation is slightly colder than the previous one through a few oscillations then there is an especially long, cold interval, followed by an especially large, abrupt warming. The latter parts of the especially cold intervals are marked by the enigmatic Heinrich layers in the North Atlantic (Heinrich, 1988).

    Heinrich layers are extensive deposits of coarse-grained sediment across the North Atlantic Ocean. Much of the material in these layers is sufficiently coarse that important transport by icebergs must have occurred. Each Heinrich layer is as much as 0.5 m thick near Hudson Strait, thinning to less than 1 cm on the east side of the Atlantic (Andrews and Tedesco, 1992 Grousset et al., 1993). The ice-rafted sediments are dominated by material with geochemical signatures indicating an origin in Hudson Bay, whereas sediments between and in the thin edges of Heinrich layers include more diverse sources (Gwiazda et al., 1996a,b). Sedimentation of thicker parts of Heinrich layers was much faster than that of surrounding sediments (McManus et al., 1998) and occurred in an anomalously cold and fresh surface ocean (Bond et al., 1993).

    Heinrich events are correlated with greatly reduced North Atlantic deep water formation (Sarnthein et al., 1994) and climate anomalies similar to, but larger than, those of the cold phases of the non-Heinrich Dansgaard/ Oeschger oscillations (reviewed by Broecker, 1994 and Alley and Clark, 1999).

    The panoply of abrupt climate change through the cooling into and warming out of the most recent global ice age and probably earlier ice ages has not been convincingly explained. However, as reviewed later, many hypotheses exist, and there is strong evidence of change in the fundamental mode of operation of parts of the coupled system of atmosphere, ocean, ice, land surface, and biosphere.


    Temperatures similar to those of the most recent 10,000 years have been reached during previous interglacials, which have occurred approximately each 100,000 years over the last 700,000 years in response to features of earth&rsquos orbit. Each of these interglacials was slightly different from the others, at least in part because the orbital parameters do not repeat exactly. The penultimate interglacial, about 125,000 years ago, is known by several names including the Eemian, Sangamonian, and marine isotope stage 5e (with the different terminologies originating in different disciplines or geographic regions and being broadly but not identically equivalent).

    As the most recent near-equivalent of the current warm period, the Eemian is of obvious interest in learning what behavior is likely during warm times (van Kolfschoten and Gibbard, 2000). The orbital parameters for the Eemian produced somewhat more incoming solar radiation than

    today in high northern latitudes, bringing warmer conditions, at least during summers (Montoya et al., 1998). This probably led to major retreat of the Greenland ice sheet, which likely explains high sea levels during that interval without major changes in the West Antarctic ice sheet (Cuffey and Marshall, 2000). Ice-core records from Greenland for this interval originally were interpreted as showing extremely large and rapid climate fluctuations, but flow disturbances are now known to have occurred and affected the records (Alley et al., 1995 Chappellaz et al., 1997).

    Much work remains to be done on intact records from the Eemian, but it is increasingly clear from many paleoclimatic archives that although the Eemian included important paleoclimatic variability and ended abruptly, the warm period was not as variable as the periods during the slide into and climb out of the ice age that followed. In this relative stability, the Eemian had much in common with the current warm period, the Holocene.

    A comprehensive survey of Eemian paleoclimatic conditions is not yet available, but a few examples of results are highlighted here. Notable variations in Eemian conditions perhaps linked to changes in oceanic circulation were documented by Fronval et al. (1998) and Bjorck et al. (2000). North Atlantic surface-water temperature fluctuations during the Eemian may have been 1-2°C, as opposed to fluctuations of 3-4°C during the cold stage that followed immediately and a deglacial warming into the Eemian of about 7°C (Oppo et al., 1997).

    European pollen records are interpreted by Cheddadi et al. (1998) as indicating one rapid shift to cooler temperatures of 6 to 10°C between 4,000 and 5,000 years after the beginning of the Eemian, followed by smaller fluctuations of 2 to 4°C and 200 to 400 mm water/yr in the following few millennia. However, Boettger et al. (2000) found that the Eemian climate as recorded in isotopic data from central Germany was relatively stable, and the Eemian climate oscillations recorded in pollen records from the Iberian Margin similarly had low amplitude (Goñi et al., 1999). Cortijo et al. (2000) found that mid-latitude North Atlantic conditions during the Eemian involved no major instabilities, but that the cooling into the following glaciation occurred abruptly in less than 400 years.

    Large fluctuations reconstructed for Lake Naivasha (Kenya) from sediment characteristics and diatom assemblages bear similarities to those observed during the Holocene (Trauth et al., 2001). This is at least suggestive of a general pattern of relatively more important fluctuations in low-latitude moisture availability during warm times and high-latitude temperatures during cold times.

    Overall, the Eemian is neither stable and boring, nor extraordinarily variable. Most regions for which good data are available record significant and important fluctuations, some of which were abrupt, but with reduced variability compared to during the cooling into and warming out of ice ages. Attention is especially focused on drought conditions in low latitudes rather than temperature in high latitudes.


    The relevance of abrupt climate change of the ice age to the modern warm climate or future warmer climates is unclear. However, although glacial and deglacial rapid shifts in temperature were often larger than those of the Holocene (the last roughly 10,000 years), Holocene events were also important with respect to societally relevant climate change (Overpeck, 1996 Overpeck and Webb, 2000). For example, there were large rapid shifts in precipitation (droughts and floods) and in the size and frequency of hurricanes, typhoons, and El Niño/La Niña events. If they recurred, these kinds of changes would have large effects on society. It is not surprising that many past examples of societal collapse involved rapid climate change to some degree (Weiss and Bradley, 2001 deMenocal, 2001a).

    This section summarizes some of the compelling evidence of rapid change during the Holocene. When we view the available evidence of abrupt climate change in the Holocene, it is apparent that their temporal and spatial characteristics are poorly understood. In addition, the causes of abrupt change are not well constrained. The lack of a mechanistic understanding regarding past abrupt climatic change is one of the unsettling aspects of the state of the art.

    Among the most widely investigated rapid climate events of the early to middle Holocene are two that took place about 8,200 and 4,000-5,000 years ago. The former event (Figures 2.3 and 2.4) has been recognized in Greenland ice, the North Atlantic, North America, Europe, Africa, and elsewhere and has been tied to a temporary reduction in the North Atlantic thermohaline circulation generated by late-stage melting of the North American ice sheets that released a large, abrupt meltwater flood from ice-marginal lakes through Hudson Strait to the North Atlantic (Bjorck et al., 1996 Alley et al., 1997 Barber et al., 1999 Gasse, 2000 Gasse and van Campo, 1994 Kneller and Peteet, 1999, von Grafenstein et al., 1999 Yu and Eicher, 1998 cf. Stager and Mayewski, 1997). If the mechanism for this event has been identified correctly, the event was a final deglacial, or

    muted Younger Dryas-like event. Changes locally might have been as large as 10°C in the North Atlantic, with changes of about 2°C extending well into Europe (Renssen et al., 2001). High-resolution pollen studies show substantial and rapid vegetation response to the event in central Europe, with early biological changes lagging climate by less than 20 years (Tinner and Lotter, 2001). Because so many Holocene climate records are available and the cause of the event is rather clear, it provides an opportunity for an especially well-documented test case of model sensitivity. The event is also important because it punctuated a time when temperatures were similar to or even slightly above more recent levels, demonstrating that warmth is no guarantee of climate stability.

    A less well-understood hydrologic event from wet to dry conditions, occurring roughly 5,000 years ago, also took place during a warm period. This event is not as well documented and suffers from less than ideal temporal resolution of available records. It is most evident in African records (Gasse and Van Campo, l994 Gasse, 2000), the North Atlantic (Duplessy et al., 1992 Bond et al., 1997 deMenocal et al., 2000b Jennings et al., in press), the Middle East (Cullen et al., 2000), and Eurasia (Enzel et al., 1999 Morrill et al., in review). Four mechanisms have been proposed to explain the event, all of which could have contributed. First, it might have been associated with a cooling in the North Atlantic, perhaps related to a slow-down in thermohaline circulation (Street-Perrott and Perrott, 1990 Gasse and van Campo, 1994 Kutzbach and Liu, 1997 deMenocal et al., 2000b). Second, it might be related to a subtle (and variable) ca. 1500-year oscillation in Atlantic variability (Bond et al., 1997) of poorly understood origin, but almost certainly involving ocean processes (Alley et al., 1999), and extending beyond the North Atlantic regions recent work (Jennings et al., in press and Morrill et al., in review) indicated that the spatio-temporal dimensions of this variability could be complex. Third, an abrupt shift in the El Niño-Southern Oscillation (ENSO) system might have led to a more widespread event at about the time in question (Morrill et al., in review). Fourth, atmosphere-vegetation feedbacks triggered by subtle changes in the earth&rsquos orbit might have triggered the event (Claussen et al., 1999) or at least amplified it (Kutzbach et al., 1996 Ganopolski et al., 1998 Braconnot et al., 1999).

    Increasing attention is also being focused on the possibility that the ENSO system has changed its pattern of variability, perhaps rapidly. The best-documented shift in the frequency of ENSO variability occurred in 1976 (Trenberth, 1990), and it was probably one of several shifts in frequency to occur over the

    last 200 years (Urban et al., 2000). Discussion continues on the statistical significance and long-term persistence of these switches and on whether they should be considered evidence of normal oscillations, of short-lived abrupt shifts, or of long-lived abrupt climate change (e.g., Rajagopalan et al., 1999 Trenberth and Hurrell, 1999a,b). Further back in the Holocene, the ENSO system might have been dramatically different from today, with much reduced variability and fewer strong events (Overpeck and Webb, 2000 Diaz and Markgraf, 2000 Cole, 2001 Sandweiss et al., 2001 Tudhope et al., 2001). Although the time at which modern ENSO variability became established is not known, there have been several model-based efforts to explain the changes, all tied to the response of the coupled atmosphere-ocean system to small orbitally induced insolation changes (Bush, 1999 Otto-Bliesner, 1999 Clement et al., 2000, 2001). The shift to more-modern ENSO variability also might have been coincident with other earth-system changes 4,000-5,000 years ago. Sandweiss et al. (2001) suggested that ENSO events were absent or substantially different from more recently between 8,800-5,800 years ago, present but reduced between 5,800-3,200 years ago, and increased to modern levels between 3,200-2,800 years ago, that would be consistent with other data that they summarize. Rodbell et al. (1999) placed the Holocene onset of El Niños at 7,000 years ago, with the beginning of modern levels reached 5,000 years ago.

    Although there are other hints of important abrupt climate changes in the Holocene record, most of them have not been studied to the degree needed to place them in a coherent context (for example, examined at multiple sites). One important observation is that the landfall frequency of catastrophic hurricanes has changed rapidly during the Holocene. For example, the period about 1,000-3,500 years ago was active on the Gulf Coast compared with the last 1,000 years and changes in North Atlantic climate could be the primary cause (Liu and Fearn, 2000 Donnelly et al., 2001a,b). The period near 1,000 years ago was also possibly marked by a substantial change in hydrologic regimes in Central and North America (Hodell et al., 1995, 2001 Forman et al., 2001).

    Climate variations within the last millennium are, in general, better resolved temporally and spatially than are variations earlier in the Holocene. This is due largely to the greater availability of annually dated records from historical documents, trees, corals, ice cores and sediments, but this availability is also due to greater emphasis on the last millenium by large paleoenvironmental science programs, such as PAst Global changES (PAGES) of the International Geosphere-Biosphere Programme (IGBP). Per-

    haps the most studied rapid temperature shift of the Holocene is the change that began in the latter half of the nineteenth century and ended the so-called Little Ice Age. The shift and later state of substantial global warming were unprecedented in the context of the last 500 years and might be due to a combination of natural (such as solar and volcanic) and human-induced (such as trace-gas) forcing (Overpeck et al., 1997 Jones et al., 1998 Mann et al., 1998, 1999, 2000 Huang et al., 2000 Crowley, 2000 Briffa et al., 2001 Intergovernmental Panel on Climate Change, 2001a).

    In contrast with the abrupt late nineteenth to early twentieth century warming, timing of the onset of the Little Ice Age is difficult to establish in that the change manifests itself as a period of slow Northern Hemisphere cooling beginning at or before ca 1000 (Mann et al., 1999 Crowley, 2000 Crowley and Lowery, 2000 Briffa et al., 2001) with several sustained cooler intervals thereafter (for example, the seventeenth century and early nineteenth century).

    There are insufficient paleoclimate records to allow complete reconstruction of the last 1,000 years of change in the Southern Hemisphere, and uncertainty remains on the amplitude of Northern Hemisphere change in this interval (e.g., Briffa et al., 2001 Huang et al., 2000). There is still debate as to whether the &ldquoMedieval Warm Period&rdquo was more than a Northern Hemisphere warm event (Mann et al., 1999 Crowley, 2000 Crowley and Lowery, 2000 Briffa et al., 2001 Broecker, 2001). Moreover, evidence is scarce outside the North Atlantic-European sector (Jennings and Weiner, 1996 Keigwin, 1996 Broecker, 2001) for medieval temperatures that were close to mean twentieth century levels. Additional annually resolved records for the last 2,000 years are needed to answer such fundamental questions.

    Holocene Droughts

    The existing temperature records, as described above, make it clear that natural variability alone can generate regional to hemispheric temperature anomalies that are sufficient to affect many aspects of human activity. However, the record of hydrologic change over the last 2,000 years suggests even larger effects: there is ample evidence that decadal, even century-scale, drought can occur with little or no warning.

    A synthesis of US drought variability over the last 2,000 years (Woodhouse and Overpeck, 1998) used records from a diverse array of proxy sources (cf. Cronin et al., 2000 Stahle et al., 1998). From this synthesis, it was concluded that multi-year droughts similar to the 1930s Dust

    Bowl or the severe 1950s southwest drought have occurred an average of once or twice per century over the last 2,000 years. Furthermore, decadal &ldquomegadroughts&rdquo have also occurred often, but at less frequent intervals. The last of these occurred in the sixteenth century, spanned much of northern Mexico to Canada, and lasted over 20 years in some regions (Woodhouse and Overpeck, 1998 Stahle et al., 2000). An earlier event in the thirteenth century also persisted for decades in some locations and involved the long-term drying of lakes in the Sierra Nevada of California (Stine, 1994) and the activation of desert dunes in parts of the High Plains (Muhs and Holliday, 1995, 2001). There is evidence of even longer droughts further back than the last millennium (Stine, 1994 Laird et al., 1996 Fritz et al., 2000), including an unprecedented multidecadal drought that has been implicated in the collapse of the Classic Mayan civilization (Hodell et al., 1995, 2001), several droughts that led to the remobilization of eolian landforms on the High Plains (Forman et al., 2001), and linkage between droughts in tropical and temperate zones (Lamb et al., 1995). An important conclusion from paleodrought research is that drought regimes can shift rapidly and without warning. A prominent example is the shift, at about 1200 BP, from a regime characterized by frequent long droughts on the High Plains to the current regime of less-frequent and shorter droughts (Laird et al., 1996 Woodhouse and Overpeck, 1998).

    Despite growing knowledge of the paleodrought record, causal mechanisms of changes are poorly understood (Woodhouse and Overpeck, 1998). Persistent oceanic temperature anomalies, perhaps related to ENSO or the North Atlantic Oscillation (NAO) as described below, have been proposed as one potential forcing mechanism (Forman et al., 1995 Black et al., 1999 Cole and Cook, 1998 Cole et al., submitted), but cause and effect have yet to be proved in the case of any decadal or longer paleodrought in North America. There is also good evidence of late Holocene multidecadal droughts outside North America (e.g., Stine, 1994 Verschuren et al., 2000 Nicholson, 2001) their causes are equally enigmatic. Thus, although we know that droughts unprecedented in the last 150 years have occurred in the last 2,000 years and so could occur in the future, we do not have the scientific understanding to predict them or recognize their onset.

    Holocene Floods

    Just as the twentieth century instrumental record is too short to understand the full range of drought, it is too short to understand how the fre-

    quency of large floods has changed (Baker, 2000). Data on past hydrological conditions from the upper Mississippi River (Knox, 2000) and from sediments in the Gulf of Mexico (Brown et al., 1999) record large, abrupt shifts in flood regimes in the Holocene, which may have been linked to major jumps in the location of the lower Mississippi (delta-lobe switching). In the western United States, there is growing evidence that flood regimes distinctly different from today, and also episodic in time, were the norm rather than the exception. The frequency of large floods in the Lower Colorado River Basin, for example, appears to have varied widely over the last 5,000 years (Ely et al., 1993 Enzel et al., 1996), with increased frequency from about 5,000-4,000 years ago, then lower frequency until about 2,000 years ago, and some abrupt shifts up, down, and back up thereafter (Ely, 1997). Those flood-frequency fluctuations and substantial fluctuations elsewhere around the world (e.g., Gregory et al., 1995 Baker, 1998 Benito et al., 1998) appear to be linked to climate shifts but in poorly understood ways. Clearly, a predictive understanding of megadroughts and large floods must await further research.

    This observation about droughts and floods applies at some level to all the abrupt climate changes recorded in proxy records. The data are clear. Ice-age events were especially large and widespread and involved changes in temperature, precipitation, windiness, and so on. Holocene events were more muted in polar regions, might have been more regionalized, and usually involved water availability, but often with important temperature changes as well. Multi-characteristic global-anomaly maps are not available for any of the abrupt changes, and additional records and proxy techniques will be required to provide such anomaly maps. Coverage gaps appear especially large in the oceans and southern latitudes, although broad gaps also exist elsewhere.


    Instrumental records from scientific monitoring programs offer the possibility of capturing directly the relevant data on abrupt climate change with greater accuracy and spatial coverage than are possible from the necessarily limited proxy records. The relatively short period of instrumental records means that they have missed most of the abrupt changes discussed above, although some droughts and the warming from the Little Ice Age have been captured rather well. Instrumental records will become more valuable as their length increases, which argues for maintenance of key

    observational data sets. Instrumental records also are critical in characterizing patterns of climate variability that might have contributed to paleoclimatic abrupt change, and might contribute to abrupt climate change in the future. It is important to the understanding of abrupt climate change that these patterns or &ldquomodes&rdquo of circulation and its variability be understood, particularly on the time-scale of decades to centuries. The abrupt changes surveyed here are smaller in strength than the extreme events of the paleoclimate record, yet they are nonetheless significant as human populations press the capacity of the environment, locally and globally.

    Atmospheric instrumental data include surface values and vertical profiles of numerous physical variables, including temperature, pressure, radiation, and winds. Surface observations, satellite radiometric observations, and the global network of regularly launched radiosonde profilers are assimilated into computer models of the atmosphere to analyze weather and climate. They capture both the conditions that cause atmospheric circulation and the resulting atmospheric motions. Much of our current understanding of climate comes from the relatively accurately observed period since 1950. More subtle are the measurements of trace chemicals, which both affect the physical state of the atmosphere, and can be used to infer its motions. The longest atmospheric time series, dating back several hundred years, are surface temperature and pressure.

    The ocean, like the atmosphere, is a thin fluid envelope covering much of the earth. Satellites are now collecting global observations of the temperature, elevation and roughness of the sea surface, which tell us the surface currents and winds fairly accurately. Crucial climate variables, such as sea-ice cover and movement (and to a lesser accuracy, ice thickness), have been measured by satellites beginning in the 1970s. Yet, oceanic data are still more restricted in coverage and duration than atmospheric data, for it is still difficult to penetrate the depths of the ocean with instruments in sufficient numbers.

    In addition to the purely instrumental problem, ocean currents and eddies are smaller in size than major atmospheric wind fields, making the mapping of ocean circulation more difficult (weather patterns are well matched in size to the spacing of major cities, which historically made their discovery possible, using simple barometers). Another contrasting property is the time for fluid to adjust fully to a change in external forcing: in the atmosphere this time is a month or two, while in the ocean it is measured in millennia. The ocean dominates the global storage of heat, carbon, and

    water of the climate system while the atmosphere dominates the rapid response of the climate system and more directly impacts human activity.

    The ocean&rsquos direct impact on the atmosphere is primarily through sea-surface temperature and ice-cover. Thus, it is fortunate that temperature records are among the longest oceanic time-series and have the best spatial coverage. Data sets include sea-surface temperatures from ocean vessels, long coastal sea-level and temperature records, and shorter or more scattered time-series of temperature and salinity from surface to sea-floor. Increasingly long time-series of directly measured ocean currents are becoming available, particularly in the tropics. The TAO (Tropical Atmosphere-Ocean) array in the Pacific, sometimes called the world&rsquos largest scientific instrument, measures equatorial temperatures, winds and currents around one-quarter of the earth&rsquos circumference (e.g., McPhaden et al., 1998). The array has given us detailed portraits of El Niño-Southern Oscillation (ENSO) cycles and equatorial general circulation.

    Over longer times other aspects of oceanic circulation, chemistry and biology become important to climate. For example, the heat storage available to the atmosphere is strongly dependent on circulation and salinity stratification of the upper ocean. The depths of the ocean become involved as the thermohaline circulation (THC) and wind-driven circulation interact to reset surface conditions. There are &ldquooverturning circulations&rdquo at many scales, from the global THC (see Plate 4) to the shallow, near-surface cells of overturning lying parallel to the equator. Direct measurements of the circulation of the deep ocean are still sparse, and indirect means are often used to infer the circulation. Water density (from measured temperature and salinity) can be combined with dynamical constraints and atmospheric observations of air-sea interaction to estimate global ocean circulation (e.g., Ganachaud and Wunsch, 2000 Reid, 1994, 1998, 2001). The results are consistent with the limited direct measurements of currents, and also with the patterns of observed chemical tracers in the ocean. The tracers include natural dissolved gases and nutrients, dynamical quantities such as potential vorticity and potential density, and chemical inputs from human activity. Transient chemical tracers, injected into the atmosphere and subsequently absorbed by the ocean, provide particularly useful images of the ocean circulation. Bomb radiocarbon, tritium and chlorofluorocarbons (CFCs), for example, allow verification and quantitative assessment of the pathways of high-latitude sinking, equatorward flow in boundary currents, and interaction with the slower flow of mid-ocean regions (Broecker and Peng, 1982 Doney and Jenkins, 1994 Smethie and Fine, 2001).

    The location, strength and depth penetration of the major sinking regions of the ocean at high latitude (see Plate 4) are known to have changed during glacial cycles, emphasizing the importance of sea-ice cover in insulating the ocean from the atmosphere, preventing deep convection and physical sinking from occurring (e.g., Sarnthein et al., 1994). The contrasting effect of freezing sea-water is that salty brine is rejected from the ice, yielding a small but very dense volume of water that can contribute to sinking events. During the twentieth century lesser yet still significant shifts of the deep circulation (e.g., Molinari et al., 1998) have been verified by tracers and direct current measurements.

    Abrupt changes in climate can occur with spatial patterns that in some way reflect the natural dynamics of atmosphere and ocean. These &ldquomodes&rdquo of circulation are seen in the seasonal, interannual and decadal variability of the system, and have great potential as an aid to understanding just how abrupt changes can occur. At work in establishing the modes are &ldquoteleconnections&rdquo both vertically, and across the globe. Various waves, particularly Rossby (or &ldquoplanetary&rdquo) waves and Kelvin waves, and unstable waves on the time-averaged circulation, are involved, as is the direct transport of climate anomalies by the circulation.

    Natural variability of climate is now occurring in the context of global warming, so the discussion of abrupt climate change during the period of instrumental records must acknowledge the presence of anthropogenic and natural change, and the possibility of strong interaction between them.


    Instrumental records show that the climate is characterized by patterns or modes of variability, such as the polar annular modes and ENSO of the equatorial Pacific, as described below. The spatial patterns can provide regional intensification of climate change in quite small geographic areas. The strong couplings and feedbacks among at least the atmosphere, oceans and sea ice, and probably other elements of the climate system, allow a pattern to persist for periods of years to many decades. The different regional modes also interact with one another. For instance, Amazonian rainfall responds to a mode of tropical Atlantic variability, which itself might be responding to ENSO or the Arctic Oscillation.

    The behavior of highly idealized models of the climate system suggests that climate change can be manifested as a shift in the fraction of the time that climate resides in the contrasting phases (for example, warm/cold or

    strong-wind/weak-wind) of such oscillations (Palmer, 1993). However, the scientific community is divided on the issue of whether analogous &ldquoregime-like&rdquo behavior exists in instrumental records related to the real climate system. Hansen and Sutera (1995), Corti et al. (1999), and Monahan et al. (2000) found evidence of such mode-shift behavior. However observational evidence has been questioned (e.g., Nitsche et al., 1994 Berner and Branstator, 2001). Also, questions remain about whether such behavior should be characteristic of an entity with as many degrees of freedom as the climate system (Dymnikov and Gritsoun, 2001).

    The possibility that mode shifts participated in or provide clues to the large, abrupt climate changes of preinstrumental times suggests common mechanisms or even common causes. Thus, the study of abrupt climate change should involve consideration of the preferred modes of the climate system.

    Annular Modes

    The annular modes&mdashthe Arctic Oscillation (AO) and the Antarctic Oscillation (AAO)&mdashprimarily affect polar to middle-latitude regions in both the North and South and are the dominant modes of climate variability in these areas, especially in the winter. The AO and AAO represent a transfer of atmospheric mass between subtropical high-pressure regions and polar lows. A strongly positive state of an annular mode is associated with intensified highs and lows driving strong atmospheric circulation. The negative state has much less difference between high- and low-pressure regions and thus is related to weaker atmospheric circulation.

    The southern annular mode is moderately symmetric about the pole, but owing to the complex geometry of northern continents, the AO is especially strong over the North Atlantic and less evident in other regions. Thus, the mode was originally described as the North Atlantic Oscillation (NAO), and an NAO index was based on the difference in atmospheric pressure between Portugal and Iceland (Hurrell, 1995). When the winter pressure difference is large, frequent strong storms take a northeasterly track across the North Atlantic, producing warm and wet weather in northern Europe, cold and dry conditions in northern Canada, and mild and wet conditions along the US East Coast. In contrast, a small pressure difference produces fewer, weaker storms, taking an easterly track to produce a moist Mediterranean, cold northern Europe, and a snowy US East Coast in response to frequent cold-air outbursts.

    ENSO and ENSO-Related Variability

    A weakening of the trade winds in the equatorial Pacific and attendant warming of the sea surface (or lack of cooling by upwelled cold water) is known as an El Niño event. Such events alternate with an opposing state, popularly referred to as &ldquoLa Niña,&rdquo with strong trade winds and upwelling of cold waters off Peru and along the Equator. The few-year oscillation between those different states is the El Niño/Southern Oscillation. The coupled oscillation of the tropical ocean and atmosphere is important in global climate, with impacts that extend far beyond the tropical Pacific to the tropical Atlantic and Indian Oceans, to the Southern Ocean, and to middle to high latitudes in the Northern Hemisphere. There are speculations that greenhouse warming is sufficient to put the world into a warmer, near-perpetual El Niño state (e.g., Timmerman et al., 1999 Federov and Philander, 2000), but there is no strong consensus.

    ENSO might be linked to another of the leading patterns of variability, the so-called Pacific North American (PNA) pattern, which exerts a strong influence on distribution of rainfall and surface temperature over western North America. Like the AO, the PNA pattern fluctuates randomly from one month to the next, but also exhibits what appear to be systematic variations on a much longer time scale. Since 1976-1977, the positive polarity of the PNA pattern&mdashmarked by a tendency toward relatively mild winters over Alaska and western Canada, below-normal rainfall and stream flows over the Pacific Northwest, and above-normal rainfall in the southwestern United States&mdashhas been prevalent, whereas during the preceding 30-year period the opposite conditions prevailed.

    The abrupt shift toward the positive polarity of the PNA pattern in 1976-1977 was coincident with and believed to be caused by a widespread pattern of changes throughout the Pacific Ocean. Sea-surface temperatures along the equatorial belt and along the coast of the Americas became warmer, while farther to the west at temperate latitudes the sea surface became cooler (Nitta and Yamada, 1989 Trenberth, 1990 Graham, 1994). An array of changes in the marine ecosystem occurred around the same time (Ebbesmeier et al., 1991). For example, salmon recruitment underwent a major readjustment toward more abundant harvests along the Alaskan coast accompanied by deteriorating conditions in southern British Columbia and the US Pacific Northwest (Francis and Hare, 1994). Another basin-wide &ldquoregime shift&rdquo that was analogous in many respects to the one that occurred in 1976-1977, but in the opposite sense, was observed during the 1940s (Zhang et al., 1997 Minobe and Mantua, 1999), and there are

    indications of prior shifts as well (Minobe, 1997). The suite of atmospheric and oceanic changes that have been linked to these basin-wide regime shifts is collectively referred to as the Pacific Decadal Oscillation (PDO) (Mantua et al., 1999).

    The sea surface temperature (SST) patterns associated with the PDO and ENSO are similar, the main distinction being that the extratropical features are somewhat more prominent in the PDO pattern. As in the few-year variations associated with the swings between El Niño (warm) and La Niña (cold) conditions in the equatorial Pacific, warm and wet decades in the equatorial zone tend to be marked by extratropical circulation patterns that favor an unusually active storm track in the mid-Pacific that splits toward its eastern end. An unusually large fraction of disturbances moves northeastward, bringing mild, wet weather to the Alaska panhandle many of the remainder track southeastward, bringing heavy rains to southern California and the US desert Southwest. The mountain ranges of British Columbia and the US Pacific Northwest, which lie directly downstream from the split in the storm track, tend to receive less than the normal amount of winter snowfall, and this reduces water supplies for the following summer season. The dynamic mechanisms responsible for the long-range &ldquoteleconnections&rdquo between the equatorial Pacific and the extratropics are better understood than the processes that control the evolution of this phenomenon on the decadal time scale. Hence, regime shifts such as the one that occurred in 1976-1977 are difficult to diagnose in real time, let alone to predict.

    There are several different schools of thought as to the nature of the interdecadal PDO variability, which has shown both the abruptness and persistence to qualify under our definition of abrupt climate change. The default hypothesis is that the PDO is merely a reflection of stochastic variability originating in the atmosphere but amplified by positive feedbacks associated with coupling between the atmosphere and ocean (Bretherton and Battisti, 2000). If this interpretation is valid, it follows that this ENSO-like variability is inherently unpredictable (i.e., that it becomes clearly evident only with the benefit of hindsight). Hopes that the phenomenon is deterministic, and therefore predictable, are based on the notion that ocean dynamics play an active role in PDO evolution, to the extent of setting the time scale for the major swings back and forth between the positive and negative polarity of the PDO pattern. One oceanic process that could conceivably set the time scale is the recirculation time for water parcels in the clockwise North Pacific and counterclockwise South Pacific subtropical

    gyres. A second subtropical gyre time scale is set by the time it takes for oceanic planetary waves to propagate to the western boundary currents, which then feed back on the atmospheric circulation. A third is the time required for water parcels subducted in the extratropical North and South Pacific at latitudes around 35°N and 25°S to reach the equatorial thermocline. Mechanisms that depend on those processes have been demonstrated to be capable of producing ENSO-like interdecadal variability in coupled atmosphere-ocean models (Latif and Barnett, 1996). Further data and model results are needed to learn the extent to which the time scales of the variability can change and whether the climate can &ldquolock into&rdquo one or another phase of the major oscillations. Mean ice-age conditions in the tropical Pacific appear to have been more La Niña-like than during the Holocene perhaps this suggests a linkage. Species and shell chemistry and isotopic ratios of planktonic foraminfera (Lee et al., 2001) and chemistry and isotopic ratios of corals (Tudhope et al., 2001) give evidence for equatorial Pacific sea-surface temperatures back at least 130,000 years. Cooler mean SST during the glaciations (

    3°C cooler than modern at the last glacial maximum in the Lee et al. study also, Patrick and Thunell, 1997 Pisias and Mix, 1997 also see Alley and Clark, 1999) and continued, yet weaker ENSO cycles are evident. Stronger glacial easterly equatorial winds are inferred (Lyle, 1988).

    Tropical Variability in the Atlantic and Indian Oceans

    Tropical variability arising from feedbacks within the Atlantic and Indian equatorial regions also contributes to regional climate modes, although of smaller global impact than ENSO, probably because of the vast width of the Pacific relative to the Atlantic or Indian. Tropical Atlantic variability correlates strongly with forcing from ENSO and the AO. The tropical Atlantic also has a mode that is symmetric about the equator with mechanisms similar to those in ENSO and might contribute to regional predictability (Amazonian and west African/Sahelian rainfall). Off-equatorial modes of tropical Atlantic variability are associated with the strength and location of the northern and southern Intertropical Convergence Zones (ITCZs) work in recent years has revealed that Northern and Southern Hemisphere SST variability are not tightly linked. Tropical Atlantic variability has a major impact on rainfall in northern Africa and northern South America and an impact on hurricane frequency and patterns in the North Atlantic.

    In the Indian tropical region, the seasonal monsoon driven by ocean-land temperature contrasts has a major impact on human life. The monsoon is perhaps the classic example of ocean-atmosphere-land interactions. During boreal summer, a northward shift of the ITCZ to the Indian sub-continent creates a major precipitation and heat source in this region. Interannual variability in the Indian monsoon correlates closely with tropical Indian Ocean SST. Indian Ocean SST is affected by ENSO and by an intrinsic Indian Ocean east-west mode of variability similar in mechanism but uncorrelated with the Pacific&rsquos ENSO.

    Extended Summer Drought

    The Northern Hemisphere&rsquos annular mode and the decade-to-decade ENSO-like variability discussed in the previous sections both affect Northern Hemisphere climate mainly during the winter season, and they involve the atmosphere&rsquos own preferred modes of month-to-month variability. In contrast, drought and desertification, when they occur in extratropical latitudes, are primarily summer phenomena whose geographic distribution and evolution are determined as much by land-surface processes as by atmospheric dynamics. Dynamical modes may still be involved, however, as the summer pattern of great anticyclones over the oceans responds to the heating of the continents. Kelvin and Rossby waves are active in determining the shape, extent, and flow of moisture in this pattern (Rodwell and Hoskins, 2001), and in turn these waves are involved in dynamical modes as noted above.

    An extended drought popularly known as the Dust Bowl affected large areas of the United States through most of the decade of the 1930s. Over parts of the Great Plains and Midwest, the 1931-1939 summers were on the average substantially warmer than the long-term climatological mean for the season, with daily maxima often in excess of 40°C, and precipitation was deficient (Borchert, 1950 Skaggs, 1975 Karl and Quayle, 1981 Diaz, 1983 Chang and Wallace, 1987 Chang and Smith, 2001). Much of the topsoil was irreversibly lost&mdashblown away in dust storms that darkened skies as far downstream as the eastern seaboard. Numerous farms were abandoned, and agricultural productivity dropped sharply. Many who lived through the Dust Bowl must have wondered whether climatic conditions would ever be suitable for farming again. Yet toward the end of the decade, the rains returned, and the region has never since been plagued by such an extended drought.

    What initiated the Dust Bowl in the early 1930s and what caused the rains to return nearly a decade later are still open questions. The prevailing view is that drought is an inherently stochastic phenomenon, initiated and terminated by random fluctuations in atmospheric circulation patterns, and sustained over long periods of time by positive feedback from the terrestrial biosphere (Namias, 1960 Rind, 1982 Shukla and Mintz, 1982 Karl, 1983 Sud and Molod, 1988 Bravar and Kavvas, 1991 Xue and Shukla, 1993 Dirmeyer, 1994 Lare and Nicholson, 1994). A few weeks of abnormally hot, dry weather are sufficient to desiccate the upper layers of the soil, reducing the water available for plants to absorb through their root systems. The plants respond by reducing the rate of evapotranspiration through leaves during the daylight hours (Dirmeyer, 1994 Radersma and de Reider, 1996 Xue et al., 1996). Reduced evapotranspiration inhibits the ability of the plants to keep themselves and the earth&rsquos surface beneath them cool during midday, when the incoming solar radiation is strongest (Somayao et al., 1980 Gardner et al., 1981). This favors higher afternoon temperatures and also reduces the humidity within the lower 1-2 km of the atmosphere (Walsh et al., 1985 Karl, 1986 Georgakakos et al., 1995 Huang et al., 1996 Dai et al., 1999). Because this boundary-layer air is the source of roughly half the moisture that condenses in summer rainstorms over the central United States, lower humidity favors reduced precipitation (Brubaker et al., 1993 Eltahir and Bras, 1996 Koster and Suarez, 1996 Findell and Eltahir, 1999 Trenberth, 1999). Higher daily maximum temperatures, lower humidity, and reduced precipitation all increase the stress on plants. If the stress is sufficiently severe and long, the physiological changes in plants become irreversible. Once the threshold is crossed, the earliest hope for the restoration of normal vegetation is the next spring growing season, which can be 6 or even 9 months away. Throughout the remainder of the summer and early autumn, the parched land surface continues to exert a feedback on the atmosphere that perpetuates the abnormally hot, dry weather conditions (Yeh et al., 1984 Huang and Van den Dool, 1993 Yang et al., 1994 Huang et al., 1996 Fennesy and Shukla, 1999).

    The wilting of the plants also affects hydrological conditions in the ground. In the absence of healthy root systems, water runs off more rapidly after rainstorms, leaving behind less to nurture the plants. Once the water table drops substantially, an extended period of near- or above-normal precipitation is required to restore groundwater (Palmer, 1965 Entekhabi et al., 1992 Bravar and Kavvas, 1991 Stamm et al., 1994). The remarkable year-to-year persistence of the 1930s drought attests to the memory of the

    vegetation and the ground. Once established, an arid climate regime, such as the one that prevailed during the Dust Bowl, appears to be capable of perpetuating itself until a well-timed series of rainstorms enables the vegetation to regain a foothold (Dirmeyer and Shukla, 1996 Wang and Eltahir, 2000a,b Clark et al., 2001).

    The onset and termination of the 1930s Dust Bowl are examples of abrupt regime shifts from a climate conducive to agriculture to a climate more characteristic of a desert region and back again. During the time covered by instrumental records, such shifts have occurred rather infrequently in the United States but more regularly in semiarid agricultural regions, such as the Sahel, northeast Brazil, and the Middle East (Nicholson et al., 1998 Street-Perrott et al., 2000). If such dry regimes are sufficiently frequent or long, the cumulative loss of topsoil due to wind erosion makes it increasingly difficult for vegetation to thrive, and difficult-to-reverse &ldquodesertification&rdquo occurs (United Nations, 1980). Thus far, the United States has experienced relatively little true desertification, but other regions of the globe have not been as fortunate. For example, it is well documented that the Sahara expanded northward and engulfed formerly productive agricultural regions of North Africa during the last few centuries of the Roman Empire (Reale and Dirmeyer, 2000) this transition might well have involved a series of prolonged drought episodes analogous to the Dust Bowl.

    Agricultural practices influence the retention of topsoil. Poor cultivation practices and overgrazing have been blamed for the desertification that has plagued North Africa, the Sahel, and other semi-arid regions (Otterman 1981 Wendler and Eaton, 1983 Balling, 1988 Bryant et al., 1990 BenGai et al., 1998 Nicholson et al., 1998 Pickup, 1998), and the planting of hedgerows designed to impede the flow of wind-blown dust has been credited with sparing much of the US Great Plains from suffering a similar fate. Whether adherence to environmentally sound agricultural practices will be sufficient to prevent further desertification is less clear.

    Global warming could render such regions as the western and central United States more vulnerable to extended drought episodes by increasing temperatures during the growing season, and thereby increasing the rate of evapotranspiration. There is no conclusive evidence of such behavior in response to the rapid warming of the last two decades, but simulations with climate models indicate that more pronounced warming like that predicted to occur by the end of the twenty-first century could serve to increase the frequency of drought episodes and the risk of irreversible desertification (Rind et al., 1989 Henderson-Sellers et al., 1995 Bounoua et al., 1999).


    The scientific consensus in the 2014 IPCC Fifth Assessment Report is that:

    A large fraction of both terrestrial and freshwater species faces increased extinction risk under projected climate change during and beyond the 21st century, especially as climate change interacts with other stressors, such as habitat modification, over-exploitation, pollution, and invasive species. Extinction risk is increased under all RCP scenarios, with risk increasing with both magnitude and rate of climate change. Many species will be unable to track suitable climates under mid- and high-range rates of climate change during the 21st century. Lower rates of climate change will pose fewer problems.

    Some predictions of how life would be affected:

    • Mediterranean Monk Seal: These animals have lost about 60% of their population in the past sixty years.
    • Miombo Woodlands of South Africa: If the temperature were to rise by at least 4.5 degrees Celsius, this area would lose about 90% of its amphibians, 86% of birds, and 80% of mammals.
    • The Amazon could lose 69 percent of its plant species.
    • In southwest Australia 89 percent of amphibians could become locally extinct.
    • 60 percent of all species are at risk of localised extinction in Madagascar.
    • The Fynbos in the Western Cape Region of South Africa, which is experiencing a drought that has led to water shortages in Cape Town, could face localized extinctions of a third of its species, many of which are unique to that region." - WorldWildLife Fund

    Temperature increase would affect the amount of rainfall and therefore the amount of drinking water animals need to survive. It would affect plant growth and desertification. This would further spread in other issues including overgrazing and loss of biodiversity. [ citation needed ]

    2004 Edit

    In one study published in Nature in 2004 found that between 15 and 37% of 1103 endemic or near-endemic known plant and animal species will be "committed to extinction" by 2050. [8] More properly, changes in habitat by 2050 will put them outside the survival range for the inhabitants, thus committing the species to extinction.

    Other researchers, such as Thuiller et al., [9] Araújo et al., [10] Person et al., [11] Buckley and Roughgarden, [12] and Harte et al. [13] have raised concern regarding uncertainty in Thomas et al. ' s projections some of these studies believe it is an overestimate, others believe the risk could be greater. Thomas et al. replied in Nature [14] addressing criticisms and concluding "Although further investigation is needed into each of these areas, it is unlikely to result in substantially reduced estimates of extinction. Anthropogenic climate change seems set to generate very large numbers of species-level extinctions." On the other hand, Daniel Botkin et al. state ". global estimates of extinctions due to climate change (Thomas et al. 2004) may have greatly overestimated the probability of extinction. " [15]

    Mechanistic studies are documenting extinctions due to recent climate change: McLaughlin et al. documented two populations of Bay checkerspot butterfly being threatened by precipitation change. [16] Parmesan states, "Few studies have been conducted at a scale that encompasses an entire species" [17] and McLaughlin et al. agreed "few mechanistic studies have linked extinctions to recent climate change." [16]

    2008 Edit

    In 2008, the white lemuroid possum was reported to be the first known mammal species to be driven extinct by climate change. However, these reports were based on a misunderstanding. One population of these possums in the mountain forests of North Queensland is severely threatened by climate change as the animals cannot survive extended temperatures over 30 °C. However, another population 100 kilometres south remains in good health. [18]

    2010 Edit

    The risk of extinction does need to lead to a demonstrable extinction process to validate future extinctions attributable to climate change. In a study led by Barry Sinervo, [19] a mathematical-biologist at the University of California Santa Cruz, researchers analyzed observed contemporary extinctions (since dramatic modern climate warming began in 1975). Results of the study indicate that climate-forced extinctions of lizard families of the world have already started. The model is premised on the ecophysiological limits of an organism being exceeded. In the case of lizards, this occurs when their preferred body temperature is exceeded in their local environment. Lizards are ectotherms that regulate body temperature using heat sources of their local environment (the sun, warm air temperatures, or warm rocks). Surveys of 200 sites in Mexico showed 24 local extinctions (= extirpations), of Sceloporus lizards. Using a model developed from these observed extinctions the researchers surveyed other extinctions around the world and found that the model predicted those observed extirpations, thus attributing the extirpations around the world to climate warming. These models predict that extinctions of the lizard species around the world will reach 20% by 2080, but up to 40% extinctions in tropical ecosystems where the lizards are closer to their ecophysiological limits than lizards in the temperate zone. [20]

    2012 Edit

    According to research published in the January 4, 2012 Proceedings of the Royal Society B current climate models may be flawed because they overlook two important factors: the differences in how quickly species relocate and competition among species. According to the researchers, led by Mark C. Urban, an ecologist at the University of Connecticut, diversity decreased when they took these factors into account, and that new communities of organisms, which do not exist today, emerged. As a result, the rate of extinctions may be higher than previously projected. [21]

    2014 Edit

    According to research published in the 30 May 2014 issue of Science, most known species have small ranges, and the numbers of small-ranged species are increasing quickly. They are geographically concentrated and are disproportionately likely to be threatened or already extinct. According to the research, current rates of extinction are three orders of magnitude higher than the background extinction rate, and future rates, which depend on many factors, are poised to increase. Although there has been rapid progress in developing protected areas, such efforts are not ecologically representative, nor do they optimally protect biodiversity. In the researchers' view, human activity tends to destroy critical habitats where species live, warms the planet, and tends to move species around the planet to places where they do not belong and where they can come into conflict with human needs (e.g. causing species to become pests). [22] [23]

    According to a long-term study of more than 60 bee species published in the journal Science said that climate change effects drastic declines in the population and diversity of bumblebees across North America and Europe. This research showed that bumblebees are disappearing at rates "consistent with a mass extinction." North America's bumblebee populations fell by 46% during the two time periods the study used, which were from 1901 to 1974 and from 2000 to 2014. North America's bumblebee populations fell by 46% because bee populations were hardest hit in warming southern regions such as Mexico. According to the study, there have been more frequent extreme warm years, which exceeded the species’ historical temperature ranges. [24]

    2016 Edit

    In 2016, the Bramble Cay melomys, which lived on a Great Barrier Reef island, was reported to probably be the first mammal to become extinct because of sea level rises due to human-made climate change. [25]

    Extinction risks of the Adelie penguin are being reported because of climate change. The Adelie penguin (Pygoscelis adeliae) species is declining and data analysis done on the breeding colonies is used to estimate and project future habitat and population sustainability in relation to warming sea temperatures. By 2060, one-third of the observed Adelie penguin colony along the West Antarctic Peninsula (WAP) will be in decline. The Adelie penguins are a circumpolar species, used to the ranges of Antarctic climate, and experiencing population decline. Climate model projections predict sanctuary for the species past 2099. The observed population is similarly proportional to the species-wide population (one-third of the observed population is equal to 20% of the species-wide population). [26]

    Sex ratios for sea turtles in the Caribbean are being affected because of climate change. Environmental data were collected from the annual rainfall and tide temperatures over the course of 200 years and showed an increase in air temperature (mean of 31.0 degree Celsius). These data were used to relate the decline of the sex ratios of sea turtles in the North East Caribbean and climate change. The species of sea turtles include Dermochelys coriacea, Chelonia myads, and Eretmochelys imbricata. Extinction is a risk for these species as the sex ratio is being afflicted causing a higher female to male ratio. Projections estimate the declining rate of male Chelonia myads as 2.4% hatchlings being male by 2030 and 0.4% by 2090. [27]

    2019 Edit

    According to the World Wildlife Fund, the jaguar is already "near threatened" and the loss of food supplies and habitat due to the fires make the situation more critical. [28]

    The fires affect water chemistry (such as decreasing the amount of dissolved oxygen in the water), temperature, and erosion rates, which in turn affects fish and mammals that depend on fish, such as the giant otter (Pteronura brasiliensis). [28]

    2020 Edit

    The unprecedented fires of the 2019–20 Australian bushfire season that swept through 18 million acres (7 million hectares) claimed 29 human lives and stressed Australia's wildlife. [29] Before the fires, only 500 tiny Kangaroo Island dunnarts (Sminthopsis aitkeni) lived on one island after half the island was burned, it is possible only one survived. Bramble Cay melomys (Melomys rubicola) became the first known casualty of human-caused climate change in 2015 due to rising sea levels and repeated storm surges the greater stick-nest rat (Leporillus conditor) may be next. [30]

    Emus (Dromaius novaehollandiae) are not in danger of total extinction, although they might suffer local extinctions as a result of bushfires in northern New South Wales, coastal emus could be wiped out by fire . [30] The loss of 8,000 koalas (Phascolarctos cinereus) in NSW alone was significant, and the animals are endangered but not functionally extinct. [31] [32]

    A February 2020 study found that one-third of all plant and animal species could be extinct by 2070 as a result of climate change. [33] [34]

    Ancient City Mysteriously Survived Mideast Civilization Collapse

    As ancient civilizations across the Middle East collapsed, possibly in response to a global drought about 4,200 years ago, archaeologists have discovered that one settlement in Syria not only survived, but expanded.

    Their next question is — why did Tell Qarqur, a site in northwest Syria, grow at a time when cities across the Middle East were being abandoned?

    "There was widespread abandonment of many of the largest archaeological sites and ancient cities in the region and also large numbers of smaller sites," said Jesse Casana, a professor of anthropology at the University of Arkansas. "At Tell Qarqur and probably at other sites also in the Orontes River Valley, where our site is located, [settlement] continues, and in our case, seems to have probably broadened [during that time]."

    Casana and Boston University archaeologist Rudolph Dornemann discovered mud-brick homes beyond the city's fortification walls, suggesting the area was thriving. [See images of the ancient city]

    "It seems like there is an intensively occupied core and fortified area, and more dispersed settlement surrounding it," said Casana. One of the team members, Amy Karoll, presented the research at the 76th annual meeting of the Society for American Archaeology in April.

    Digging up history

    Tell Qarqur was occupied for about 10,000 years, between 8,500 BC and AD 1350 While excavations have taken place off and on for nearly three decades now, only a small portion of the city has been excavated so far. The long history of the site makes digging down to the 4,200-year-old remains difficult. To compensate, the team has used Ground Penetrating Radar to help map structures beneath the surface.

    One of the most interesting excavated finds is a small temple or shrine made out of stone that also dates back 4,200 years. "It's a small stone building with a whole series of plastered basins inside the building that were used probably in some kind of libation ritual," said Casana.

    The team also found large standing stones, bones from baby sheep, cult stands used for incense and decorative figurines, some of which are now on display in a local museum.

    Global climate change

    Environmental data gathered from numerous sources, including ocean sediment cores and plant remains, suggests that there was a climate event that rocked the Middle East and much of the planet 4,200 years ago. [10 Surprising Results of Climate Change]

    "At 4,200 years ago, there was an abrupt climate change, and abrupt drying, and abrupt deflection of the Mediterranean westerly winds that transport humid air into the eastern Mediterranean region," Harvey Weiss of Yale University told LiveScience.

    Weiss has been researching the phenomenon, working with other scholars to figure out how broad an event this was and what its effects were.

    "That deflection of those winds reduced the annual precipitation across western Asia for about 300 years," he said, with rainfall being reduced somewhere between 30 and 50 percent. This meant that cities in the Middle East that depended on rain-fed crops had a difficult time surviving.

    Along with the Mesopotamian and eastern Mediterranean societies that met their demise, Old Kingdom Egypt, a civilization that built the Great Pyramids, collapsed. "A different weather system reduced the flow of the Nile River at the same period so the Nile was affected," Weiss said.

    Casana cautioned that not all scholars are convinced that climate change was the main cause for the collapse of cities in the Middle East.

    "It's a pretty thorny question," Casana said.

    Some researchers "simply don't like the sort of one-to-one causal story that that kind of narrative tells, in which the rain stopped falling and everybody died," he said, adding that the way people were farming and using the land may also have played an important role.

    Another factor is the shaky political stability that large states sometimes endure. "There are other scholars who simply think that the decline of these civilizations, at that time, is kind of part and parcel of the story of civilization itself," Casana said.

    Why did Tell Qarqur survive?

    The question now is why Tell Qarqur is different. Why did the site survive and expand while so many others collapsed? Casana said that until more excavation is done, the jury will still be out as to why.

    Weiss believes that the Orontes River, on which the city is located, is the key to answering this question. He pointed out that other archaeological sites on the river, including Qatna and Nasriyah, also appear to have prospered during this time of collapse.

    "The Orontes River is fed by a huge underground chamber of water, which is called a Karst," Weiss said. "That huge underground source of water continued to flow and to feed the Orontes River during this period when rainfall was diminished."

    There are other questions. Before the collapse hit, Tell Qarqur was within the sphere of influence of a powerful kingdom known as Ebla. That kingdom was destroyed sometime prior to 4,200 years ago. This likely changed the way the city was governed and managed, something that future excavations may reveal.

    "What happens to the political realities of the community at Qarqur I don't know," said Casana. "I'm sure there must have been some change."

    Weiss said that the discovery of cities that grew during climate collapse offers a new frontier for archaeologists and scientists to investigate.

    "I think that the early bronze four [the scientific name for this period of collapse] culture of the Orontes is only just now emerging for our attention and that it's going to provide an extremely interesting example of cultural growth in unique environments during this period," he said.

    Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

    Abrupt Climate Change May Have Rocked the Cradle of Civilization - History

    Posted on 02/15/2004 11:18:28 AM PST by blam

    Mesopotamian climate change

    Geoscientists are increasingly exploring an interesting trend: Climate change has been affecting human society for thousands of years. At the American Geophysical Union annual meeting in December, one archaeologist presented research that suggests that climate change affected the way cultures developed and collapsed in the cradle of civilization — ancient Mesopotamia — more than 8,000 years ago.

    Archaeologists have found evidence for a mass migration from the more temperate northern Mesopotamia to the arid southern region around 6400 B.C. For the previous 1,000 years, people had been cultivating the arable land in northern Mesopotamia, using natural rainwater to supply their crops. So archaeologists have long wondered why the ancient people moved from an area where they could easily farm to begin a much harder life in the south. “The challenge to us as paleoclimatologists is to develop much more detailed and well-dated records.” -Peter deMenocal, Columbia University

    One reason could be climate, said Harvey Weiss, an archaeologist at Yale University, at the meeting in December. The climate record in ancient Mesopotamia and around the world shows an abrupt climate change event in 6400 B.C., about 8,200 radiocarbon years before present. A period of immense cooling and drought persisted for the next 200 to 300 years.

    When the severe drought and cooling hit the region, there was no longer enough rainwater to sustain the agriculture in the north, Weiss says. And irrigation was not possible due to the topography, so these populations were left with two subsistence alternatives: pastoral nomadism or migration.

    Archaeologists first start seeing evidence of settlements in southern Mesopotamia shortly after 6400 B.C. In the south, an area too arid to have sustained rain-fed agriculture, irrigation from the Tigris and Euphrates rivers would have been possible where the rivers flow at plain level, Weiss says. Irrigation farming took three to four times the labor effort of rain-fed farming, but irrigation agriculture would have made surplus production easier because the yield was double that of rain-fed agriculture. Surplus production meant that people could begin specializing in full-time crafts rather than relying exclusively on farming, Weiss says, thus giving rise to the first class-based society and the first cities.

    "It's perhaps too extreme to say that climate change caused all of the advanced society collapses," says Peter deMenocal, a paleoceanographer at Columbia University's Lamont-Doherty Earth Observatory. "But it's also too extreme to say that climate change has had no effect. The challenge to us as paleoclimatologists is to develop much more detailed and well-dated records," he says.

    The most fundamental question in Mesopotamian archaeology, Weiss concludes, "is, 'why is there a Mesopotamian archaeology?'" Having already tied the Early Bronze Age collapses from the Aegean to the Indus to the abrupt climate change event 4,200 years before present, Weiss believes he can now tie the changes of lifestyle and migration that were essential for early class formation and urban life in Mesopotamia to an abrupt, multi-century shift toward drier conditions which occurred near 8,200 years before present.

    Ancient city survived as civilizations collapsed

    As ancient civilizations across the Middle East collapsed, possibly in response to a global drought about 4,200 years ago, archaeologists have discovered that one settlement in Syria not only survived, but expanded.

    Their next question is — why did Tell Qarqur, a site in northwest Syria, grow at a time when cities across the Middle East were being abandoned?

    "There was widespread abandonment of many of the largest archaeological sites and ancient cities in the region and also large numbers of smaller sites," said Jesse Casana, a professor of anthropology at the University of Arkansas. "At Tell Qarqur and probably at other sites also in the Orontes River Valley, where our site is located, (settlement) continues, and in our case, seems to have probably broadened (during that time)."

    Casana and Boston University archaeologist Rudolph Dornemann discovered mud-brick homes beyond the city's fortification walls, suggesting the area was thriving.

    "It seems like there is an intensively occupied core and fortified area, and more dispersed settlement surrounding it," said Casana. One of the team members, Amy Karoll, presented the research at the 76th annual meeting of the Society for American Archaeology in April.

    Digging up history
    Tell Qarqur was occupied for about 10,000 years, between 8,500 B.C. and A.D. 1350. While excavations have taken place off and on for nearly three decades now, only a small portion of the city has been excavated so far. The long history of the site makes digging down to the 4,200-year-old remains difficult. To compensate, the team has used Ground Penetrating Radar to help map structures beneath the surface.

    One of the most interesting excavated finds is a small temple or shrine made out of stone that also dates back 4,200 years. "It's a small stone building with a whole series of plastered basins inside the building that were used probably in some kind of libation ritual," said Casana.

    The team also found large standing stones, bones from baby sheep, cult stands used for incense and decorative figurines, some of which are now on display in a local museum.

    Global climate change
    Environmental data gathered from numerous sources, including ocean sediment cores and plant remains, suggests that there was a climate event that rocked the Middle East and much of the planet 4,200 years ago.

    "At 4,200 years ago, there was an abrupt climate change, and abrupt drying, and abrupt deflection of the Mediterranean westerly winds that transport humid air into the eastern Mediterranean region," Harvey Weiss of Yale University told LiveScience.

    Weiss has been researching the phenomenon, working with other scholars to figure out how broad an event this was and what its effects were.

    "That deflection of those winds reduced the annual precipitation across western Asia for about 300 years," he said, with rainfall being reduced somewhere between 30 percent and 50 percent. This meant that cities in the Middle East that depended on rain-fed crops had a difficult time surviving.

    Along with the Mesopotamian and eastern Mediterranean societies that met their demise, Old Kingdom Egypt, a civilization that built the Great Pyramids, collapsed. "A different weather system reduced the flow of the Nile River at the same period so the Nile was affected," Weiss said.

    Casana cautioned that not all scholars are convinced that climate change was the main cause for the collapse of cities in the Middle East.

    "It's a pretty thorny question," Casana said.

    Some researchers "simply don't like the sort of one-to-one causal story that that kind of narrative tells, in which the rain stopped falling and everybody died," he said, adding that the way people were farming and using the land may also have played an important role.

    Another factor is the shaky political stability that large states sometimes endure. "There are other scholars who simply think that the decline of these civilizations, at that time, is kind of part and parcel of the story of civilization itself," Casana said.

    Why did Tell Qarqur survive?
    The question now is why Tell Qarqur is different. Why did the site survive and expand while so many others collapsed? Casana said that until more excavation is done, the jury will still be out as to why.

    Weiss believes that the Orontes River, on which the city is located, is the key to answering this question. He pointed out that other archaeological sites on the river, including Qatna and Nasriyah, also appear to have prospered during this time of collapse.

    "The Orontes River is fed by a huge underground chamber of water, which is called a Karst," Weiss said. "That huge underground source of water continued to flow and to feed the Orontes River during this period when rainfall was diminished."

    There are other questions. Before the collapse hit, Tell Qarqur was within the sphere of influence of a powerful kingdom known as Ebla. That kingdom was destroyed sometime prior to 4,200 years ago. This likely changed the way the city was governed and managed, something that future excavations may reveal.

    "What happens to the political realities of the community at Qarqur I don't know," said Casana. "I'm sure there must have been some change."

    Weiss said that the discovery of cities that grew during climate collapse offers a new frontier for archaeologists and scientists to investigate.

    "I think that the early bronze four (the scientific name for this period of collapse) culture of the Orontes is only just now emerging for our attention and that it's going to provide an extremely interesting example of cultural growth in unique environments during this period," he said.

    Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on .