Recent Periods of Abrupt Climate Change

Human civilization began to emerge as the glaciers of the last ice age receded. In the Holocene, the period since the end of the last glacial approximately 11,500 years ago, abrupt changes take on a new significance. Many of them have been linked to the times of societal collapse or upheaval. This has provided archeologists with new information for understanding changes in human societies. Paleoclimatic data come from a wide range of environmental records that serve as proxies for changes that occurred in climate. These paleoclimatic records demonstrate dramatic shifts in the climate system that exceed any seen during the brief instrumental record. Many of these likely had significant impacts on human systems, particularly agriculture.

Figure 15. Scientists reconstructed changes in the balance between evaporation and precipitation using oxygen isotope data from the shells of gastropods and ostracods (Hodell et al. 1995).

Severe Drought and the Collapse of Civilizations

Climate has played an important role in the development of human civilization. One reason for this was the need for more intensive agriculture to support larger and more stationary populations as urbanization increased.

The ancient Maya civilization occupied a vast area of Mesoamerica between the time period of 4,600 B.P. and 800 B.P. After the peak in the classic Maya culture around 1400-1200 B.P. (A.D. 600-800), the civilization collapsed. Evidence now indicates that the rapid onset of an intense drought corresponds with the collapse of the Classic Maya civilization between 1200-1100 B.P. (A.D. 800-900).

Scientists reconstructed the past climate of the Maya civilization by studying lake sediment cores from the Yucatan Peninsula. By using oxygen isotope data from the shells of gastropods and ostracods (Figure15) scientists reconstructed changes in ratios of evaporation to precipitation through time (Hodell et al. 1995). Distinct peaks that represent arid climate conditions coincide with important archeological periods, such as the "Maya Hiatus," the early/late Classic boundary (a time of reduced monument building and social upheaval) and the collapse of Classic Maya civilization.

Figure 16. This comparison of data from marine sediments, archeological findings, and isotopes in cave deposits shows the synchrony of the collapse of the Akkadian empire with severe climatic changes, including the abrupt onset of drought.

These findings support a rather strong correlation between times of drought and major cultural discontinuities in Classic Maya civilization ( Hodell et al. 1995, Gill, 2000, DeMenocal 2001). While we do not know the details of these events, they appear to correspond with one definition of an abrupt climate change: "an abrupt change is one that takes place so rapidly and unexpectedly that human or natural systems have difficulty adapting to it" (National Research Council 2002).

In southern Mesopotamia, much of the Late Uruk society collapsed suddenly at about 5,200-5,000 BP. Paleoclimatic data now indicate that a severe, multi-decadal drought contributed to this collapse (Bar-Matthews et al. 2003, Cullen et al. 2000, Weiss and Bradley 2001).

Many ancient civilizations throughout the Middle East and Mediterranean reached their economic peak at about 4,300 BP. However, before 4,200 BP a catastrophic drought and cooling event contributed to regional abandonment, collapse, and movement of societies to areas with more favorable climate.

Paleoclimatic data from numerous sites document significant changes in climate throughout the region, including precipitation reductions of up to 30% that had severe impacts on agricultural production ( Bar-Matthews et al. 2003, Cullen et al. 2000, Weiss and Bradley 2001). Figure 16 shows the the correlation between sediments from the Gulf of Oman, isotopes from the Soreq cave, and archeological evidence from the region.