The previous case studies have illustrated the range and severity of man-made impacts to the environment and landscapes of different culture areas in prehistory. Over the last 10 years, intriguing new theories have been presented by scientists from several disciplines about how climate change has influenced the rise and fall of ancient civilizations in both the Old and New Worlds. In particular, these theories have focused on droughts and, in one well-publicized case, floods, and most recently, the role of sea-level rise. The availability of high-resolution and well-dated paleoclimatic and archaeological sequences are providing new insights and alternate interpretations about how and why complex urban societies formed and collapsed.
Most of the following cases involve debates over the identification and cultural significance of long-term (300-500 year) mega-droughts on several well known but unexplained prehistoric economic and demographic collapses and disappearances. Peter deMenocal of the Lamont-Doherty Earth Observatory at Columbia University notes the independent record of significant episodes of climate changes over the last 3000years of the postglacial Holocene period. Instead of being uniform and climatically stable, as once thought, it was punctuated by a series of widespread, continent-wide, periods of cooling and desiccation and droughts, throughout North America, that lasted for approximately 1500 years over the last 10 000 years. He also noted that where identified, these long-term droughts emerged suddenly, often in less than a decade. And as we well see below, he also argues that these episodes of long-term climatic change had devastating consequences for prehistoric and historic peoples of the Americas. These arguments fall into two subcategories: (1) studies that point to the negative effects of severe droughts; and (2) those that point to the onset of drought conditions as catalysts for major population movements and/or adaptations to changing landscape and environmental conditions. Only recently, high-resolution environmental sequences of climatic and botanical changes, often now resolved down to 20-50-year intervals, have become available with sufficient chronological precision to be matched to comparable archaeological sequences of cultural growth, decline, or disappearance. Readers must be cautious of ‘single source’ explanations of culture change. Brian Fagan has warned that the ‘correlation’ of high-resolution data on environmental change (from sediment, lake, or ice cores) does not demonstrate a ‘causal relationship’. And as he underscores, these traumatic episodes of culture change are complex and difficult to ascribe to a single guiding force.
Ancient Egyptian Droughts
Archaeologists have long been aware that significant population migrations played a role in the development of early sedentary agricultural settlements in both the Nile valley and Mesopotamia, as well as in the formerly sparsely populated and dryer southern region of the Tigris and Euphrates river drainages. Additionally, a number of scientists have argued that climate change and a shift to dryer, more arid conditions played a significant role in bringing about the demise or collapse of early urban centers in both Egypt and Mesopotamia. Recently, alternative arguments are now being put forth that the onset of more arid conditions may have served as an important ‘trigger’ or causal factor for the emergence of early civilizations in both regions. All of these arguments are based on the chronological correlation of recently available high-resolution sequences of environmental and climate data, generally from marine and terrestrial cores, with longstanding archaeological sequences from different culture areas.
For ancient Egypt, the 1000-year period between c. 5800 and 4800 BC saw the emergence of first goat and sheep herding, and then early mixed herding and farming communities in the Nile Delta. Beginning around 3300 BC, a period referred to as the Old Kingdom emerged as a period of prosperity and growth distinguished by the first urban centers, centralized state rule, large-scale construction of government and religious complexes, and the first pyramids. It was also was a period of continual and reliable Nile floods. However, sometime around 2200 BC, the Old Kingdom appears to have suddenly suffered social and economic anarchy and collapse. Scholars have spent their entire careers promulgating theories as to why, but generally without much evidence.
Work by Fekri Hassan initially suggested that drought may have been a factor in the collapse of early Egyptian civilization. Hassan based this possible explanation on the simple stratigraphic observation that an important basin fed by the Nile River, the Fayum Depression, lacked any lake-bottom sediments dating to the period of the Old Kingdom. This suggested to him that, instead of the traditional 18-20 m of water, the depression had dried out and, that where once present, the sediments from this period had blown away.
Multiple lines of recent geological and sedimento-logical evidence are substantiating Hassan’s thesis that drought may have brought about the demise and collapse of the Old Kingdom. The recovery of high-resolution sediment cores from the Nile Delta by Jean-Daniel Stanley, has recorded a band of red wind-blown sand, suggestive of severe drought, dating to c. 2150 BC, or at the same time as the collapse of these earliest Egyptian urban and religious centers. Other sediment cores from elsewhere in Egypt and North Africa have recorded the chemical and mineralogical fingerprints of similar layers of wind-blown sand of the same age. Scientists of the French National Center for Scientific Research (CNRS) have also reported that high-resolution pollen and diatom sequences have identified parallel periods of drying and drought dating to around 2100 BC from Ethiopia, Rwanda, Uganda, and equatorial Africa. These multiple lines of evidence now appear to have world-wide implications, with comparable effects on the settlement and culture histories of other regions, including the Near East and the New World.
Hassan also argued that just as drought appears to have brought an end to the Old Kingdom, the beginning of the subsequent Middle Kingdom between c. 1900 and 1800 BC was distinguished by centralized efforts to control the short-term effects of drought. He points to the initial building of canals, a large dam and reservoir, and the dredging of the channel of the Nile linking the river to the Fayum Depression. These constructions both buffered against periods of drought and served to regulate the water level within the Fayum Depression in support of extensive irrigation agriculture surrounding it. He theorized that subsequent Nile floods breached the dam, inundated the depression, and rendered it unusable for agriculture until the third-century BC, or a period of abandonment lasting at least 1500 years.
Mesopotamian Droughts
Similar work on sudden climate change by Dr. deMenocal has used deep-sea sediment cores taken from the Mediterranean to argue that abrupt climate change to cooler and dryer conditions brought about the sudden collapse of the Akkadian civilization in northern Mesopotamia around 4200 BP. Detailed chemical and particle analysis identified a thick lens of air-borne sand in the column beginning (dating to 4170 BP), which appears to have lasted for at least 300 years. This and other lines of archaeological and climatic evidence were combined to contend that, as was the case for the Old Kingdom of Egypt, the collapse of the Akkadian culture and the previously identified migration of its population to the south coincided with the onset of a major long-term period of drought in the region.
As was the case for the advent of irrigation and water control systems in the Middle Kingdom of Egypt, others have used paleoclimatic evidence to suggest that a regional shift to more arid conditions also helped trigger the onset of both irrigation agriculture and urbanism in southern Mesopotamia. In 2004, two scientists - Nick Brooks and Harvey Weiss - independently used similar evidence to theorize that increased desiccation was not only responsible for the collapse and migration southward of the Akkadian peoples.
In addition, they both argued that drought may have played a role in the subsequent population increases, urbanism, social stratification and onset of irrigation agriculture in southern Mesopotamia, as well.
North American Droughts
In a synthesis of both archaeological and climatic evidence from both New World and Old World archaeological chronologies Dr. deMenocal has highlighted both Old and New World examples of longterm drought relative to a number of archaeological culture sequences. His examples illustrate prehistoric shifts in settlement patterns, the abandonment of urban centers, and in the case of the Pueblo and Mayan cultures of North and Central America, their complete disappearance. He used evidence from sediment cores and tree ring dating to explain the long-debated decline of both the first European colonies in North America and, quite clearly, provided credence to the arguments by several archaeologists that drought was also the culprit behind the abandonment of the great Pueblo centers.
Archaeological evidence and tree ring dates have also provided convincing evidence that the 1587 settlement of the Lost Colony on North Carolina’s Roanoke Island had disappeared during the most severe drought in 800 years. Similarly, the high mortality and near loss of the 1607 settlement of Jamestown, the first permanent English settlement of North America, coincided with the driest period of drought in the prior 770 years of the local tree ring record.
Finally, the correlation of sequences of climatic change with the pivotal benchmark dates of prehistoric culture change has provided convincing evidence to explain the thirteenth-century abandonment of the Anasazi ‘cliff house’ settlements in the American Southwest. Although often without much evidence to go on, earlier archaeological theories had suggested their demise through droughts, war, internecine conflict, or religious chaos. Recently, the availability of high-resolution sequences of climate change now presents a tightly dated record indicating that drought was indeed the primary cause of decline. Regional tree ring chronologies and drought distribution maps document that the thirteenth-century abandonment of Anasazi settlements coincided with the advent, around 1280 AD, of a significant, generation-long drought lasting at least 26 years.
Central American Droughts
Similar lines of evidence from dated sediment cores help to explain the sudden decline of the Maya as well. Two came from land-locked lakes in the Yucatan peninsula. A third was recovered from a segment of a 500-foot-deep core drilled off the coast of Venezuela, to the south of the Maya heartland in the Caribbean. Using different lines of evidence, the core sequences together provided redundant records that drought played a key, if not a primary, role in the collapse of Maya urban centers and the depopulation of the peninsula between AD 800 and 1000. Radioisotope analysis of minute fresh water shells from the inland lake cores indicated that the onset of an extended, but ill-defined (c. 200 year) period of dryer conditions and reduced rainfall had occurred sometime around AD 900.
More recently, based on radiocarbon determinations from the Venezuelan sediment column, Dr. Larry Peterson of the University of Miami was able to define a 200-year series of climate ‘signals’ - or indices of sudden climate fluctuations - dating from AD 750 to 950. Work by Gerard Haug used X-ray fluorescence to measure the relative amounts of trace titanium in a series of dated sediment samples. Titanium was tested for because it accumulated in the basin sediments as a small but measurable by-product (‘trace element’) of stream run-off from the Yucatan peninsula; the lower the titanium levels, the lower the rainfall. Four periods of reduced titanium measurements dated to AD 760, 810, 860, and 910. Each was interpreted to be an indicator of episodes of apparent drought, which correlated suggestively with the collapse of the Maya centers sometime after AD 800. These independent core-derived findings support the earlier thesis of an independent Texas archaeologist, Richard Gill, that drought was responsible for the abandonment of the Maya cities sometime after AD 800. Gill based his suggestion on a survey of the latest dates carved into monuments before different centers were abandoned. Despite a fudge factor of ±30 years in the core-derived radiocarbon determinations, his archaeological chronology of calendar dates for the demise of the Maya matched the dates of the titanium ‘proxies’ of climate change with uncanny precision.
Andean Droughts
A similar sudden climatic event in around the thirteenth-century AD has also been presented to explain the decline of the Middle Horizon urban centers in the Bolivian Andes (and by extension in highland Peru). Although somewhat less convincing than the wealth of tree ring data for the American Southwest, samples from a single ice core have been used by some to argue that the collapse of the pre-Inca urban center of Tiahuanaco on the Bolivian altiplano (11 000 feet elevation), surrounding Lake Titicaca, may also have occurred as the result of drought.
Counted and measured like tree rings, the availability of a finely dated ice core from the Quelccaya area of the southern Andes, 200 km to the north, provided a tightly dated 1500-year sequence of annual ice accumulation. The dated ice sequence indicated that a lessening of ice accumulations and a shift to significantly drier conditions began around AD 1040 and lasted for at least 200 years, or until the thirteenth century. This suggested change to dryer conditions also corresponded with a 30 foot drop in the level of Lake Titicaca. Archaeologists and climatologists have used this single line of dated climatic evidence to argue that the sudden and long-term climate shift to dryer conditions would have been of sufficient magnitude and duration to destroy or radically diminish the productivity of the traditional raised field system.
In addition, new research on the Quelccaya ice cap used radiocarbon dating of ancient plants recently exposed by the retreating glacier to argue that the onset of the Late Holocene around 5000 BP was marked by a measurable shift to cooler conditions. This evidence of dated climatic change is also identical in age to comparable climactic events in the Old World. As discussed above, the climatic transition at 5200 BP is also coincident with parallel shifts and the emergence of urbanism in Egypt, Mesopotamia, and elsewhere. This independent ice core data indicated that a climate shift also occurred in the Americas at the same time as the wind-borne sand was recorded in sediment cores from the Mediterranean. These temporal parallels suggest that the shift may have been a world-wide event, with world-wide cultural consequences.
World History
