Studies of the relationship between Holocene climate variability and human societies have often been marked by antithetical perceptions within the social science and physical science communities (Oldfield and Dearing 2003). Arche-ological and anthropological research encompass interpretations of socioeconomic and cultural change, resource use, and subsistence practices (Pringle 1997; Redman 1999), but the direct evidence for potentially damaging climate change is usually derived independently from different archives and lines of evidence (Cullen et al. 2000; Hodell et al. 2001). As a result, even where temporal correlations can be proposed between major societal changes and shifts in climate, they could be viewed as little more than coincidences. Taken to its extreme, the "cultural" view attributes major changes in past societies, even the collapse of ancient civilizations, entirely to human actions. Although human actions are clearly important, too one-sided interpretation is not supported by research which ascribes the collapse of civilizations as diverse as the classic Akkadian (Cullen et al. 2000), Mayan (Hodell et al. 1995, 2001), and Anasazi (Dean et al. 1985, 1999) cultures to abrupt and persistent climatic changes. O Figure 12.9 presents a selection of paleoclimate examples where the combination of environmental and cultural history, coupled with rigorous chronological constraints, points to a strong link between the incidence of drought and the collapse of human cultures. Such studies do not discount the role of societal factors but assert that, at times, climate variability has been a critical factor influencing societal stability.
Four thousand years ago Yu the Great, founder of the first, semimythical, Xia dynasty of China is said to have spent more than a decade battling floods continuously inundating the valley of the Yellow river. According to Confucius' Shu Jing (Book of History), written much later in the fifth century bc but supposedly collated from ancient sources, Yu's great insight was to discontinue the practice of building ever-higher levees and dikes and instead dig a vast network channels to drain the great volumes of water east to the sea. Flooding, up until the present day, has continued to be a major concern for the region. A million people are thought to have died when the Yangtze broke through flood defenses in 1877. But flooding is not the only problem; the flow of the Yellow River has been in continual decline for the last 50 years. In 1997, the river ran dry in places during 226 days of the year. Much of China's agriculture and population depends on water from the Yangtze and is greatly vulnerable to drought.
The proposed modern day solutions, multibillion dollar engineering projects, are not so very different from those proposed in the time of Yu the great.
Two examples of paleorecords where the combination of environmental and cultural history, coupled with rigorous chronological constraints, point to a strong link between the incidence of drought and the collapse of human cultures. In the upper panel, drought in Yucatan, Mexico inferred from changes in the stable isotope ratios in two species of ostracod is shown to coincide with the collapse of Mayan civilization (Hodell et al. 2001). In the lower panel, a steep fall in carbonate percentage in a marine sediment record from the Gulf of Oman, representing a major episode of dust deposition, is directly linked to drought conditions associated with the demise of the Akkadian civilization (Cullen et al. 2000; Adapted from Alverson and Oldfield 2003)
One, the famous Three Gorges Dam, in addition to generating electricity, is intended to control flooding. A second, intended to ameliorate vulnerability to drought, is to divert water to the north, primarily from the Yangtze river basin, by digging a series of south to north canals in the western, central, and eastern parts of China. Will these gigantic engineering projects fulfill their goals? Will they reduce human vulnerability to climatic variability and change?
The answers to these questions are a matter of much debate, but one salient fact is often overlooked in that debate. Paleoenvironmental science provides a detailed, quantitative record of human society, climate variability, and their interaction over the past several thousand years, both in the Yangtze valley region and over the rest of the globe. Such information from the past is directly relevant to questions of climate variability and human vulnerability today and therefore directly relevant to the modern debate.
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