<!--intro-->Contrary to common beliefs, societal collapses of the past have been caused by sudden climate change, not only by social, political and economic factors, Yale anthropologist Harvey Weiss reports in a new study published in this week's Science. <!--/intro--> "Our conclusions are both surprising and challenging because in the past, archaeologists and anthropologists have commonly explained collapsed societies as the result of social, economic and political forces combined," said Weiss, professor of Near Eastern archaeology at Yale. For their study, Weiss and his colleague, Raymond S. Bradley of University of Massachusetts, Amherst, summarized and synthesized recent archaeological and paleoclimatological research. This allowed them to understand that repeated incidents of societal collapse in the archaeological and historical past have been the product of abrupt, natural climate changes. "These data force a change in some general social science understandings," said Weiss. "The data are also important because they underscore the difference between past climate changes and present-future climate change. Past climate changes were unrelated to human activities. In contrast, present and future climate change will involve both natural and anthropogenic forces and will be increasingly dominated by the latter." The climatic events Weiss describes in the study were abrupt, involved new conditions that were unfamiliar to the inhabitants of the time, and persisted for decades or centuries. They were therefore highly disruptive, Weiss said, leading to societal collapse-an adaptive response to otherwise insurmountable stresses. The study describes well-documented examples of societal collapse dating back to about 12,500 to 11,500 years ago with the Natufian communities in southwest Asia. This community suddenly abandoned seasonally nomadic hunting and gathering activities that required relatively low inputs of labor to sustain low population densities and replaced these with new labor-intensive subsistence strategies of plant cultivation and animal husbandry. Weiss said a major difference from the past is that we are now able to foresee the results of these climate changes and are able to understand the technological and social innovations which could allow us to address them. "We also know where the population growth will be greatest," Weiss adds. "We must use this information to design strategies that minimize the impact of climate change on societies that are at greater risk. This will require substantial international cooperation, without which the 21st century will likely witness unprecedented social disruptions."