Last month's Science article, coauthored by Harvey Weiss of Yale University and Raymond Bradley of the University of Massachusetts at Amherst, explored other prehistoric, ancient, and premodern societies, and found natural climatic variations often corresponded with the societies' collapse.

In Mesopotamia, a canal-supported agricultural society collapsed about 3,400 years ago. The paleoclimatic record, write the authors, suggest a severe 200-year drought may have caused the society to collapse.

With wetter conditions, civilizations thrived in the Mediterranean, Egypt, and west Asia. Ten years after their economic peak in 2300 B.C., however, catastrophic drought and cooling hurt agricultural production and caused regional abandonment and collapse.

Society at Risk

The worst case scenerio, said Will Burns of California's Pacific Institute for Studies in Development, Environment, and Security, is if the world does not make any changes in its activities—inaction that would cause an 11-degree temperature change. "That would have some pretty horrific implications," he said.

The scientists note that although some of the world's population—expected to reach up to 10 billion by 2050—lives in a different manner than past societies, most of the world's people live as subsistence or small-scale farmers.

Fluctuations in the weather, the scientists write, can have a dramatic impact on these agricultural societies. Like civilizations of the past, today's may be unable to cope with droughts or monsoons that hinder crop growth.

Thousands of years after the collapse of the societies profiled in the Science article, the authors note that an increased human population may cause coming climate changes to have a greater—and more dangerous—impact. Whereas past societies could have migrated to unaffected areas, the crowded Earth may hinder migration.

"We do, however, have distinct advantages over societies in the past because we can anticipate the future," the scientists write. "We must use this information to design strategies that minimize the impact of climate change on societies that are at greatest risk."