The study offers an intriguing window on the future, the scientists suggest. Raymond Bradley and Harvey Weiss offer details in the Jan. 26 issue of the journal Science. Bradley is head of the geosciences department at UMass. Weiss, a Yale University archeologist, holds an adjunct post at UMass.

"In general, climate is a major factor when societies unravel, but at times in the past, it's been the decisive factor," said Bradley, whose studies on global warming and greenhouse gases have received extensive notice in recent years.

The theory differs from widely held views that societies were resilient and adaptable in the face of changes in the external environment, but failed due to combinations of social, political, and economic forces.

The two scientists combined their expertise, comparing when various societies fell apart with occurrences of major climate changes. Scientists are able to determine the climate of past centuries with high accuracy, by studying sediments that are layered along lake bottoms, and the chemistry of ice cores, as well as of stalagmites and stalactites in caves. "We felt there were sufficient examples to point out the links," Bradley said.

The societies they considered existed during prehistoric and early historic times, and were located all around the world, including Africa, the Mediterranean, and North and South America.

The team defined collapse as an abandonment of an established community, due to a lack of available food. In many cases, farming and harvesting efforts faltered during prolonged droughts. Lacking adequate food, the societies became nomadic and followed the rains.

"Societies that have been close to subsistence levels had certain expectations about weather conditions, such as amounts of rainfall, and their patterns of existence -- their infrastructures -- were built on those expectations," Bradley said.

"Such expectations would have been handed down for generations. Thus, a sudden climate shift, such as a drought, would have presented completely unfamiliar conditions. If a major climate shift persisted, it would have caused unprecedented disruption in their ability to secure food."

When societies did collapse, the team found, several elements related to climate change were generally present: the change was abrupt; was persistent over decades or even centuries; and was unprecedented in the experiences of the people living during those times. "The change had to be of a sufficient magnitude to threaten the food supply," said Bradley.

Turning to the future, Bradley said: "It's fairly inarguable that the population is going to grow from 6 billion today to nearly 9-12 billion by the year 2050, according to the United Nations. A lot of the developing world lives at subsistence levels, and is already vulnerable to year-to-year variations in climate."

The combination of accelerated population growth and projected changes in the climate "make for a potent mix for real problems on a global scale," he suggested. Furthermore, although scientists can reasonably project population and temperature, it's harder to determine how and where rainfall patterns will change during the next half-century. "Due to the modern political systems, people may not be able to follow the rains as they once did."

The concern extends to the developed world, as well, he said: "Much of our infrastructure -- our hydroelectric dams, our levees, and coastal construction -- were built based on weather patterns that we expect to continue.

But if you have a hydroelectric dam, and you can't meet the society's demand for electricity, that's a problem," said Bradley, pointing to the energy crisis in California. "We're somewhat insulated by technology, and we're not going to starve, but even in the developed world there may be disruptions."

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