But new evidence shows that periods of gradual change in Earth's past were punctuated by episodes of abrupt change, including temperature changes of about 10 degrees Celsius, or 18 degrees Fahrenheit, in only a decade in some places.

Severe floods and droughts also marked periods of abrupt change.

A new report from the National Academies' National Research Council says greenhouse warming and other human alterations of the climate system may increase the possibility of large, abrupt, and unwelcome regional or global climatic events.

Researchers do not know enough about such events to accurately predict them, so surprises are inevitable.

If the planet's climate is being forced to change -- as is currently the case -- it increases the number of possible mechanisms that can trigger abrupt events, the report says. And the more rapid the forced change that is taking place, the more likely it is that abrupt events will occur on a time scale that has immediate human and ecological consequences.

There is no need for undue alarm, however, about the possibility of sudden climate change, because societies have learned to adapt to these changes over the course of human history, said the committee that wrote the report. The committee was chaired by Dr. Richard B. Alley of Pennsylvania State University.

The committee said research into the causes, patterns and likelihood of abrupt climate change is the best way to reduce its impact. Overall, research should be aimed at improving modeling and statistical analysis of abrupt changes.

An important focus of the research should be on mechanisms that lead to sudden climate changes during warm periods, with an eye to providing realistic estimates of the likelihood of extreme events. Poor countries may need more help preparing for abrupt climate change since they lack scientific and economic resources, the report noted.

The planet's past climate record also needs to be understood better, according to the report. Scientists have a variety of means to study what the climate was like thousands of years ago. For example, researchers look at tree rings to examine the frequency of droughts and analyze gas bubbles trapped in ice cores to measure past atmospheric conditions.

With such techniques, scientists have discovered repeated instances of especially large and abrupt climate changes over the last 100,000 years during the slide into and climb out of the most recent ice age. For instance, the warming at the end of the last ice age triggered an abrupt cooling period, which finished with an especially abrupt warming about 12,000 years ago.

Since then, less dramatic -- though still rapid -- climate changes have occurred, affecting precipitation, hurricanes, and the El Niño events that occasionally disrupt temperatures in the tropical Pacific.

Examples of abrupt change in the past century include a rapid warming of the North Atlantic from 1920 to 1930 and the Dust Bowl drought of the 1930s.

Simulating abrupt climate changes using computer models is particularly difficult because most climate models respond in a linear manner in which a doubling of the factor forcing change -- greenhouse gases, for instance -- doubles the response.

However, abrupt climate changes show that a small forcing may cause a small change, or may force the climate system across a threshold and trigger huge change. A massive discharge of fresh water from lakes dammed by melting ice sheets, which suddenly changes climate conditions worldwide, is an example of threshold-crossing.

Chaotic behavior in the climate also may push it across a threshold without any apparent external forcing.

The collapse of some ancient civilizations has been associated with abrupt climate changes, especially severe droughts, but humans have shown great resilience as well.

Fast changes make adaptation more difficult, so research should be pursued to identify strategies that reduce vulnerabilities and increase the adaptability of economic and ecological systems, the committee said. It noted that many proactive policies might provide benefits regardless of whether abrupt climate change occurs.

Some steps that deserve careful scrutiny include reducing emissions to slow global warming, improving climate forecasting, slowing biodiversity loss, and improving water, land and air quality.

The report was sponsored by the U.S. Global Change Research Program, with additional support from the National Bureau of Economic Research Program on International Environmental Economics at Yale University.

The National Research Council is the principal operating arm of the National Academy of Sciences and the National Academy of Engineering. It is a private, nonprofit institution that provides science and technology advice under a congressional charter.

(Editor's Note:ABRUPT CLIMATE CHANGE: INEVITABLE SURPRISES is available on the Internet at this URL. Copies of the report will be available for purchase early next year from the National Academy Press.)

13-Dec-2001