How quickly has climate changed in the past? What is the evidence that these abrupt climate changes were due to changes in ocean circulation? How big were these changes? What are the prospects for an abrupt change in climate and/or ocean circulation in the future? Might these prospects be changed in a warmer world? What are the human and ecological implications of an abrupt cessation of the ocean conveyor belt?

INTRODUCTION: Dr. Robert W. Corell, Assistant Director for Geosciences, National Science Foundation, and Chair, Subcommittee for Global Change Research.

SPEAKER: Dr. Wallace S. Broecker, Columbia University, Lamont-Doherty Earth Observatory, New York.

Records from ice cores suggest that elements of the global climate in the latter part of the last ice age changed significantly over periods as short as a decade or two. Could such abrupt changes occur in the future? Are there likely to be such climatic surprises in a greenhouse-warmed world?

According to the climate record, the last ten thousand years of Earth history, during which civilization developed, are unusual. This period, called the Holocene, was a time marked by weather more consistent and equable than any similar time span in the last one hundred thousand years. However, in the period prior to the Holocene, ice cores from Greenland reveal a series of alternating cold and warm periods, each lasting a thousand years or more, that raised or lowered the average winter temperature in northern Europe by as much as 10 degrees C, with the changes occurring over a time span as short as a decade.

The last of these one thousand year cold periods, which is known as the Younger Dryas, ended about 11,000 years ago. Evidence suggests that the effects of this cold period were global in scope: the warming of Antarctica came to a halt for 1,000 years; New Zealand's mountain glaciers advanced; the proportions of certain species of plankton changed drastically in the South China Sea; and the atmosphere's concentration of methane (a greenhouse gas) dropped by 30%.

A critical question is what caused these abrupt changes in climate. While uncertainties remain, a number of models suggest that the circulation of heat and salt through the world's ocean - "The Great Ocean Conveyor Belt" - changed suddenly, causing major changes in global and regional climates. Large circulation cells span the length of each ocean. In the Atlantic, warm surface water flows northward, bringing warmth to Europe. In the vicinity of Greenland the Arctic air cools this warm surface water, causing these waters to sink and form a deep ocean current that flows all the way to the vicinity of Antarctica. There, cold North Atlantic bottom water rises to the surface because it is warmer and thus less dense than the frigid surface water. At the surface, the North Atlantic water is chilled even further and sinks back into the ocean depth as Antarctic bottom water. Tongues of this Antarctic bottom water flow northward into all of the major ocean basins, where it is gradually warmed and forced to the surface to repeat this cycle of oceanic circulation.

The Atlantic Ocean's conveyor circulation, which has a flow equal to 100 Amazon Rivers, transports an enormous amount of heat northward, providing the heat to create the warm climate enjoyed by Europe.

Ice core records and models suggest that this conveyor circulation pattern is susceptible to perturbations as a result of injections of excess fresh water into the North Atlantic, which were probably caused by melting icebergs in the past, but may also be altered by precipitation changes in the future. Were the conveyor to stop, winter temperatures in the North Atlantic would abruptly fall by 5 or more degrees C, and the change could occur in ten years or less. Such changes would, for example, cause Dublin to acquire the climate of Spitsbergen. Ice core record suggest that past cessations in the conveyor belt dropped temperatures by 7 degrees C. Ice core records and models also reveal that the conveyor circulation eventually reestablished itself, but only after hundreds or thousands of years had elapsed.

Since 1977, Dr. Broecker has been the Newberry Professor of Geology at Columbia University's Lamont-Doherty Earth Observatory. He is widely acclaimed for his pioneering work in paleoclimatology, ocean chemistry, ocean circulation, and environmental science. Dr. Broecker popularized the notion of the world oceans operating as giant conveyor-like circulation cells having different modes of operation, each mode having important climatic and ecological implications. He was elected as a member of the National Academy of Sciences in 1979, and has been a member of the American Academy of Arts and Sciences since 1976. Dr. Broecker has been the recipient of the Maurice Ewing Medal in 1979, the Arthur L. Day Medal in 1984, the A.G. Huntsman Award for Excellence in the Marine Sciences in 1985, the Urey Medal, the Alexander Agassiz Medal, and the U.M. Goldschmidt Award in 1986, the Vetlesen Award in 1987, the Joseph Priestly Award and the Wollaston Medal in 1990, and the American Geophysical Union's Roger Revelle Medal in 1995. He received his A.B. Degree from Columbia University in 1953, and his Ph.D. from Columbia University in 1958.