Paleoclimatic record holds clues to explaining abrupt climate changes

Ray Pierrehumbert

A growing body of scientific evidence suggests that abrupt climate change may be more typical of Earth’s history than the relatively stable conditions humans have enjoyed not only for the last few centuries, but the last 10,000 years. Yet most scientists, economists and policy-makers pay more attention to the issue of gradual climate change, wrote a University climatologist and 10 of his colleagues in a journal article published Friday, March 28, in Science.

“We’re drawing attention to the vast body of literature accumulating, which says when it comes to global warming, we may not be just looking at a different climate, but one that is more variable from year to year than our present climate,” said Ray Pierrehumbert, Professor in Geophysical Sciences and the College. “Think about what would happen if one year we had 105-degree heat waves, then the next decade we had unusually cold winters, and then we had 50 years of drought. It would be very hard to adapt to that kind of climate.”

Much of the scientific literature on abrupt climate change originates from paleoclimatology, the study of ancient climates. Modern meteorological records only cover the past century or so, but ice cores from Greenland and Antarctica push the paleoclimatic record back half a million years.

“The chemical composition of the water in this ice and other things trapped in the bubbles, too–methane and other gases–tell us a lot about what past climates were like,” Pierrehumbert said.

The Science article summarizes the findings that were presented in a panel report on abrupt climate change, which the National Research Council issued in December 2001. The National Research Council is a private, not-for-profit institution that provides science and technology advice to the U.S. government under a congressional charter.

Pierrehumbert and his Science co-authors, who all served on the council’s panel, recommend that the nation’s funding agencies place a greater emphasis on abrupt climate change research than they have in the past. Paleoclimatic studies are particularly important to understanding abrupt climate change, yet the federal government’s strategic plan for the U.S. Climate Change Science Program largely overlooks the paleoclimate issue, he said. The government plan, announced last November, attempts to facilitate climate change research across 13 federal agencies.

“One of the real shortcomings of the U.S. Climate Change Science Program report is that it was almost completely ignorant of the role of paleoclimate, especially in understanding climates that are different from what we’re familiar with,” Pierrehumbert said.

The ice cores of Greenland have revealed that the climate had been warming for thousands of years following the last ice age and before the onset of the Holocene, the epoch of warmth that began 10,000 years ago. But pollen from sediments in Europe signaled an abrupt climate change called the Younger Dryas, named for a cottony flower that grows in tundra.

“The Younger Dryas cool event was discovered in Europe when the Dryas flower suddenly cropped up again long after the Ice Age was supposed to have ended,” Pierrehumbert said. “In as little as perhaps 10 years, the whole planet reverted to nearly full glacial conditions. It stayed there for maybe a thousand years, then it popped back up and resumed its rise into the Holocene.”

Yet the reasons behind the climatic stability that gave birth to the rise of agriculture and civilization remain elusive. Climatologists know that rising carbon dioxide in the atmosphere has been associated with global warming in the past, and that carbon dioxide levels are growing today. They also know the climatic system is sensitive to the amount of fresh water entering the oceans from river runoff, melting glaciers and the like.

Nevertheless, something is missing. Computer simulations have failed to reproduce the climatic changes that led to the Younger Dryas. Instead, the simulations show only localized, moderate cooling.

“Our understanding of climate is poor enough that if we did not have the paleoclimate data, saying that the Younger Dryas did happen, we would never have anticipated anything like that,” Pierrehumbert said.