Photograph: Wallace S. Broecker
Undeniable evidence that earth's climate has shifted abruptly many times should compel an urgent effort to learn whether the buildup of heat-trapping greenhouse gases could spark another radical shift, a Columbia professor and leading expert on climate change told the American Association for the Advancement of Science Monday.
In a plenary lecture at the AAAS's national meeting in Atlanta, Ga., Wallace S. Broecker of Columbia's Lamont-Doherty Earth Observatory said that because abrupt climate changes have not occurred during the past 10,000 years, human civilization has flourished--with a false sense of security in the stability of earth's climate system.
"The earth's climate system is endowed with a disturbing characteristic: It is able to jump from one mode of operation to another," Broecker said. In his lecture, "Abrupt Climate Change: Is One Hiding in the Greenhouse?" he chronicled the current, rapidly developing research effort to piece together how the earth's atmosphere, oceans and ice sheets interact to produce global climate change.
Evidence from seafloor and lake sediments, ice cores, fossilized pollen, corals, Andean glacial movements and other records demonstrate unequivocally that during the last ice age from 70,000 to 10,000 years ago, earth's climate system changed frequently, often within the span of a lifetime. The shifts had global consequences: In the region around the North Atlantic, average temperatures rose and fell 10 degrees F or more; tropical rainfall changed radically; the world's ocean circulated in a different way; icebergs and sea ice periodically proliferated, and atmospheric concentrations of gases varied. Such global changes, if they occurred today, would have profound impacts on agriculture and living conditions.
"What lies behind this curious behavior?" Broecker asked. "The honest answer is that we don't know. We have some very powerful clues, but we have yet to pin down the origin of these abrupt climate changes. This leaves us in limbo with regard to predicting future climate.
"Might the anthropogenically driven buildup of greenhouse gases trigger yet another reorganization of the ocean-atmosphere system?" he asked. "The observation that these jumps have been confined to times when the northern Atlantic was surrounded by huge ice sheets could be taken as an indication that the likelihood is small. But on the other hand, as the greenhouse nudge promises to be far larger than any other stress experienced during intervals between ice ages, there is no certainty that the system is immune to disruption.
"Even if the probability is small, the impacts of such climate jumps would be very large on a world already struggling to maintain ecosystems and wildlife that are under grave attack and bulging with people threatened by hunger and disease. So it behooves us to take this possibility seriously. We must pull out all stops in an attempt to better understand the chaotic behavior of our climate system."
Broecker is the Newberry Professor of Geological Sciences at Columbia and a scientist at Lamont-Doherty. He is a member of the National Academy of Sciences and winner of the Vetlesen Medal, the most prestigious award for earth sciences. A geochemist, he has explored ocean waters, corals, deep-sea sediments and polar ice cores for chemical clues to understand the workings of earth's complex, delicately balanced climate system and its susceptibility to change in the past and future.