Abrupt Climate Change: Inevitable Surprises

Abstract

Climate change is now well recognized as a potentially significant factor in the human future, affecting ecological systems, agriculture, health, and settlement patterns. The scientific consensus, periodically assessed by the Intergovernmental Panel on Climate Change, foresees a long‐term atmospheric warming of a few degrees over a century. The accumulating knowledge of the world's climate history, however, suggests that any implied stability of trend may be illusory. The evidence comes from ice and seabed cores and tree rings, which can yield year‐by‐year information on atmospheric conditions and time‐series of mean temperatures extending back for millennia. These data show many episodes of a sudden rise or fall in temperature in particular regions, sometimes of 10°C or more in ten years, with the new mean lasting for decades or centuries. There is thus the likelihood—even the inevitability—of comparably large and abrupt changes occurring in coming years, a very different prospect from the “greenhouse warming” scenario projected by current large‐scale climate models. Moreover, the two phenomena may be connected: a continued slow temperature rise may at some point trigger an abrupt shift in climate regime through mechanisms such as the effect on ocean circulation. In 2002 a committee set up by the US National Research Council reviewed what is known about this subject in a report, Abrupt Climate Change: Inevitable Surprises (Washington, DC: National Academy Press). The Council is the operating arm of the US National Academy of Sciences; the Committee on Abrupt Climate Change comprised mostly oceanographers and climate experts and was chaired by Richard B. Alley, Professor of Geosciences, Pennsylvania State University, a glaciologist and author of The Two‐Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future (Princeton, 2000). A recent spate of media attention to the subject has been occasioned less by the NRC report itself than by a dramatic scenario exercise derived from it, prepared under US Defense Department auspices, and by release of a disaster film with an instant‐ice‐age theme. The Pentagon study, An Abrupt Climate Change Scenario and Its Implications for United States National Security, authored by Peter Schwartz and Doug Randall, was issued in October 2003. It explores the hypothetical geopolitical consequences of a repetition in the near future of an event experienced 8,200 years ago (as recorded in an ice core from central Greenland): a sudden cooling of some 2–3° C, lasting for a century, punctuating climatic conditions broadly similar to those of the present day. In the scenario, the collapse of ocean heat‐conveying currents causes rapid cooling in the northern hemisphere and warming in the southern hemisphere. Outcomes include resource wars, large population movements, and “a significant drop in the human carrying capacity of the Earth's environment.” In February 2004 the NRC reasserted the more measured voices of its committee by issuing a four‐page summary of the 2002 report, under the same title as the full document. This “report in brief” is reproduced below.