Global warming trend may be sudden rather than slow

SCIENTISTS concerned about man-made climate change generally think in terms of 50 to 100 years. That's the usual time scale for global warming due to the buildup of carbon dioxide (CO2) in the air. But what if it happens more quickly? That possibility worries one of the world's leading earth scientists - Wallace S. Broecker of Columbia University's Lamont-Doherty Geological Observatory. Writing in the journal Nature, he explains that, rather than changing slowly and smoothly, Earth's climate can switch abruptly. He warns that significant effects of the so-called ``greenhouse'' warming could happen so suddenly that people wouldn't have time to adapt.

It's a warning worth heeding.

Broecker is no more certain of his climatic expectations than are scientists who have been expecting a gradual change. But he makes the point that, since the climate might surprise us, nations should mount a more intensive and better focused research effort to understand the CO2-induced greenhouse effect than has been carried out so far. This, at least, might reduce the element of surprise. It could also help identify the most likely climatic changes and the degree of urgency with which people should prepare to cope with them.

There's little doubt that we will continue to load the air with CO2. It's released when we burn fossil fuels. It's released when we clear forests, since burned or decaying trees usually give up more CO2 than the plants that replace them absorb. Fuel conservation can slow down the CO2 buildup. But, since the processes that release CO2 are built into the world economy in fundamental ways, the buildup will likely continue.

Among the possible effects that are usually discussed, there's the famous rise in sea level as warming sea water expands and, perhaps, some of the Antarctic icecap melts. More abundant CO2 in the air might help at least some plants grow more vigorously. On the other hand, climate changes could shift precipitation patterns, drying out the present North American wheat lands, among other changes.

Such possible effects are predicted by computer-run mathematical models of Earth's climatic system. The models suggest that there will be enough lead time for people to adapt to the new conditions. Broecker warns that these models can mislead us. He explains that scientists do not know how to incorporate important aspects of the CO2 problem - especially the role played by the oceans - into these models.

Furthermore, Broecker explains, studies of the climatic record contained in North Atlantic seabed sediments and other ``records,'' such as Greenland's icecap, have disturbing implications. They record ecological changes that indicate rapid climatic fluctuation. During glacial time, he says, ``climate changed frequently and in great leaps.'' And the end of that glacial time appears to have come abruptly. It's as though a key element of the climatic system were switching between stable modes of operation rather than changing gradually. Many oceanographers think that global ocean circulation is such a key factor.

Currently, the overall circulation of shallow and deep currents through the Atlantic and Pacific is such as to maintain the present temperate climate in middle Northern Hemisphere latitudes, especially over Europe, where the warming influence of the Gulf Stream is legendary. But the climatic record suggests that this circulation has another mode, which shuts off the warming effect. Broecker says that many climatic leaps seen in the ice cores probably represent flips between these two stable modes of the ocean circulation system.

This raises questions about the likelihood that greenhouse warming could jolt the climatic system out of its present mode and what the effects might be. ``Unfortunately,'' Broecker observes, ``we have little basis for answering this critical question.'' Hence his call for a more aggressive research effort.

In the United States, relevant research is carried out by several agencies. Some fields - such as atmospheric science and climatic aspects of oceanography - are vigorous, while other fields important to this issue, such as climatic studies on land, are relatively weak. Broecker suggests creation of a special institute to coordinate and fund a research program that would tackle the CO2 climate problem comprehensively.

It's quite possible that any greenhouse climate changes will come slowly. As Broecker notes, however, ``there is an equal possibility that they will arrive suddenly and dramatically.'' The need to clear up this uncertainty is reason enough to beef up CO2-climate research.

A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.

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