WITH SPEED AND VIOLENCE

These whirlpools are often mentioned in connection with the Gulf Stream (known in scientific circles as the thermohaline circulation) that keeps northern Europe warmer than the same latitudes in the U.S. and Canada. The thermohaline circulation is a globe-girdling current, driven by density differences in sea water, that carries warm tropical waters north toward Scandinavia. Theory says that if the thermohaline circulation shuts down, temperatures in Europe would drop. What could shut it down? According to the theory, a massive influx of fresher water freed from the Greenland ice cap. A warmer world could cause massive quantities of ice to melt, resulting in catastrophic cold in Europe.

This is just one of the phenomena known as "tipping points" or thresholds in the climate system. Another is the carbon cycle, in which CO₂ is constantly released and trapped by various means. It is released by volcanoes from carbonate rock, and trapped anew in carbonate rock formed from trillions of seashells. On shorter time scales, animals breathe out CO₂, while decay and many industrial processes release it. Part of this CO₂ is absorbed by ocean waters and by soils. Plants incorporate it into their tissues, keeping it out of the atmosphere while they live. Some plant matter gets buried as peat moss, petroleum, or coal, sequestering carbon for longer periods. Humans free this in the form of CO₂ as they plow up the peat, power vehicles with petroleum products, and heat homes with coal. Also, clearing land for cities and cutting down forests to make lumber or plant crops adds more CO₂ to the atmosphere.

Methane too is produced by living creatures, and massive quantities of it are trapped in swamps, in peat bogs, and in undersea clathrates.

Both CO₂ and methane are greenhouse gases, and the wild card in our current climate situation is that more of both gets released as the world's temperature rises. Warm soils hold less CO₂ than cooler soils. The same is true of ocean water. When permafrost melts, the peat held in it releases methane if it stays wet, CO₂ if it dries out. Clathrates too are apt to release their stored methane as they warm up, and much of this will turn into CO₂ over the course of years. Theory suggests that this sort of scenario may underlie the observed fact that temperature consistently rises about 800 years before atmospheric CO₂. It is plausible that the triggering event may be a smaller increase in temperature that cannot be read from the proxy data. (Alternatively, an effective increase in solar radiation could be the cause.)

Solar radiation may be more important than previously understood. Recent research1 suggests that, while the Sun's emission of visible light varies only slightly, ultraviolet radiation may vary ten times as much. This UV may have a profound effect near the poles, driving circulating jet streams that pull warm winds from further south to melt ice and snow cover. The soil exposed would absorb more visible light, adding its own warming effect.

Interaction among these positive feedbacks has the net effect of amplifying any increase in temperature. This is cutting edge research; there is much that is not understood about the processes involved.2 The apparent 800-year lag between temperature and CO₂ increases probably means there is no reason for urgent action. However, the fact that ongoing research makes it ever more certain that global warming will cause major problems in the future, and that it could happen faster than expected, makes prudent action imperative. And more climatology research is among the most imperative.

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This page was last modified on 13 September 2024.