All these lovely disaster prognostications predicated on something yet to be observed. We still don’t know whether increasing atmospheric carbon dioxide actually increases net greenhouse effect or merely displaces absorption already occurring with previously abundant greenhouse gases.
Mr. Scheffer and other scientists are now trying to identify the early-warning signals for climate that precede abrupt transitions. Tim Lenton, a climate scientist at the University of Exeter in England, has identified a handful of climate systems that could reach tipping points in the not-too-distant future. These are not so much related to global average temperatures — the main metric for climate-change arguments — as they are to patterns of climate that repeat themselves each year.
El Niño is one such pattern — a gigantic blob of warm water that sloshes around in the Pacific Ocean, causing weather changes across wide swaths of the globe. Another is the West African monsoon, which brings rain to the west coast of the continent. Each is subject to behaving like dynamical systems — which means they are prone to “flip” from one state to another, like one of Mr. Scheffer’s ponds, over time periods that vary from a year to a few hundred.
The most frightening prospect that Mr. Lenton has found is the vulnerability of the Indian monsoon. More than a billion people depend on this weather pattern each year for the rain it brings to crops. The monsoon, though, is being affected by two conflicting forces: the buildup of carbon dioxide in the atmosphere is adding energy to the monsoons, making them more powerful. On the other hand, soot from fires and coal plants acts to blocks the sun’s energy, weakening the monsoons.
This opposition creates potential instability and the possibility that the atmospheric dynamics that bring the monsoons could change suddenly. Mr. Lenton’s analysis shows this could occur in a remarkably short time. The monsoons could be here one year, then gone the next year.
Other possible tipping points are the melting of the North Pole’s sea ice, Greenland’s glaciers and the Antarctic ice sheets, and the destruction of the Amazon rain forest and Canada’s boreal forests.