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Even if temperatures rise only moderately, some scientists fear, the climate would reach a "tipping point"--a point at which even a tiny additional increase would throw the system into violent change. If peat bogs and Arctic permafrost warm enough to start releasing the methane stored within them, for example, that potent greenhouse gas would suddenly accelerate the heat-trapping process.
By contrast, if melting ice caps dilute the salt content of the sea, major ocean currents like the Gulf Stream could slow or even stop, and so would their warming effects on northern regions. More snowfall reflecting more sunlight back into space could actually cause a net cooling. Global warming could, paradoxically, throw the planet into another ice age.
Even if such a tipping point doesn't materialize, the more drastic effects of global warming might be only postponed rather than avoided. The IPCC's calculations end with the year 2100, but the warming won't. World Bank chief scientist, Robert Watson, currently serving as IPCC chair, points out that the CO2 entering the atmosphere today will be there for a century. Says Watson: "If we stabilize [CO2 emissions] now, the concentration will continue to go up for hundreds of years. Temperatures will rise over that time."
That could be truly catastrophic. The ongoing disruption of ecosystems and weather patterns would be bad enough. But if temperatures reach the IPCC's worst-case levels and stay there for as long as 1,000 years, says Michael Oppenheimer, chief scientist at Environmental Defense, vast ice sheets in Greenland and Antarctica could melt, raising sea level more than 30 ft. Florida would be history, and every city on the U.S. Eastern seaboard would be inundated.
In the short run, there's not much chance of halting global warming, not even if every nation in the world ratifies the Kyoto Protocol tomorrow. The treaty doesn't require reductions in carbon dioxide emissions until 2008. By that time, a great deal of damage will already have been done. But we can slow things down. If action today can keep the climate from eventually reaching an unstable tipping point or can finally begin to reverse the warming trend a century from now, the effort would hardly be futile. Humanity embarked unknowingly on the dangerous experiment of tinkering with the climate of our planet. Now that we know what we're doing, it would be utterly foolish to continue.
--Reported by David Bjerklie, Robert H. Boyle and Andrea Dorfman/New York and Dick Thompson/Washington
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