PARIS--Decades of climate studies have made some progress Researchers have convinced themselves that the world has indeed warmed by 0.6°C during the past century. And they have concluded that human activities--mostly burning fossil fuels to produce the greenhouse gas carbon dioxide (CO_2)-- have caused most of that warming. But how warm could it get? How bad is the greenhouse threat anyway? For 25 years, official assessments of climate science have been consistently vague on future warming. In report after report, estimates of climate sensitivity, or how much a given increase in atmospheric CO_2 will warm the world, fall into the same subjective range. At the low end, doubling CO_2--the traditional benchmark-- might eventually warm the world by a modest 1.5°C, or even less. At the other extreme, temperatures might soar by a scotching 4.5°, or more warming might be possible, given all the uncertainties.
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