Could Global Warming Be Worse Than You Think?: BLOG: SciAm Observations: "One of the questions that came up in the earlier global warming thread was whether climate models have been tested against historical data. As I collect my thoughts on this issue, I wanted to share with you one observation. Climatologists who think global warming is serious and human-driven actually agree with skeptics who say that models have not been adequately tested. But whereas the skeptics think that the models overstate the threat, the mainstream researchers think they could understate it.
Their concern stems from one simple fact: the projected increase in temperature over the coming decades would take us out of the range encountered in the natural ice-age cycle of recent geologic history. It could 'imply changes that constitute practically a different planet,' climate scientist Jim Hansen told the Washington Post in January, and neither climate models nor humanity is up to it.
Update (April 20th): Models do a pretty good job at matching climate variations in the relatively recent past. A Nature paper today compares models to climate data for the period from 1270 to 1850. Volcanic eruptions, identified by the residue they left in ice cores, provided a particularly good set of test cases. Combining this comparison with one for the years from 1950 to 2000, the authors conclude that doubling the concentration of carbon dioxide would lead to a global temperature increase of about 2.5 degrees Celsius, with an error range from 1.5 and 6.2 degrees. The trouble, which I discuss below, is that the present warming is greater than anything observed over the past thousand years, so new effects might crop up that neither models nor recent history captures. "
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