NY Times quotes Pierrehumbert, who compares trace amounts of carbon dioxide to a "thermonuclear warhead"

Clouds’ Effect on Climate Change Is Last Bastion for Dissenters - NYTimes.com

polls say 97 percent of working climate scientists now see global warming as a serious risk...“Clouds really are the biggest uncertainty,” said Andrew E. Dessler, a climate researcher at Texas A&M. “If you listen to the credible climate skeptics, they’ve really pushed all their chips onto clouds...In the high projection, some polar regions could heat up by 20 or 25 degrees Fahrenheit — more than enough, over centuries or longer, to melt the Greenland ice sheet, raising sea level by a catastrophic 20 feet or more. Vast changes in  rainfall, heat waves and other weather patterns would most likely accompany such a large warming.

“The big damages come if the climate sensitivity to greenhouse gases turns out to be high,” said Raymond T. Pierrehumbert, a climate scientist at the University of Chicago. “Then it’s not a bullet headed at us, but a thermonuclear warhead.”