James E. Hansen of NASA's Goddard Institute for Space Studies looked at how Earth's climate responded to past natural changes to answer a fundamental question raised by ongoing climate change: "What is the dangerous level of global warming?"
While some international leaders have suggested a goal of limiting warming to 2 degrees Celsius from pre-industrial times to avert catastrophic change, Hansen said a warming of 2 degrees C would lead to drastic changes such as significant ice sheet loss in Greenland and Antarctica.
Hansen made his remarks at a meeting of the American Geophysical Union in San Francisco Tuesday.
Hansen compared the current climate with previous similar "interglacial" epochs when polar ice caps existed but glaciers did not dominate the world.
Hansen found global mean temperatures during the Eemian period, which began about 130,000 years ago and lasted about 15,000 years, were less than 1 degree Celsius warmer than today but still caused sea level rises of 13 to 20 feet.
"The paleoclimate record reveals a more sensitive climate than thought, even as of a few years ago. Limiting human-caused warming to 2 degrees is not sufficient," Hansen said. "It would be a prescription for disaster."