New research from NASA into the Earth's paleoclimate history indicates we could be facing rapid climate change this century, including sea level rises of many meters.
And while international leaders have suggested a goal of limiting global warming to two degrees Celsius from pre-industrial times, Goddard Institute for Space Studies director James E Hansen says that even this would lead to drastic changes.
The Earth's average global surface temperature has already risen by 0.8 degrees Celsius since 1880, says Hansen, and is now increasing by more than 0.1 degree Celsius every decade.
At the current rate of fossil fuel burning, the concentration of carbon dioxide in the atmosphere will have doubled from pre-industrial times by the middle of this century, causing an eventual warming of several degrees, he says.
Hansen and his colleague Makiko Sato compared the climate of today, the Holocene, with previous similar interglacial epochs. By studying cores from both ice sheets and deep ocean sediments, they found that global mean temperatures during the Eemian period, which began about 130,000 years ago and lasted about 15,000 years, were less than one degree Celsius warmer than today.
If temperatures were to rise two degrees Celsius over pre-industrial times, global mean temperature would far exceed that of the Eemian, when sea level was four to six meters higher than today, says Hansen.
"The paleoclimate record reveals a more sensitive climate than thought, even as of a few years ago. Limiting human-caused warming to two degrees is not sufficient," he says. "It would be a prescription for disaster."
Two degrees Celsius of warming would make Earth much warmer than during the Eemian - indeed, similar to Pliocene-like conditions, when sea level was about 25 meters higher than today, says Hansen.
However, that sea level increase due to ice sheet loss would be expected to occur over centuries, and large uncertainties remain in predicting it accurately.
"We don’t have a substantial cushion between today's climate and dangerous warming. Earth is poised to experience strong amplifying feedbacks in response to moderate additional global warming," says Hansen.
"Humans have overwhelmed the natural, slow changes that occur on geologic timescales."