Ancient Climate History Foreshadows Rapid Future Climate Change

SAN FRANCISCO, California, December 11, 2011 (ENS) – At the rate the greenhouse gas carbon dioxide is increasing in the atmosphere, Earth is likely to experience a rise in average temperatures of several degrees as well as ice sheet loss that could cause the seas to rise several meters in this century, warns the director of NASA’s Goddard Institute for Space Studies.

To answer the fundamental question, “What is the dangerous level of global warming?” Dr. James Hansen looked at how the Earth’s climate responded to natural changes in the distant past.

“We don’t have a substantial cushion between today’s climate and dangerous warming,” Hansen said. “Earth is poised to experience strong amplifying feedbacks in response to moderate additional global warming.”

The human-caused release of increased carbon dioxide into the atmosphere presents climate scientists with something they have never seen in the 65 million year record of carbon dioxide levels – a drastic rate of increase that makes it difficult to predict how rapidly the Earth will respond, NASA said in a statement announcing the results of Dr. Hansen’s research.

Many scientists have recommended limiting climate warming to two degrees Celsius from pre-industrial times to avert catastrophic damage, extreme weather, droughts, floods and the northward movement of tropical diseases.

International heads of state and government adopted this two degree Celsius limit as a upper limit at the United Nations climate change conference last year in Cancun, Mexico and reconfirmed it today as this year’s UN climate conference closed in Durban, South Africa.

Dr. James Hansen is interviewed by Climate One founder Greg Dalton, October 2010. (Photo by Climate One)

But Hansen told reporters at a meeting of the American Geophysical Union in San Francisco Tuesday that warming of two degrees Celsius would lead to drastic changes, such as ice sheet loss in Greenland and Antarctica.

“Humans have overwhelmed the natural, slow changes that occur on geologic timescales,” Hansen said. “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 said.

“It would be a prescription for disaster,” said Hansen, who has marched at a number of climate justice demonstrations and was arrested protesting the TransCanada Keystone XL tar sands pipeline outside the White House in August.

Based on Hansen’s temperature analysis work at the Goddard Institute for Space Studies, the Earth’s average global surface temperature has already risen .8 degrees Celsius since 1880, and is now warming at a rate of more than .1 degree Celsius every decade.

This warming is driven by increased greenhouse gases in the atmosphere, particularly carbon dioxide, emitted by the burning of fossil fuels at power plants, in cars and in industry.

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. A doubling of carbon dioxide would cause an eventual warming of several degrees, Hansen said.

In recent research, Hansen and co-author Makiko Sato, also of the Goddard Institute for Space Studies, compared the climate of today, the Holocene, with previous similar “interglacial” epochs – periods when polar ice caps existed but the world was not dominated by glaciers.

In studying cores drilled from ice sheets and also cores from 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, Hansen said.

The paleoclimate record suggests that every degree Celsius of global temperature increase will equate to 20 meters of sea level rise over centuries, he said, noting that “large uncertainties” remain in predicting how that ice loss would unfold.

Hansen says ice sheet disintegration will be a non-linear process as seen in vulnerable places such as Pine Island Glacier in West Antarctica, where the rate of ice mass loss has continued accelerating over the past decade.

Data from NASA’s Gravity Recovery and Climate Experiment, GRACE, satellite is already consistent with a rate of ice sheet mass loss in Greenland and West Antarctica that doubles every 10 years.

The GRACE record is too short to confirm this with great certainty; however, the trend in the past few years does not rule it out, Hansen said, adding that this continued rate of ice loss could cause multiple meters of sea level rise by 2100.

In periods when carbon dioxide has increased due to natural causes, the rate of increase averaged about .0001 parts per million per year – in other words, one hundred parts per million every million years.

Scientists say that fossil fuel burning is now causing carbon dioxide concentrations to increase at two parts per million per year.

The ice and ocean sediment cores from the polar regions indicate that temperatures at the poles during previous epochs – when sea level was tens of meters higher – is not too far removed from the temperatures Earth could reach this century on what Hansen called a “business as usual” trajectory.

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