SAN FRANCISCO, Dec. 11, 2011 -- Ice-age geologic records suggest Earth's climate will warm faster than expected, pushing the global sea level perhaps more than 3 feet higher within this century, a panel of scientists warned Tuesday.
Speaking at the American Geophysical Union annual meeting here, federal and academic scientists said they reviewed ice core measures spanning more than 500,000 years of Ice Ages and subsequent warming periods to warn that ice sheets in the past had quickly melted once temperatures reached tipping points.
"It's like the ice on your windshield suddenly starting to melt all at once," says Eelco Rohling of the United Kingdom's University of Southampton. "The ice takes a little kicking and then it melts."
Sea-level rise has long been a point of contention among climate scientists, who overwhelmingly agree that humanity adding greenhouse gases such as carbon dioxide to the atmosphere has raised global average temperatures about 1.4 degrees Fahrenheit worldwide in the last century, according to a 2010 National Academy of Sciences report.
Exactly how much hotter it will get by 2100 if humanity doubles the amount of atmospheric carbon dioxide — projected to happen by 2060 at present rates by the 2007 Intergovernmental Panel on Climate Change (IPCC) — remains to be seen. Estimates range from roughly 4 to 9 degrees warmer.
Carbon dioxide concentrations in the air are 39% higher now than pre-industrial levels.
That number grows every year because the greenhouse gas remains for centuries in the sky. Each degree of warming added to the atmosphere by the increase, the panel members warned, raises the risk of more sea-level rise from melting ice sheets on Greenland and Antarctica.
"We cannot double carbon dioxide," said NASA climate scientist James Hansen, who has been a central figure among climate scientists calling for actions to curb greenhouse gas emissions since 1988. "We will be sending the climate back to a state very different from what humanity is used to."
In particular, Hansen and climate scientist Ken Caldeira of Stanford University pointed to an era 55 million years ago when the globe was ice-cap free and temperatures reached heights far above today's.
"The difference was that took millennia to happen. We're doing it on a much shorter time scale," Caldeira said.
The panel of scientists took issue with a recent Science magazine report led by Andreas Schmittner of Oregon State University that looked at pollen and seafloor records of the last Ice Age that was more than 19,000 years ago.
That study concluded that doubling carbon dioxide was very unlikely to increase global average surface temperatures more than 4.7 degrees. "Virtually impossible to go higher," Schmittner said. Rohling says other estimates see it climbing 8.6 degrees or more.
The 2007 IPCC projected less than 2 feet of sea-level rise from warming in this century, partly because the report called such sudden ice sheet melts too hard to project for reliable estimates.
All of the studies of carbon dioxide doubling effects show temperatures increasing, regardless, Caldeira says.
Because carbon dioxide mostly remains in the atmosphere for centuries, the current high level of carbon dioxide means that a millennia from now there will be a sea-level rise of about 80 feet hitting coasts worldwide, "at some point."