A Point of No Return for Greenland's Ice

July 25, 2008 -- Every molecule of carbon dioxide added to the atmosphere brings Greenland's ice sheet closer to irreversible melting -- and a sea level rise of more than 20 feet.

A new analysis suggests that if we pass a certain threshold of total emissions, the ice sheet will melt completely, no matter how high or low a peak CO2 concentration is reached or how quickly emissions are reduced afterward.

"A peak warming for a very short period will have an impact, but it might not be enough to cause long-term melting," said John Church of the Center for Australian Weather and Climate Research in Hobart, Australia, who was not a part of the study. "It's a matter of getting the temperatures up and keeping them up."

"We show that it's not really a question of how much CO2 in terms of 700 or 800 ppm [parts per million] in the atmosphere," said study author Gilles Ramstein of the Centre National de la Recherche Scientifique in Gif-sur-Yvette, France "It's really about cumulative doses. That means you can get a deglaciation of Greenland at 700 ppm if you reach this value and stay on that value for a long time."

The researchers used a climate model designed to reach over very long timescales -- tens of thousands of years -- to test the effect of different emissions scenarios on the extent of Greenland ice melting over millennia.

Their results indicate that regardless of the peak CO2 concentration, if total emissions surpass 3,800 billion tons of carbon, the Greenland ice sheet will melt completely over thousands of years. So far, humans have emitted about 380 billion tons of carbon from fossil fuel combustion, according to the researchers.