A team of geophysicists is warning that the massive polar ice sheets are even more vulnerable to global warming than previously believed, and could trigger a sea level rise of six to nine metres.
The scientists from Princeton and Harvard universities say that just two degrees Celsius of global warming, which is now widely expected to occur in coming decades, could be enough to commit the planet to inundation.
"The time to avoid disastrous outcomes may run out sooner than expected," says Princeton's Michael Oppenheimer.
He is co-author of a ominous new report on what happened the last time global temperatures rose a couple of degrees - the Greenland and West Antarctic ice sheets melted away so extensively that sea level rose between 6.6 and 9.4 metres.
If emissions of greenhouse gases are not reduced soon, they scientists say the planet could be committed to comparable melting, which might be unstoppable.
They say low-lying regions around the world could be inundated by more than a metre of sea level rise this century, followed by many more metres in coming centuries. Low-lying areas like Bangladesh and Florida would be hard hit, and Canadian communities from Tuktoyaktuk to Vancouver to Charlottetown could all expect to see waters rise. A one-metre rise in sea level would immediately affect 145 million people around the world.
The geophysicists' report in the journal Nature Thursday is a new - and much more "startling," according to a commentary accompanying the report - assessment of what occurred the last time polar temperatures were three to five degrees Celsius warmer than today. The so-called "last interglacial stage" 125,000 years ago is considered an analogue or guide to what might happen if global warming continues on its current path.
The new assessment is based on sea level indicators such as coral and beach records as well as changes to Earth's gravity and surface as massive ice sheets melted away. It concludes that during the last interglacial, global sea level peaked more than 6.6 metres higher than today and may have risen 9.4 metres. That is higher than previous estimates, and indicates much of the Greenland and Antarctic ice sheets melted away.
The "disconcerting message" is that the planet's response to 1.5-2 degree C of global warming could be an increase in sea level of seven to nine metres, Peter Clark at Oregon State University writes in a commentary.
Given current rates of fossil fuel consumption and greenhouse gas emissions, Princeton co-author Robert Kopp notes that Earth is on track to have "significantly more warming by the end of century than occurred during the last interglacial."
The researchers caution that it is not clear from their study how long temperatures had to stay high to commit the planet to six to nine metres of sea level rise last time around.
Despite the uncertainties, Oppenheimer says the findings should send a "strong message" to the governments negotiating in Copenhagen about the need to reduce emissions.