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Tuesday, March 27, 2012

Global Warming threshold for Greenland Ice Sheet collapse reduced to 1.6 degrees C

New research from the Potsdam Institute for Climate Impact Research and Universidad Complutense de Madrid has lowered the best estimate for the irreversible collapse of the Greenland Ice Sheet down to 1.6 °C, making the ice sheet more vulnerable than previously thought to global warming. The previous best estimate was 3.1 °C. As we currently have 0.8 °C of global warming, by the middle of the century we could easily pass this new threshold unleashing an ultimate sea level rise of several metres.

Already we have seen record summer melting in Greenland in 2010, and near record mass loss in 2011. According to NOAA the melt season in 2011 lasted up to 30 days longer than average and it affected 31 percent of the ice sheet surface, making 2011 one of just three years since 1979 where melt area exceeded 30 percent. Polar regions are warming much faster and to a greater degree than any other latitude on earth.

Previous best estimates of the threshold leading to complete melting were 3.1 °C (1.9-5.1 °C, 95% confidence interval) above the preindustrial climate temperatures. The study - Multistability and critical thresholds of the Greenland ice sheet (abstract) - says in part:

We estimate that the warming threshold leading to a monostable, essentially ice-free state is in the range of 0.8-3.2 °C, with a best estimate of 1.6 °C. By testing the ice sheet's ability to regrow after partial mass loss, we find that at least one intermediate equilibrium state is possible, though for sufficiently high initial temperature anomalies, total loss of the ice sheet becomes irreversible.

"The more we exceed the threshold, the faster it melts," says Alexander Robinson, lead-author of the study that has just been published in Nature Climate Change. If greenhouse-gas emissions continue on a business-as-usual approachm we could be looking at 8 degrees Celsius of global warming. This would result in 20 per cent of the ice sheet melting within 500 years and a complete loss in 2000 years, according to the study. "This is not what one would call a rapid collapse," says Robinson. "However, compared to what has happened in our planet's history, it is fast. And we might already be approaching the critical threshold."

"Our study shows that under certain conditions the melting of the Greenland ice sheet becomes irreversible. This supports the notion that the ice sheet is a tipping element in the Earth system," says team-leader Andrey Ganopolski of PIK. "If the global temperature significantly overshoots the threshold for a long time, the ice will continue melting and not regrow - even if the climate would, after many thousand years, return to its preindustrial state."

Feedbacks between the collapsing ice sheet and climate make this an important tipping element in the Earth climate system. The Ice sheet is over 3000 metres thick and elevated cooler altitudes. Once melting starts reducing the altitude of the ice sheet, higher temperatures will kick in accelerating the melting further. As ice melts and glacial water pools, the albedo of the ice sheet will change, absorbing more radiation with more warming.

The study utilised a computer simulation which incorporated the various climate feedback mechanisms of the ice sheet and the regional climate. The model correctly simulated observations of the ice sheet and the ice sheet behaviour in previous glacial cycles.

Global climate Negotiations since Copenhagen in 2009 have adopted 2°C as the 'safe' limit of global temperature increase to attempt to not surpass. However some climate scientists such as NASA climatologist James Hansen have been warning for some time that even 2°C is too high and that perhaps we should be aiming at a much lower limit. "The paleoclimate record reveals a more sensitive climate than thought, even as of a few years ago. Limiting human-caused warming to 2 degrees is not sufficient," said NASA climatologist James Hansen at the American Geophysical Union meeting on December 6 2011

Most of the small island states that are vulnerable to rising sea levels have voiced strong concern at the 2°C temperature limit as being too high and will result in their countries being innundated. The Climate Vulnerable Countries'Forum in 2009 called for "ambitious emission reduction targets consistent with limiting global average surface warming to well below 1.5 degrees Celsius above preindustrial levels and long-term stabilization of atmospheric greenhouse gas concentrations at well below below 350 p.p.m."

This study confirms that 2°C is no longer safe in regard to maintaining the Greenland Ice Sheet, but is likely to lead to the eventual disintegration of the ice sheet raising sea levels by approximately seven metres.


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