The Greenland ice sheet is likely to be more vulnerable to global warming than previously thought.
A new study by scientists from the Potsdam Institute for Climate Impact Research (PIK) and the Universidad Complutense de Madrid, shows that the temperature threshold for melting the ice sheet completely is in the range of 0.8 to 3.2 degree Celsius global warming, with a best estimate for 1.6 degrees above pre-industrial levels.
Today, already 0.8 degrees global warming has been observed.
Substantial melting of land ice could contribute to long-term sea-level rise of several meters, potentially affects the lives of many millions of people. The time it takes before most of the ice in Greenland is lost strongly depends on the level of warming.
“The more we exceed the threshold, the faster it melts,” says Alexander Robinson, lead-author of the study published in Nature Climate Change. “In a business-as-usual scenario of greenhouse gas emission, in the long run humanity might be aiming at 8 degrees Celsius of global warming.”
According to the study, this would result in one-fifth of the ice sheet melting within500 years and a complete loss in 2,000 years.
“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.”
In contrast, if global warming would be limited to 2 degrees Celsius, complete melting would happen on a timescale of 50,000 years. Still, even within this temperature range often considered a global guardrail, the Greenland ice sheet is not secure.
Previous research suggested a threshold in global temperature increase for melting the Greenland ice sheet of a best estimate of 3.1 degrees, with a range of 1.9 to 5.1 degrees. The new study’s best estimate indicates about half as much.
“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 thousands of years, return to is pre-industrial state.”
This is related to feedbacks between the climate and the ice sheet: The ice sheet is over 3,000 meters thick and, thus, elevated into cooler altitudes. When it melts its surface comes down to lower altitudes with higher temperatures, which accelerates the melting. Also, the ice reflects a large part of solar radiation back into space. When the area covered by ice decreases, more radiation is absorbed and this adds to regional warming.
The scientists achieved their insights by using a novel computer simulation of the Greenland ice sheet and the regional climate. This model performs calculations of these physical systems including the most important processes, for instance climate feedbacks associated with changes in snowfall and melt under global warming.
The simulation proved able to correctly calculate both the observed ice sheet of today and its evolution over earlier glacial cycles, increasing the confidence that it can properly assess the future. All this makes the new estimate of Greenland temperature threshold more reliable than previous ones.