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Scientists identify melting threshold for Greenland ice sheet

  • 12 Mar 2012, 16:50
  • Verity Payne

A new scientific study suggests that the Greenland ice sheet is more sensitive to rises in global temperature than previously thought.

Researchers calculated that another 0.9°C of global temperature rise from today's levels could commit the Greenland ice sheet to completely melting. They have also come up with a time frame - anywhere from thousands to tens of thousands of years, depending on how much the Earth eventually warms and how long that warming is sustained for.

Much of the recent research about the Greenland ice sheet focuses on how fast it is melting, and how this will affect sea level rise in the short-term - mostly over the coming century. A lot less is known about how man-made climate change might affect Greenland beyond 2100.

The new study, published in the journal Nature Climate Change, examines how much temperature rise would be needed to melt the Greenland ice sheet completely. It suggests that the most likely temperature threshold is 1.6°C warmer than pre-industrial times, with a range of results from 0.8°C to 3.2°C.

Because global average temperature is currently nearly 0.8°C higher than pre-industrial times, reaching 1.6°C would require a further rise in global average temperature of around 0.9°C.

The figure of 1.6°C is notable because it is around half the value of previous estimates. Further research will be necessary to reconcile the results or back up the new findings, but this paper may mean that previous research has overestimated Greenland's stability in the face of rising global temperatures.

The researchers used a climate model simulating regional climate around Greenland and the important processes happening in the ice sheet - including its surface mass balance, melt dynamics and climate feedbacks. The model simulations tally with observations of the ice sheet today, and of how it has changed back in time, leading the researchers to express confidence in their model's ability to represent the ice sheet and any changes to it accurately.

Irreversible melting 

Study co-author Andrey Ganopolski, of the Potsdam Institute for Climate Impact Research (PIK), says that the research confirms that there is a critical threshold temperature beyond which the loss of the Greenland ice sheet could be irreversible:

"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. 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."

The team also considered the possibility that there might be 'equilibrium states', where the ice sheet stabilises after partial melting. Model simulations suggested that at least one 'equilibrium state' might be possible, but only if there isn't too much warming:

"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."

Rate of ice sheet melt depends on warming

The paper stresses that how quickly the ice sheet melts will be determined by how much future temperature overshoots the threshold temperature.

For example, they calculate that if temperature rise is limited to just 2°C above pre-industrial climate, it will take 50,000 years to melt the entire ice sheet. On the other hand, if global temperature rises by 8°C compared to pre-industrial climate, under a scenario where greenhouse gas emissions just keep going up, the paper suggests it would take significantly less time - around 2000 years - for the ice sheet to fully melt.

Lead-author of the study, Alexander Robinson of PIK, points out:

"This is not what one would call a rapid collapse. However, compared to what has happened in our planet's history, it is fast. And we might already be approaching the critical threshold [temperature]."

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