SPIEGEL ONLINE: The Sermeq Kujalleq glacier, where we are standing right now, has retreated by 15 kilometers (nine miles) in recent years. The ice that breaks off contributes to sea-level rise. How worried should the world be about a collapse of the Greenland ice sheet?
Andreas Peter Ahlstrøm: The Greenland ice sheet is, like some regions in Antarctica, an awakening giant. In the coming centuries it will add significantly to the global sea-level rise. It matters to a huge number of people around the globe, as there are definitely regions that are very sensitive to even the slightest changes in the global sea level. People in the Indian Ocean island nation of the Maldives, for example, are already worried right now.
SPIEGEL ONLINE: In its last report, the United Nations' Intergovernmental Panel on Climate Change seems to have underestimated the importance of what is happening with Greenland's icecap. What is the current situation?
Ahlstrøm: By the time the last IPCC report was finished in 2005, it was simply not possible to project what will happen. We had already seen dramatic changes by that time, but we had to leave them out. We simply didn't know how to model the ice sheet. But we are in a better position right now. New estimates that we are about to finalize suggest a potential sea-level rise from the Greenland ice sheet that could be 0.35 meters (14 inches) over the next century.
SPIEGEL ONLINE: How much ice is Greenland losing every year?
Ahlstrøm: Among colleagues, we still have problems agreeing on estimates. An average from different people would probably be 130 cubic kilometers (31 cubic miles) per year at the moment. But some experiments, such as those done using the GRACE satellite, which maps variations in the Earth's gravity field, indicate much higher losses of up to 170 cubic kilometers per year.
SPIEGEL ONLINE: What makes it so difficult to pinpoint the exact number?
Ahlstrøm: We are simply dealing with a huge piece of ice, about 1.7 million square kilometers (0.66 million square miles) of it. The sheer size of the ice makes it difficult to track changes. We still have a scarcity of observations. And there are a lot of different processes involved that are difficult to keep track of: You have precipitation over this enormous area, then you have the outlet glaciers, the ice streams, that are difficult to assess, and on top of that you have to estimate the melt at the margins.
SPIEGEL ONLINE: Measurements indicate that there is more and more snow falling in the middle of the ice sheet. Do all of your colleagues share the view that the ice in Greenland is losing mass?
Ahlstrøm: There is indeed additional snow in the middle of the ice sheet. But it is easily outweighed by the increased melt and the increased loss from icebergs. I don't see assessments around that see the complete mass-balance of the ice sheet being positive. It is all about to what extent we are losing mass.
SPIEGEL ONLINE: Some of your colleagues argue that phases of increased outflow from the glaciers could be just a short episode that regularly reoccurs.
Ahlstrøm: We have very detailed records which show that Sermeq Kujalleq has been constantly retreating for at least 150 years. Normally, the glacier retreats slowly and gradually. And then, sudden jumps in speed occur in between. The latest acceleration now seems to be permanent. I don't think it will accelerate much more, but it will stay at the high rate of loss. With the atmosphere and the oceans warming, the glacier "tongues" simply can't extend again. The higher rates of mass loss from the outlet glaciers is likely to stay at a very high level.
SPIEGEL ONLINE: The Greenland ice sheet will play a big symbolic role for the climate talks in Copenhagen in December. What do you expect as an outcome from these talks?
Ahlstrøm: I am a scientist, not a politician. As a scientist it is, nevertheless, interesting to see that the politicians are trying to figure out what would be feasible politically. Currently, trying to cap global warming at two degrees is something that they can seemingly sell to the public. But as a scientist I have to really stress that two degrees is an absolute maximum. It is not something to be negotiated, like 2.7 or 2.5 degrees. We are at the very maximum already, I believe.