Investigating Sea Ice Decline

Chances that the 2008 ice extent will fall below last year's record minimum is about 8 percent, researchers forecast after having run a number of different models predicting the fate of the Arctic sea ice this summer. But there is still reason for concern; the scientists are almost certain the ice extent will fall below the minimum of 2005, which was the second lowest year on record. With a probability of 80% the minimum ice extent in 2008 will be in the range between 4.16 and 4.70 million km².

“After the strong decrease of the Arctic ice during the last summer, climate scientists all around the world are constantly asked: how will the ice develop in the next years?” described Prof. Dr. Rüdiger Gerdes from the Alfred Wegener Institute his motivation. “To answer this question, we did not want to guess, but to rely on sound calculations.”

Scenarios of the long-term development of sea ice clearly indicate a de-creased ice cover - exact prognoses for the following summer, however, are not yet possible. This is mainly due to the fact that the short-term development of sea ice depends strongly on the actual atmospheric conditions, namely the weather and in particular wind, cloud cover and air temperatures.

Because the exact atmospheric conditions which determine the weather patterns in the Arctic Ocean during the coming months are not predictable, Rüdiger Gerdes and his team have entered atmospheric data of the last twenty years into an ocean sea ice model.

“Through this, we are still not able to make a definitive statement on sea ice cover in September. This 'trick' enables us to compute the bandwidth of possible ice covers, and to quantify the probability of extreme events”, said Gerdes.

Apart from the variability of atmospheric quantities during the melting season, ice thickness at the beginning of the season determines the new ice minimum. Accordingly, computations of ice thickness enter the models of the researchers. Start conditions from June 27th 2008 were used for their current prognosis.

Different from long-term prognoses, the researchers' forecasts can quickly be checked by reality.

“It is a first test, and all participating researchers are eager to know how their prognosis has fared at the end of the summer. In the end, this small competition serves the optimisation of our models, so that we are able to improve our predictions concerning short-term seasonal fluctuations. It has to be added, however, that even perfect models would not be able to rule out a component of chance regarding the atmosphere. These forecasts will always be about probability, and not exact prognoses”, Rüdiger Gerdes concluded.

Eleven research cruises will be carried through as part of the DAMOCLES programme (Developing Arctic Modelling and Observing Capabilities for Long-term Environment Studies) during the summer and autumn 2008.