Winds from Siberia reduce Arctic sea ice cover, Norwegian researchers find

In recent years, satellite images have shown large variations in the ice cover around the North Pole. The images have also shown that the ice cover in the Arctic has diminished considerably over the past 30 years, with the most drastic reductions occurring in recent years.

Many experts believe that it is now only a matter of decades before climate change results in a totally ice-free Arctic during parts of the year. For instance, the UN Intergovernmental Panel on Climate Change (IPCC) projects that this may occur by the end of this century.

How much of the change in ice cover is caused by dramatic changes in the climate, and how much is the result of other factors? And what is causing the ice cover in the Arctic to disappear even faster than the climate models project?

The Arctic climate paradox

A few years ago, US researchers discovered what they termed the "Arctic climate paradox." Since 1980, the researchers had been observing a decrease in ice cover. They explained this through a slow process of climate change combined with fluctuations in patterns of atmospheric pressure and air currents over the Arctic. It was believed that the positive phase of the Arctic Oscillation (AO) was a major cause of the receding ice cover.

The AO is normally influenced by three pressure systems located over the Azores, Iceland and the Northern Pacific Ocean. Since 2000 the AO has been in a negative phase. As a result, researchers predicted that the pace of reduction in the ice cover would slow down.

Instead it accelerated.

Unknown factor

"The US researchers argued that the ice was responding to something else, another factor that nobody had considered," explains Asgeir Sorteberg, Associate Professor at the Geophysical Institute at the University of Bergen. He has been investigating this phenomenon along with his colleagues in the project entitled the Norwegian Component of the Ecosystem Studies of Sub-Arctic Seas (NESSAS).

When the Norwegian researchers began their work, they noticed in particular a dramatic change in the weather pattern in the Arctic beginning about the year 2000. The change corresponded to the point in time when the reduction of ice cover in the Arctic began to accelerate.

The researchers began to analyze the circulation patterns over the Arctic.

"We found that these patterns can explain in large part why the ice cover decreased so much more rapidly after 2000. Wind patterns depend on the position of major high-pressure and low-pressure systems. We discovered that months with very little ice cover and high temperatures corresponded with crucial variations in the wind patterns," explains Mr Sorteberg.

"Up until 2000, the Arctic Oscillation (AO) had the greatest impact on the winter ice cover in the Arctic. But the change around 2000 meant that more of the weather and wind over the Arctic after that year was determined by high-pressure and low-pressure systems in northern Russia. In other words, the AO, which was usually so crucial, played a much less important role."

Ice is pushed away

"We have now managed to document what has occurred in connection with this change," says Mr Sorteberg.

The changed wind direction pushes large ice masses away from the Arctic and down along the eastern coast of Greenland. At the same time, less ice forms when the winds over the Arctic are determined by the pressure systems in northern Russia rather than those over the North Atlantic and the Pacific Ocean, as is normally the case.

The conclusion from this research is that we should be cautious about using the extent of the ice cover as an indicator of the ice's climatic "state of health."

The extent of the ice cover is highly dependent on the wind direction, and short-term changes in the ice cover give very little indication of whether climate change is occurring in the Arctic.

"The dramatic changes in the extent of Arctic sea ice in recent years have mainly been caused by atmospheric circulation patterns that have tended to reduce ice cover, combined with a slow process of climate change. Variations in the circulation patterns are part of the natural fluctuations in the weather. In certain periods these fluctuations will reinforce human-made changes, while at other times they will mask them," says Mr Sorteberg.

Climate change leads to thinner ice

Mr Sorteberg believes we should be cautious about interpreting the dramatic decrease in Arctic ice cover in the past decade as an indication that the Arctic will be ice free in 10 to 20 years.

However, he emphasizes that he and his colleagues do not reject the assertion that climate change is affecting Arctic ice cover or that the IPCC is wrong when it states that the Arctic may be nearly ice free in summer towards the end of this century.

"There is no doubt that the Arctic sea ice has become thinner in recent years. The thickness of the sea ice is a much better indicator than the extent of the ice cover if we want to study how climate change may affect the ice in the Arctic," says Mr Sorteberg.