The Arctic summer nears its end and the minimum extent of sea ice is reached. The Arctic ice cover amounted to 4.5 million square kilometres on September 12th.
This is slightly more than the lowest ice cover ever measured: 4.1 million square kilometres in the year 2007. Scientists are anxious about the development of sea ice because the long-time mean is 2.2 million square kilometres higher.
This development did not come about completely unexpectedly, however. A model calculation conducted at the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association in early summer showed that the ice minimum of the year 2008 would lie below that of the year 2005 with almost one hundred per cent probability. A new minimum below that of the year 2008, however, was expected with a probability of just eight per cent.
"If we regard the sea ice cover from the beginning of satellite observations in the year 1979, the measurement of 2008 comes as a little surprise, because summers with a low ice cover like 2007 are usually followed by winters with a heavy ice production," explains Prof. Rüdiger Gerdes, physical oceanographer at the Alfred Wegener Institute. From 1979 to 2004, about 6 to 7.5 million square kilometres of the Arctic were usually covered with ice. Now it is the second year in a row with less than 4.5 million square kilometres. However, the following summers still have to show if this trend is to continue.
"A hitherto unanswered question is whether the sequence of two extreme years presents a transition towards a new regime of Arctic sea ice, which makes the return to former states of sea ice cover difficult," continues Gerdes. Transitions of this kind show up in coupled climate models. However, they are prognosticated for the late 21st century. The key factor in the model simulations for Arctic sea ice is ice thickness. If the mean thickness falls below a certain threshold value, the main part of sea ice will melt. This means that vast ice-free areas will emerge each summer.
Compared to the extent of ice - which can be measured pretty well by means of satellites - the distribution of ice thickness in the Arctic Ocean is less well-known. The Alfred Wegener Institute contributes to the estimation of the Arctic ice volume and its variability with measurement equipment, which is hauled by helicopters. This way, data from more than fifteen years are at hand and account for a decrease of ice thickness in the central Arctic. However, these measurements do not cover all relevant parts of the Arctic Ocean - the range of the helicopters is too limited. "It cannot be excluded that sea ice is simply mechanically reallocated," reports Gerdes. "Our model computations show that ice transport from eastern to western Arctic waters caused by wind was an important factor in the great ice-free areas of the Siberian Shelf in the year 2007."
RV Polarstern currently benefits from the low sea ice cover in the Arctic Ocean. Scientists on board can measure the seafloor and sample sediment probes in areas, which were unreachable a few years ago. Polarstern cruises through close pack ice with a northerly course in the direction of 80° latitude; however, since it is mostly one year old thin sea ice, it can easily be broken. So far, the participants of the expedition were able to conduct their activities without hindrance.
The Alfred Wegener Institute carries out research in the Arctic and Antarctic as well as in the high and mid latitude oceans. The institute coordinates German polar research and makes available to international science important infrastructure, e.g. the research icebreaker "Polarstern" and research stations in the Arctic and Antarctic. AWI is one of 15 research centres within the Helmholtz Association, Germany's largest scientific organization.
Margarete Pauls | idw
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