Sea ice can be observed reliably by means of several satellites, according to scientists at the Finnish Meteorological Institute. Their first observations were made in 1979. The observations can be used to calculate the sea area covered by ice or the extent of the entire ice field. The calculation of the ice field size only considers areas where the ice concentration accounts for over 15 per cent of the total area.
Prior to 2000, the minimum extent of sea ice ranged from 6.2 to 8.0 million square kilometres. In September this year, the average ice cover is only about 3.6 million square kilometres, which is the lowest figure throughout the whole measurement history. This is already the sixth week when the extent of ice has been lower than the previous minimum value measured in 2007.
"The extent of sea ice has been decreasing throughout the measurement period, but its melting has accelerated within the last ten years. The shrinking is in line with global warming," says Jari Haapala, Head of Group at the Finnish Meteorological Institute.
On the basis of all observations, the rate of change has been -0.91 million square kilometres in ten years. According to the observations made during the last ten years, the rate of change has been -2.03 million square kilometres in a decade. This corresponds to about five times the area of the Baltic Sea.
The sea ice field is not a uniform slab melting and growing at its edges. Instead, it is a mosaic of ice floes that differ in size and thickness. In particular, the thickening of ice depends on air temperature, thermal radiation and the riding of ice. The factors affecting melting include solar radiation, air temperature and the heat flux from the sea. The ice field is transported by winds and ocean currents.
Many individual factors have affected the shrinking and thinning of sea ice, but the ultimate reason for the changes is warming of the Arctic region. Owing to warmer winter months, sea ice no longer grows as thick as before, and the ice field melts more quickly during the summer season. The increase in ice-free sea areas compounds warming because ice-free seas absorb more than 90 per cent of solar radiation. Correspondingly, snow-covered ice reflects 90 per cent of solar radiation back into space. Ice that survives the melting season becomes multi-year ice. During the past ten years, the amount of ice older than five years, in particular, has declined sharply. The thinning of ice has also made the ice field more brittle. In consequence, its drift speed has intensified and ice drifts more rapidly from the Arctic Ocean to the Northern Atlantic.
In summer 2012, the melting of sea ice was particularly rapid in August, when a strong low-pressure system formed on the Pacific side of the region. The resulting storm broke the ice field and accelerated melting. Low-pressure systems are common in the Arctic Ocean, but their impact on the melting of sea ice is now greater since the thinner sea ice is more vulnerable to breaks caused by storms.
Sea ice has shrunk more quickly than predicted
The loss of sea is a clear indication of the progress of climate change. One of the basic findings of climate models is that the Arctic will get warmer more rapidly than other regions and that the sea ice will disappear with rising concentrations of greenhouse gases in the atmosphere. The changes observed in the Arctic Ocean are in agreement with this finding, which was already made some decades ago. However, what gives cause for concern is that the sea ice has been shrinking much more rapidly than what the models have estimated.
It is expected that sea ice will decrease in the coming decades as well. "Estimates of an ice-free Arctic Ocean vary greatly. A conservative estimate is that the Arctic Ocean will be without ice every summer at the earliest in 2030-2050. The last areas of multi-year ice would be found in the northern parts of Canada and Greenland. The rest of the areas covered by multi-year ice, such as the North Pole region, may be ice-free within the next five years. In winter, ice will cover nearly areas nearly as large as at present, but sea ice will be considerably thinner than it is now," says Haapala.
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