The 2009 Arctic sea ice extent is the third lowest on record. The downward spiral is reinforced by thick multi-year sea ice being replaced by younger and thinner ice.

The Arctic sea ice extent is in a downward spiral. Photo: Ragnvald Nærø

The Arctic sea ice extent is in a downward spiral. Photo: Ragnvald Nærø

The Arctic sea ice cover in 2007 was the lowest ever recorded. In 2008, the sea ice extent was slightly bigger than 2007, but still dramatically low.  The 2009 data shows that the sea ice extent is bigger than the two previous years. However, this does not mean that the Arctic sea ice is recovering, rather the opposite.

“While 2007 was an extreme situation, this year’s ice extent is consistent with the downward trend we have observed in the Arctic over the previous decades”, said Cecilie Mauritzen, project co-leader in DAMOCLES.

Towards an ice free summer-Arctic

Dwindling summer time sea-ice extent is a prime indicator of climate change at high latitudes, which over the past decades, have warmed faster than most other parts of the planet. As ice reflects most of the sunlight that falls on the surface while dark ocean water absorbs this energy, reduced ice cover further accelerates the warming trend.

Figure 1. Daily Arctic sea ice extent on September 12 was 5.10 million square kilometers (1.97 million square miles). The orange line shows the 1979 to 2000 median extent for that day. The black cross indicates the geographic North Pole. Credit: National Snow and Ice Data Center

Figure 1. Daily Arctic sea ice extent on September 12 was 5.10 million square kilometers (1.97 million square miles). The orange line shows the 1979 to 2000 median extent for that day. The black cross indicates the geographic North Pole. Credit: National Snow and Ice Data Center

“We're entering a new period of sea-ice melt in the Arctic Ocean due to climate change”, said Peter Wadhams, an oceanographer at the University of Cambridge, UK, who is conducting research in the Fram Strait off Greenland.

“In five years' time, most of the sea ice could be gone in summer with just an 'Alamo of ice' remaining north of Ellesmere Island”, Wadhams said to the journal Nature.

Cecilie Mauritzen is more modest and refers to the IPCC:

“It will probably take more than five years before we see an ice free summer-Arctic, but there is little scientific doubt that the Arctic is entering a new state this century”, said Mauritzen.

More than one million square kilometres below the average minimum

The Arctic sea ice extent reaches its annual minimum in mid-September. On September 12, 2009 sea ice extent dropped to 5.10 million square kilometres.

The 2009 minimum is the third-lowest recorded since 1979, 580,000 square kilometres above 2008 and 970,000 square kilometres above the record low in 2007.

“Even though it may seem that the sea ice has recovered substantially in 2009, it is important to bear in mind that the 2009 sea ice extent is 1.28 million square kilometres below the thirty-year 1979 to 2008 average minimum”, said Cecilie Mauritzen.

Figure 2. The graph above shows daily sea ice extent as of September 15, 2009. The solid light blue line indicates 2009; dark blue shows 2008, dashed green indicates 2007; light green shows 2005, and solid gray indicates average extent from 1979 to 2000. The gray area around the average line shows the two standard deviation range of the data. Credit:
National Snow and Ice Data Center

Figure 2. The graph above shows daily sea ice extent as of September 15, 2009. The solid light blue line indicates 2009; dark blue shows 2008, dashed green indicates 2007; light green shows 2005, and solid gray indicates average extent from 1979 to 2000. The gray area around the average line shows the two standard deviation range of the data. Credit: National Snow and Ice Data Center

Annual weather conditions determine

Local weather conditions to a large extent determine the annual variations in the sea ice melting. The typical combination is a high-pressure system over the Beaufort Sea and low pressure over eastern Siberia, which carries a lot of warm air across the Siberian Sea and pushes the ice edge towards the pole.

This year, the minimum extent did not fall as low as the minimums of the last two years, because temperatures through the summer were relatively cooler due to cloudy skies, according to The National Snow and Ice Data Center (NSIDC). The Chukchi and Beaufort seas were especially cool compared to 2007. Winds also tended to disperse the ice pack over a larger region.

“Both these factors contributed to the fact that we did not see a new record low in 2009”, said Mauritzen.

An important and uncertain piece: ice thickness

Arctic sea ice has become thinner over the last decades. Thick multi-year sea ice is being replaced by thinner, younger ice which melts at lower temperatures. This process reinforces the downward spiral, and makes it difficult for the multi-year sea ice to recover.

 

Ice thickness is an important part of the puzzle. Scientists use satellites to measure ice age, a proxy for ice thickness. This year, younger (less than one year old), thinner ice, which is more vulnerable to melt, accounted for 49 percent of the ice cover at the end of summer. Second-year ice made up 32 percent, compared to 21 percent in 2007 and 9 percent in 2008 (Figure 5). Only 19 percent of the ice cover was over 2 years old, the least in the satellite record and far below the 1981-2000 average of 52 percent. Earlier this summer, NASA researcher Ron Kwok and colleagues from the University of Washington in Seattle published satellite data showing that ice thickness declined by 0.68 meters between 2004 and 2008.

NSIDC Scientist Walt Meier said, “We've preserved a fair amount of first-year ice and second-year ice after this summer compared to the past couple of years. If this ice remains in the Arctic through the winter, it will thicken, which gives some hope of stabilizing the ice cover over the next few years. However, the ice is still much younger and thinner than it was in the 1980s, leaving it vulnerable to melt during the summer.”

 

Sep 24, 2009
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