National Snow & Ice Data Center - Arctic Melt Season May Have Finally Turned

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Figure 1: Figure 1: Sea ice extent for September 16, 2007, compared to previous record



Sea ice conditions at the annual minimum on September 16, 2007

Figure 1 compares the five-day mean sea ice extent on September 16, 2007, this year’s apparent minimum, with minimum sea ice extent on September 20–21, 2005, the previous record low. The magenta line indicates the mean September extent based on data from 1979 to 2000. The apparent five-day 2007 minimum was 4.13 million square kilometers (1.59 million square miles), compared to 5.32 million square kilometers (2.05 million square miles) in 2005.

The long-term average minimum, based on averaging data from 1979 to 2000, is 6.74 million square kilometers (2.60 million square miles) and occurs on September 12. Compared to this average, five-day mean ice extent for September 16, 2007, was lower by 2.61 million square kilometers (one million square miles), an area approximately equal to the size of Alaska and Texas combined, or the size of ten United Kingdoms.

The minimum for 2007 shatters the previous five-day minimum set on September 20–21, 2005, by 1.19 million square kilometers (460,000 square miles), roughly the size of Texas and California combined, or nearly five United Kingdoms.

Figure 2: Summer melt season Arctic sea ice extent



2007 sea ice minimum in context

Figure 2 is the updated time series of daily sea ice extent for 2007, which can be compared to the time series for 2005 and to the 1979 to 2000 average. The lowest point on each year’s time series indicates the annual absolute minimum. With the onset of autumn and lower temperatures each year, sea ice extent begins to increase. As winter progresses, the sea ice “recovers,” usually reaching its maximum in March before the melting cycle begins again.

A word of caution on calling the minimum

Determining with certainty when the minimum has occurred is difficult until the melt season has decisively ended. For example, in 2005, the time series began to level out in early September, prompting speculation that we had reached the minimum. However, as Figure 2 illustrates, the sea ice contracted later in the season, again reducing sea ice extent and causing a further drop in the absolute minimum.

We mention this now because the natural variability of the climate system has frequently been known to trick human efforts at forecasting the future. It is still possible that ice extent could fall again, slightly, because of either further melting or a contraction in the area of the pack due to the motion of the ice. However, we have now seen five days of little change or slight gains in extent, so that seems unlikely at this time. Due to the variability of sea ice at this time of year, the National Snow and Ice Data Center determines the minimum using a five-day running mean value.

At the end of the melt season, in the first few days of October, we will be issuing a full analysis of the 2007 melt season, including final numbers and a discussion of the causes and effects of this year’s stunning record low. At that time, we will also know what the monthly September sea ice extent was in 2007—the measure scientists often rely on for the most accurate analysis.

Figure 3: September 9, 2007, sea ice extent compared to animation of Septembers 1979 to 2006



Comparing this year’s minimum to previous years

Figure 3 tells the story of this year’s sea ice eloquently. On the right is recent sea ice extent (on September 9, 2007, one of the lowest days of the melt season). On the left is an animation that shows a slide show of September sea ice extents, beginning with 1979 and ending with 2006. The animation shows two aspects of sea ice change: First, it shows that as the years unfold, the September sea ice is gradually dwindling. Second, it shows that even compared to the very lowest years on record, 2007 sea ice has visibly declined.

A final note: The Northwest Passage is still open but is starting to refreeze. The Northeast Passage (the Northern Sea Route) along the coast of Siberia, is still closed by a narrow band of sea ice.

EDIT/END

http://nsidc.org/news/press/2007_seaiceminimum/20070810_index.html