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Carbon Brief Staff

Carbon Brief Staff

20.09.2012 | 12:00pm
ScienceThinning ice probably explains record low Arctic sea ice coverage
SCIENCE | September 20. 2012. 12:00
Thinning ice probably explains record low Arctic sea ice coverage

Scientists from the US National Snow and Ice Data Center (NSIDC) have announced that Arctic sea ice has reached its minimum extent for the year – 3.41 million square kilometres. The minimum extent marks the lowest summer extent since the satellite record began in 1979, and continues the decline in Arctic sea ice coverage over the last four or five decades.

Here’s a video showing how this year’s melt season has looked, compared to the average minimum extent over the past 30 years:

 

The NSIDC scientists do stress that the declaration of the seasonal low is preliminary, saying:

“Changing winds could still push ice floes together, reducing ice extent further.”

The team will provide a more thorough analysis in early October, once they have collected data for the whole of September.

How does this compare to previous years?

The last six years (2007 to 2012) have seen the six lowest summer minimum ice extents in the satellite record:

NSIDC previous summer minima

Details of summer minima from the last six years. Source: NSIDC

The previous record low, reached in 2007, was significantly below anything seen before in the satellite record, jarring with the expectations of scientists who had predicted a more gradual decline, and raising questions about whether a new phase of rapid ice collapse had begun. The NSIDC team says:

“This year’s minimum was 760,000 square kilometers (293,000 square miles) below the previous record minimum extent in the satellite record, which occurred on September 18, 2007.  This is an area about the size of the state of Texas. The September 2012 minimum was in turn 3.29 million square kilometers (1.27 million square miles) below the 1979 to 2000 average minimum, representing an area nearly twice the size of the state of Alaska. This year’s minimum is 18% below 2007 and 49% below the 1979 to 2000 average.”

Here’s how this year’s melt season looks on a graph:

NSIDC sea ice extent graph 190912

Why is ice extent so low this year?

Scientists think that five years ago, the unprecedented very low level of seasonal sea ice in 2007 was down to a combination of factors, including rough weather conditions which acted to break up the ice more swiftly than normal. NSIDC scientists say that the conditions this year “were not as extreme”, and that although temperatures were warmer than average, it was still cooler than 2007.

It is likely that a powerful storm in early August helped break up the sea ice, but the NSIDC team think that the storm merely “sped up the loss of the thin ice that appears to have been already on the verge of melting completely.” In other words, the ice continues to get thinner.

Claire Parkinson, a climate scientist at NASA Goddard Space Flight Center, agrees, saying:

“The storm definitely seems to have played a role in this year’s unusually large retreat of the ice. But that exact same storm, had it occurred decades ago when the ice was thicker and more extensive, likely wouldn’t have had as prominent an impact, because the ice wasn’t as vulnerable then as it is now.” 

This video shows the impact of the storm on the Arctic sea ice:

What about sea ice volume?

Sea ice extent has undergone a dramatic decline this year, but it doesn’t necessarily follow that the volume of sea ice has declined dramatically. Sea ice volume is more difficult to measure, but scientists at the Polar Science Center study it by combining computer modelling and measurements of sea ice coverage from microwave satellite data, producing an assessment called PIOMAS (Pan-Arctic Ice Ocean Modeling and Assimilation System).

Axel Schweiger, principal oceanographer at the Polar Science Center told us:

“With respect to ice volume from PIOMAS for this summer (and we are not finished), the change from 2011 to 2012 looks actually much less dramatic than the ice extent change and rather in line with previous year to year losses.”

If the amount of older, thicker sea ice continues to shrink – as it is projected to – Arctic storms such as this year’s are likely to have a big impact on summer ice loss in years to come. Back in October last year, University of Bremen scientists argued that the unexpected 2007 melt “was not a one off” – now it seems they have been proved right. 


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