Seemorerocks: Arctic sea ice

Sunday 10 May 2015

Arctic sea ice

April sea-ice extent near record low

As the melt season took hold last month, the sea-ice extent hit its second-lowest level ever. Meanwhile, the amount of thick, old ice also continues to decline

The April extent compared with the average. See full image below (Image: NSIDC

Arctic Journal,

7 May, 2015

Sea-ice extent in April was the second-lowest level for the month since satellite measurements began in 1979, according to monthly data from the National Snow and Ice Data Center, a Colorado-based research institute.

Since 1979, April sea-ice extent has declined 2.4% per decade.

The figure continues a streak of near-record low monthly extents in 2015. (See graph below.) In March the sea-ice extent hit a record low for the month.

Sea-ice extent for April 2015 averaged 14.0 million square km, a loss of 862,000 square km on the previous month. The figure places the April extent some 810,000 square km below the 1981 to 2010 long-term average of 15.0 million square kilometres.

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The record for the month, set in 2007, was 80,000 square km less than the April 2015 extent.

April typically marks the start of the melting season. Two indicators of how much ice can be expected to melt are the thickness and age of existing ice, and the NSIDC says it will begin offering data on both starting this month.

While this month’s data is insufficient to give an indication of how the 2015 melt season will develop, the spring ice thickness does plays an important role in how robust ice cover is as melting begins, explains Rasmus T Tonboe, of DMI, the Danish Met.

Summer weather, he says, has a larger impact on determining the September minimum than it did in the past, when the ice was thicker and more robust.

SEE RELATED: Annual sea-ice maxiumum comes early, sets record low

The average annual ice thickness over the central Arctic Ocean, according to the findings of a recent paper published in The Cryosphere and cited in the NSIDC data, has declined 65% over the past four decades. Average thickness fell from 3.59 metres in 1978 to just 1.25 meters in 2012.

The amount of multi-year ice, which is more resilient to melting, has also declined since the 2014 summer minimum, the NSIDC said. The amount of second-year ice fell by more than 30%, while ice older than three years declined by 10%.