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
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.
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.
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%.
The
April extent compared with the average
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