Surface snowmelt in Antarctica in 2008, as derived from spaceborne
passive microwave observations at 19.35 gigahertz, was 40% below the
average of the period 1987–2007. The melting index (MI, a measure
of where melting occurred and for how long) in 2008 was the second-smallest
value in the 1987–2008 period, with 3,465,625 square kilometers
times days (km2 × days) against the average value of 8,407,531
km2 × days (Figure 1a). Melt extent (ME, the extent of the area
subject to melting) in 2008 set a new minimum with 297,500 square kilometers,
against an average value of approximately 861,812 square kilometers.
The 2008 updated melting index and melt extent trends over the whole
continent, as derived from a linear regression approach, are –164,487
km2 × days per year (MI) and –11,506 square kilometers per
year (ME), respectively.
Negative trends for the period 1987–2008 of the number of
melting days (Figure 1b) over the Antarctic Peninsula are observed at
a rate down to –2 days per year for internal areas and about –0.7
days per year for coastal areas. Contrarily, positive trends (up to
approximately +0.25 days per year) are observed on part of the Larsen
Ice Shelf.
(Article continues below)
In East Antarctica, positive trends are observed over the Amery,
West, Shackleton, and Voyeykov ice shelves, with values of up to +0.7
days per year for Shackleton and +0.8 days per year for Amery. Interestingly,
the latter shows negative trends (down to –0.3 days per year)
for internal areas but positive values for coastal areas.
Large-scale monitoring of ice shelves is an important task for
many reasons: Though ice shelves do not contribute directly to sea level
rise, they play an important role in keeping the warm marine air at
a distance from glaciers; and recent observations also suggest the buttressing
effect of ice shelves in preventing acceleration of ice sheets. An increasing
surface snowmelt over ice shelves might lead to persisting melt ponds,
which, in turn, might contribute to ice shelf disintegration as liquid
water fills small surface cracks. Depending on the amount of water and
the depth of a crack, the water can deepen the crack and eventually
wedge through the ice shelf. Along with surface processes, it is imperative
to focus on verifying hypotheses regarding those processes occurring
at the ice-ocean boundaries, such as, for example, the thinning of ice
shelves driven by ocean-induced melting.
A color version of Figure 1 can be viewed in the electronic supplement
to this Eos issue (http://www.agu.org/eos_elec).
This report seems to have otherwise been missed by the media (I could
not find articles in a search on google news), but it consistent with
the recent colder than average tropospheric temperatures in the Southern
Hemisphere. The finding that
“The melting index (MI, a measure of where melting occurred and
for how long) in 2008 was the second-smallest value in the 1987–2008
period…”
is quite an important observation and further reinforces what is at least
a short term cooling of the climate system.
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