Environmental Engineering Reference
In-Depth Information
day (12 July), satellite observations showed that 97% of the ice sheet surface was in a melt-
ing condition. For the technically inclined, the ELA was 3.7 times the standard deviation
above the 21-year mean ELA value. Ice cores from the ice sheet suggest that similar melt-
ing events have occurred in the past, with a frequency of about once in 150 years. The last
was in 1889, so the big question is: “Will this frequency change?”
I suspect it will. The entire oceanographic and atmospheric environment around
Greenland seems to have been changing in a way that brings warmer air over the ice sheet
and warmer water under the floating outfall glaciers (that is, the floating portions of glaci-
ers that reach and extend into the sea). The atmospheric conditions that helped promote the
record2012surfacemeltoftheGreenlandicesheetwasaprolongednegativeNorthAtlant-
ic Oscillation (NAO) index. The North Atlantic Oscillation is the name given to variations
in the relative strengths and locations of the Azores High and the Icelandic Low areas of
atmospheric pressure. A large difference in pressure between the two systems (known as
an index of NAO
+
) leads to increased moist westerly winds, giving cool summers and mild
wet winters in Europe. In the later section on “Climate Change Impacts on Arctic Ecosys-
tems”, we will see how a NAO
+
condition can lead to feeding difficulties for reindeer in
winter in Fennoscandia.
When the pressure difference is low (NAO
−
), the moist westerly winds weaken, res-
ulting in cold winters in Europe. In spring 2012, a negative NAO index resulted in lower
snow cover over much of the Arctic, including Greenland. This reduced the surface albedo
of the surface of glaciers and the Greenland ice sheet. In summer, the area was again under
the influence of a negative NAO index for the entire season - a situation perhaps encour-
aged by the general lack of surrounding sea ice. Consequently, sea level atmospheric pres-
sure was unusually high over the ice sheet, causing warm air to be swept from the South
over western Greenland, as has happened every year since 2007. This helped create the
perfect conditions for the record-breaking summer temperatures over the ice sheet.
The fate and activity of glacial meltwater within the ice sheet and its exit glaciers
is an interesting question. A portion of the melt is frozen in pore spaces within the per-
colation zone (immediately below the snow) where firn (compacted snow that has been
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