A guest post from climate scientists Dr Ruth
Mottram and Dr Peter
Langen from the Danish Meteorological
Dramatic decline in
Arctic sea ice has become an eye-catching barometer of our
warming climate. But the Greenland ice sheet, which is often
confused with sea ice, has exhibited no less dramatic - but less
talked about - changes in the past few years.
Stretching out over 1.7m
square kilometres, the Greenland ice sheet is the second
largest land ice mass on the planet, after Antarctica. It's an area
of solid ice and snow about the size of Libya.
The end of August heralds the completion of the summer
melt season for another year and is an opportune time for an annual
checkup. So is the Greenland ice sheet in rude health, or looking
under the weather?
Give and take
An ice sheet is a vast, slow-moving mass of fresh
water. It differs from a glacier or an ice cap because it isn't
confined by the land it sits on. But, like all glaciers, it gains
ice by the accumulation of snowfall in the winter and loses it from
surface melting during summer and the 'calving' of icebergs that
break off into the sea at its edges.
The melting and calving of ice is collectively called
"ablation". If more snow accumulates than is ablated away, the ice
sheet will grow and advance. Conversely, if more ice melts and
calves than is replenished, it will retreat and shrink.
It's worth noting that melting at the surface doesn't
necessarily mean that an ice sheet will lose mass. The meltwater
percolates through to the lower layers of the sheet and often
refreezes. But when snow and ice melts, it gives off heat, which
warms up its surroundings making subsequent melting more
Surface melting also affects how much of the Sun's
energy the ice sheet reflects - known as the albedo effect.
Bright white fresh snow reflects sunlight more efficiently than the
much darker bare glacier ice underneath. So, if the snow melts, the
ice sheet surface absorbs more energy and warms up more
Melt can occur at surprisingly high elevations on
Greenland ice sheet. For example, observation stations regularly
record melting at an altitude of 1,800m near Kangerlussuaq in
western Greenland - even during cool summers.
In 2012, which was a record year for melting on
Greenland, about 97% of the ice sheet was observed to be melting at
some point - even at the summit at an altitude of 3,200m.
Aerial shot of eastern Greenland. Credit:
Comparing the total snowfall and surface melt against
each other gives you the "surface mass balance". At the Danish
Meteorological Institute (DMI), we combine data from observation
stations on Greenland with output from a numerical weather
forecasting model to estimate the surface mass balance for the
whole ice sheet for every single