Greenland's
ice melt accelerating as surface darkens, raising sea levels
Winnowing
away of the ice, exacerbated by soot blown on to the ice from
wildfires, means Greenland’s ice sheet is stuck in a ‘feedback
loop’
Oliver Milman
Thursday 3 March
2016 18.35 GMT
Greenland’s vast
ice sheet is in the grip of a dramatic “feedback loop” where the
surface has been getting darker and less reflective of the sun,
helping accelerate the melting of ice and fuelling sea level rises,
new research has found.
The snowy surface of
Greenland started becoming significantly less reflective of solar
radiation from around 1996, the analysis found, with the ice
absorbing 2% more solar energy per decade from this point. At the
same time, summer near-surface temperatures in Greenland have
increased at a rate of around 0.74C per decade, causing the ice to
melt.
This winnowing away
of the ice, exacerbated by soot blown on to the ice from wildfires,
means that Greenland’s ice is stuck in what is known as a “feedback
loop” that will make it ever more vulnerable to warming global
temperatures. The study predicts that the ice surface reflectivity,
or albedo, will drop by 10% or more by the end of the century, which
will trigger further melting.
“It’s melting
cannibalism, basically – it’s melting that’s feeding itself,”
said lead author Marco Tedesco, of Columbia University’s
Lamont-Doherty Earth Observatory. “Rising temperatures are
promoting more melting, and that melting is reducing albedo, which in
turn is increasing melting.
It’s
worrying because if the ice sheet continues to get darker, it becomes
more sensitive to atmospheric warming
Marco Tedesco
“It’s worrying
because if the ice sheet continues to get darker, it becomes more
sensitive to atmospheric warming. The impact of two weeks of sunshine
with no clouds, for example, is far greater than it was 20 years ago.
The ice is going to melt much more quickly, with more water flowing
off on to the sea.”
In recent years,
scientists have began to pick apart the complex, interrelated forces
at play in the Arctic, which has experienced a 13.4% drop in minimum
ice extent per decade, on average, since the 1980s. More than half of
the Greenland ice sheet melted last summer, the largest annual melt
since 2012 and well beyond the average melting seen over the past 35
years.
Tedesco’s research
shows that as the surface of Greenland’s ice melts, old impurities,
such as dust from erosion or soot that has been entombed for years,
start to appear, darkening the surface.
If the summer is
warm enough to remove all the snow, these dark impurities begin to
spread across the surface, providing a far more heat-absorbent
environment. At the same time, as this snow melts and then refreezes,
the grains of snow get larger. These larger grains, invisible to the
eye but detected by satellite’s infrared instruments, also create a
less reflective surface.
These two processes
are turning Greenland into a store, rather than a reflector, of solar
energy, with consequences far beyond the icy wilderness. Water from
the melting flows into the sea, contributing to rising oceans around
the world. This process is unlikely to reverse given the increasing
concentrations of greenhouse gases in the atmosphere.
“As warming
continues, the feedback from declining albedo will add up,” Tedesco
said. “It’s a train running downhill, and the hill is getting
steeper.”
The research,
published in the European Geosciences Union journal The Cryosphere,
looked at satellite data from 1981 to 2012. The drop in reflectivity
from 1996 was probably due to a change in atmospheric circulation
that favoured warmer, moist air from the south. The scientists found
there was no significant increase in soot from forest fires since
1997 to explain the darkening of the surface.
This article was
amended on 3 March 2016. An earlier version referred to the European
Geophysical Union, instead of the European Geosciences Union.
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