By Verity Payne
25 Jul 2012 - Over several days at the
start of July this year nearly the whole surface of the Greenland
ice sheet thawed, according to new NASA satellite data.
Typically around half of the ice sheet's surface melts during
summer, so scientists were surprised when by July 12th this year an
estimated 97 percent of the ice sheet surface had thawed.
Extent of surface melt over
Greenland's ice sheet on July 8 (left) and July 12 (right).
In the image, the areas classified as "probable melt" (light
pink) correspond to those sites where at least one satellite
detected surface melting. The areas classified as "melt" (dark
pink) correspond to sites where two or three satellites detected
surface melting. Credit: Nicolo E. DiGirolamo, SSAI/NASA GSFC, and
Jesse Allen, NASA Earth Observatory.
It's worth pointing out that this satellite data only indicates
widespread thawing at the surface of the ice - the Greenland ice
sheet reaches a depth of around three kilometres at its thickest
point. Also, much of the melt water over the thickest parts of the
ice sheet refreezes quickly, so it's only near the coast that some
of this meltwater is lost to the ocean.
The news has received
media coverage, although some media outlets are not
too sure whether the unexpected surface melting is linked to recent
climate change. The Washington Post, for example, says:
"The unusual amount of melt [...] has
highlighted the extent to which warming temperatures are affecting
But rather confusingly continues:
"Researchers said it is too early to
connect the new readings with broader climate change."
So - is the melting down to climate change?
The NASA scientists who collected the data suggest the
unexpected surface melting might be down to recent weather
patterns, noting that the unusual melting happened when a high
pressure ridge, or 'heat dome' was over Greenland.
Interestingly this Greenland high pressure ridge may be linked
to the UK's decidedly inclement weather at the start of July, as BBC Weather explains:
"An unusually strong area of high
pressure over Greenland has been pushing a northwesterly jet stream
over the British Isles, [...] bringing us the wettest April and
June on record and much wetter weather than normal in July,
particularly in the southwest."
Scientists from the US Snow and Ice Data Center (NSIDC) think
that this year's northern hemisphere weather patterns also contributed to above-average temperature
in parts of the region and low Arctic sea ice extent this year.
But are these weather patterns down to climate change? It has
been suggested that the decline in Arctic sea ice associated with
man-made climate change may be linked to northern hemisphere
weather patterns in winter, and that earlier melt of snow on Arctic and sub-Arctic
land in the spring may be affecting summer weather patterns.
It's early days for these theories, and other natural
influences, such as solar activity, may also be playing a role. As
is often the case with linking particular events and wider change
in the climate system, the jury is still out.
Is the melting unprecedented?
The NASA press release is headlined 'Satellites See
Unprecedented Greenland Ice Sheet Surface Melt', and says:
"For several days this month,
Greenland's surface ice cover melted over a larger area than at any
time in more than 30 years of satellite observations"
Jay Zwally, a glaciologist at Nasa's Goddard Space Flight
Center, told the Guardian "I think it's fair to say that this
[melting] is unprecedented."
Ruth Mottram, glaciologist at the Danish Meteorological
Institute, seems to suggest that it's a close run thing though, tweeting:
"[I]nterestingly, 2002 had almost as
wide a melt extent (briefly) but 2007, 2010 and poss [sic] 2011
bigger in total melt terms"
The satellite record only goes back for three decades - beyond
that information about how the ice sheet has behaved comes from ice
cores. Lora Koenig, glaciologist at Nasa's Goddard Space Flight
Center, suggests the melting may not be unprecedented
if you consier the ice core record:
"Ice cores from Summit show that melting
events of this type occur about once every 150 years on average.
With the last one happening in 1889, this event is right on
So the description of 'unprecedented' needs to be qualified - on
the latest satellite measurements it's certainly the case, but
longer-term records suggest this kind of melt may have happened
Should we be concerned?
Recent research suggests that surface melting can
affect how much sunlight the ice sheet can reflect, so the sort of
widespread surface thawing that we've seen this year could
potentially lead to further warming in the region.
But, while the melting could have some impacts on future
warming, it remains to be seen whether this year's melting is a one
off or the start of a trend. As Koenig explains:
"[I]f we continue to observe melting
events like this in upcoming years, it will be worrisome."
This article originally appeared in
the Carbon Brief and is reprinted here with
the kind permission of the publisher.