Greenland 97 per cent surface melting - what does it mean?
- 25 Jul 2012, 15:00
- Verity Payne
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
widespread
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
the Arctic..."
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 (who tweets in a personal capacity), seems to suggest
that it's a close run thing though, telling
twitter followers:
"[I]nterestingly, 2002 had almost as
wide a melt extent (briefly) but 2007, 2010 and poss [sic] 2011
bigger in total melt terms"
The summer surface melting of 2002 was indeed more
extensive than normal, although not as widespread
as this year's, and lasted for just a few days. Scientists
suggest that 2002's extensive surface thaw was
probably down to a similar 'heat dome' to that contributing to this
year's unusual thawing.
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
time"
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
before.
Should we be concerned?
Climate scientist Edward Hanna, University of Sheffield,
explains in
the Guardian that Greenland's summers have been getting warmer
over the last 20 years, and:
"The last six summers since have seen
successive record warmth and surface melt and runoff of that
meltwater signalling increased mass loss from the ice sheet [...]
Although we cannot yet reliably predict how the ice sheet will
respond to ongoing global warming the general prognosis is not
good: more warmth clearly means more melting."
Scientists are keen to work out how much ice the Greenland ice
sheet loses because, unlike when Arctic sea ice melts, ice sheet
meltwater adds to global sea level rise. But this year's widespread
summer melt doesn't necessarily mean that the amount of total ice
melt from the ice sheet this year will be unusually large - we'll
have to wait until the melt season is over before scientists can
work out if this year's total ice melt is affected.
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."
Updated 11:05 27/07/12 to reflect comments from Dr Ruth
Mottram and include quote from Dr Edward Hanna.