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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:

NASA Greenland surface ice melt

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.

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