[Reader-list] Fwd: Greenland ice sheet melted at unprecedented rate during July

[Nagraj Adve]

http://www.guardian.co.uk/environment/2012/jul/24/greenland-ice-sheet-thaw-nasa


Greenland ice sheet melted at unprecedented rate during July

Scientists at Nasa admitted they thought satellite readings were a mistake
after images showed 97% surface melt over four days


   - Suzanne Goldenberg<http://www.guardian.co.uk/profile/suzannegoldenberg> US
   environment correspondent
   - guardian.co.uk <http://www.guardian.co.uk/>, Tuesday 24 July 2012
   22.48 BST

   Photograph: Nasa

The Greenland <http://www.guardian.co.uk/world/greenland> ice sheet melted
at a faster rate this month than at any other time in recorded history,
with virtually the entire ice sheet showing signs of thaw.

The rapid melting over just four days was captured by three satellites. It
has stunned and alarmed scientists, and deepened fears about the pace and
future consequences of climate
change<http://www.guardian.co.uk/environment/climate-change>
.

In a statement posted on Nasa <http://www.guardian.co.uk/science/nasa>'s
website on Tuesday, scientists admitted the satellite data was so
striking<http://www.nasa.gov/topics/earth/features/greenland-melt.html>
they
thought at first there had to be a mistake.

"This was so extraordinary that at first I questioned the result: was this
real or was it due to a data error?" Son Nghiem of Nasa's jet propulsion
laboratory in Pasadena said in the release.

He consulted with several colleagues, who confirmed his findings. Dorothy
Hall, who studies the surface temperature of Greenland at Nasa's space
flight centre in Greenbelt, Maryland, confirmed that the area experienced
unusually high temperatures in mid-July, and that there was widespread
melting over the surface of the ice sheet.

Climatologists Thomas Mote, at the University of Georgia, and Marco
Tedesco, of the City University of New York, also confirmed the melt
recorded by the satellites.

However, scientists were still coming to grips with the shocking images on
Tuesday. "I think it's fair to say that this is unprecedented," Jay Zwally,
a glaciologist at Nasa's Goddard Space Flight Center, told the Guardian.

The set of images released by Nasa on Tuesday show a rapid thaw between 8
July and 12 July. Within that four-day period, measurements from three
satellites showed a swift expansion of the area of melting ice, from about
40% of the ice sheet surface to 97%.

Zwally, who has made almost yearly trips to the Greenland ice sheet for
more than three decades, said he had never seen such a rapid melt.

About half of Greenland's surface ice sheet melts during a typical summer,
but Zwally said he and other scientists had been recording an acceleration
of that melting process over the last few decades. This year his team had
to rebuild their camp, at Swiss Station, when the snow and ice supports
melted.

He said he was most surprised to see indications in the images of melting
even around the area of Summit Station, which is about two miles above sea
level.

It was the second unusual event in Greenland in a matter of days, after an
iceberg the size of Manhattan broke off from the Petermann Glacier. But the
rapid melt was viewed as more serious.

"If you look at the 8 July image that might be the maximum extent of
warming you would see in the summer," Zwally noted. "There have been
periods when melting might have occurred at higher elevations briefly –
maybe for a day or so – but to have it cover the whole of Greenland like
this is unknown, certainly in the time of satellite records."

Lora Koenig, another Goddard glaciologist, told Nasa similar rapid melting
occurs about every 150 years. But she warned there were wide-ranging
potential implications from this year's thaw.

"If we continue to observe melting events like this in upcoming years, it
will be worrisome." she told Nasa.

The most immediate consequences are sea level rise and a further warming of
the Arctic. In the centre of Greenland, the ice remains up to 3,000 metres
deep. On the edges, however, the ice is much, much thinner and has been
melting into the sea.

The melting ice sheet is a significant factor in sea level rise. Scientists
attribute about one-fifth of the annual sea level rise, which is about 3mm
every year, to the melting of the Greenland ice sheet.

In this instance of this month's extreme melting, Mote said there was
evidence of a heat dome over Greenland: or an unusually strong ridge of
warm air.

The dome is believed to have moved over Greenland on 8 July, lingering
until 16 July.
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