By ScienceDaily, October 25, 2011
Marco Tedesco standing on the edge of one of four moulins (drainage holes) he and his team found at the bottom of a supraglacial lake during the expedition to Greenland in the summer, 2011. (Credit: P. Alexander) |
The Greenland
ice sheet can experience extreme melting even when temperatures don't hit
record highs, according to a new analysis by Dr. Marco Tedesco, assistant
professor in the Department of Earth and Atmospheric Sciences at The City
College of New York. His findings suggest that glaciers could undergo a
self-amplifying cycle of melting and warming that would be difficult to halt.
"We are finding
that even if you don't have record-breaking highs, as long as warm temperatures
persist you can get record-breaking melting because of positive feedback
mechanisms," said Professor Tedesco, who directs CCNY's Cryospheric
Processes Laboratory and also serves on CUNY Graduate Center doctoral faculty.
Professor Tedesco
and his team collected data for the analysis this past summer during a
four-week expedition to the Jakobshavn Isbræ glacier in western Greenland.
Their arrival preceded the onset of the melt season.
Combining data
gathered on the ground with microwave satellite recordings and the output from
a model of the ice sheet, he and graduate student Patrick Alexander found a
near-record loss of snow and ice this year. The extensive melting continued
even without last year's record highs.
The team recorded
data on air temperatures, wind speed, exposed ice and its movement, the
emergence of streams and lakes of melt water on the surface, and the water's
eventual draining away beneath the glacier. This lost melt water can accelerate
the ice sheet's slide toward the sea where it calves new icebergs. Eventually,
melt water reaches the ocean, contributing to the rising sea levels associated
with long-term climate change.
The model showed
that melting between June and August was well above the average for 1979 to
2010. In fact, melting in 2011 was the third most extensive since 1979, lagging
behind only 2010 and 2007. The "mass balance," or amount of snow
gained minus the snow and ice that melted away, ended up tying last year's
record values.
Temperatures and an
albedo feedback mechanism accounted for the record losses, Professor Tedesco
explained. "Albedo" describes the amount of solar energy absorbed by
the surface (e.g. snow, slush, or patches of exposed ice). A white blanket of
snow reflects much of the sun's energy and thus has a high albedo. Bare ice --
being darker and absorbing more light and energy -- has a lower albedo.
But absorbing more
energy from the sun also means that darker patches warm up faster, just like
the blacktop of a road in the summer. The more they warm, the faster they melt.
And a year that
follows one with record high temperatures can have more dark ice just below the
surface, ready to warm and melt as soon as temperatures begin to rise. This
also explains why more ice sheet melting can occur even though temperatures did
not break records.
Professor Tedesco
likens the melting process to a speeding steam locomotive. Higher temperatures
act like coal shoveled into the boiler, increasing the pace of melting. In this
scenario, "lower albedo is a downhill slope," he says. The darker
surfaces collect more heat. In this situation, even without more coal shoveled
into the boiler, as a train heads downhill, it gains speed. In other words, melting
accelerates.
Only new falling
snow puts the brakes on the process, covering the darker ice in a reflective
blanket, Professor Tedesco says. The model showed that this year's snowfall
couldn't compensate for melting in previous years. "The process never
slowed down as much as it had in the past," he explained. "The brakes
engaged only every now and again."
The team's
observations indicate that the process was not limited to the glacier they
visited; it is a large-scale effect. "It's a sign that not only do albedo
and other variables play a role in acceleration of melting, but that this
acceleration is happening in many places all over Greenland," he
cautioned. "We are currently trying to understand if this is a trend or
will become one. This will help us to improve models projecting future melting
scenarios and predict how they might evolve."
Additional
expedition team members included Christine Foreman of Montana State University,
and Ian Willis and Alison Banwell of the Scott Polar Research Institute, Cambridge,
UK.
Professor Tedesco
and his team provide their preliminary results on the Cryospheric Processes
Laboratory webpage (http://greenland2011.cryocity.org/). They will will be presenting further results at
the American Geophysical Union Society (AGU) meeting in San Francisco on
December 5 at 9 a.m. and December 6 at 11:35 a.m.
The
research was supported by the National Science Foundation and the NASA
Cryosphere Program. The World Wildlife Fund is acknowledged for supporting
fieldwork activities.
Story Source:
The above story is reprinted from materials provided by City College of New York.
1 comment:
Interesting and disturbing! Another reminder - It is constantly revealed that climate change projections are too conservative - so many effects & feedbacks we don't yet understand - These Moulins were explained in the recent documentary '6 degrees' which is worth a watch - A particularly alarming +ve feedback is the massive methane deposits on the ocean beds that will likely manifest as methane burps if the oceans warm by ...x? amount. BW Martin
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