Alister Doyle, Reuters 12 Nov 09;
OSLO (Reuters) - Greenland's ice losses are accelerating and nudging up sea levels, according to a study showing that icebergs breaking away and meltwater runoff are equally to blame for the shrinking ice sheet.
The report, using computer models to confirm satellite readings, indicated that ice losses quickened in 2006-08 to the equivalent of 0.75 mm (0.03 inch) of world sea level rise per year from an average 0.46 mm a year for 2000-08.
"Mass loss has accelerated," said Michiel van den Broeke, of Utrecht University in the Netherlands, who led the study, in Friday's edition of the journal Science.
"The years 2006-08, with their warm summers, have seen a huge melting," he told Reuters of the study with colleagues in the United States, the Netherlands and Britain.
"The underlying causes suggest this trend is likely to continue in the near future," Jonathan Bamber, a co-author at the University of Bristol, said in a statement.
The computer models matched satellite data for ice losses -- raising confidence in the findings -- and showed that losses were due equally to meltwater, caused by rising temperatures, and icebergs breaking off from glaciers.
"This helps us to understand the processes that affect Greenland. This will also help us predict what will happen," van den Broeke said. Until now, the relative roles of snowfall, icebergs and thawing ice have been poorly understood.
Greenland locks up enough ice to raise world sea levels by 7 meters (23 ft) if it ever all thawed. At the other end of the globe, far-colder Antarctica contains ice equivalent to 58 meters of sea level rise, according to U.N. estimates.
COPENHAGEN
About 190 governments will meet in Copenhagen from December 7-18 to try to agree a U.N. pact to slow global warming, fearing that rising temperatures will bring more powerful storms, heatwaves, mudslides and species extinctions as well as rising sea levels.
The study said losses of ice from Greenland would have been roughly double recent rates but were masked by more snowfall and a re-freezing of some meltwater before it reached the sea.
In total, Greenland lost about 1,500 billion tons of ice from 2000-08, split between icebergs cracking into the sea from glaciers and water runoff. "The mass loss would have been twice as great," without offsetting effects, Van den Broeke said.
The U.N.'s Intergovernmental Panel on Climate Change estimated in 2007 that world sea levels could rise by 18-59 cms by 2100. A natural expansion of water as it warms would account for most of the rise, rather than melting ice.
Greenland's current rate, of 0.75 mm a year, would be 7.5 cms if continued for 100 years. "This is...much more that previous estimates of the Greenland contribution," van den Broeke said.
Greenland ice loss 'accelerating'
Richard Black, BBC News 12 Nov 09;
The Greenland ice sheet is losing its mass faster than in previous years and making an increasing contribution to sea level rise, a study has confirmed.
Published in the journal Science, it has also given scientists a clearer view of why the sheet is shrinking.
The team used weather data, satellite readings and models of ice sheet behaviour to analyse the annual loss of 273 thousand million tonnes of ice.
Melting of the entire sheet would raise sea levels globally by about 7m (20ft).
For the period 2000-2008, melting Greenland ice raised sea levels by an average of about 0.46mm per year.
Since 2006, that has increased to 0.75mm per year.
"Since 2000, there's clearly been an accelerating loss of mass [from the ice sheet]," said lead researcher Michiel van den Broeke from Utrecht University in the Netherlands.
"But we've had three very warm summers, and that's enhanced the melt considerably.
"If this is going to continue, I cannot tell - but we do of course expect the climate to become warmer in the future."
In total, sea levels are rising by about 3mm per year, principally because seawater is expanding as it warms.
Sea change
Changes to the Greenland sheet and its much larger counterpart in Antarctica are subjects commanding a lot of interest within the scientific community because of the potential they have to raise sea levels to an extent that would flood many of the world's major cities.
The 2007 Intergovernmental Panel on Climate Change (IPCC) report projected a sea level rise of 28-43cm during this century.
But it acknowledged this was almost certainly an underestimate because understanding of how ice behaves was not good enough to make reliable projections.
By combining different sources of data in the way it has, and by quantifying the causes of mass loss, the new study has taken a big step forwards, according to Roger Barry, director of the World Data Center for Glaciology at the University of Colorado in Boulder, US.
"I think it's a very significant paper; the results in it are certainly very significant and new," he said.
"It does show that the [ice loss] trend has accelerated, and the reported contribution to sea level rise also shows a significant acceleration - so if you multiply these numbers up it puts us well beyond the IPCC estimates for 2100."
Professor Barry was an editor on the section of the IPCC report dealing with the polar regions.
On reflection
An ice sheet can lose mass because of increased melting on the surface, because glaciers flow more quickly into the ocean, or because there is less precipitation in the winter so less bulk is added inland.
The new research shows that in Greenland, about half the loss comes from faster flow to the oceans, and the other half from changes on the ice sheet itself - principally surface melting.
Another analysis of satellite data, published in September, showed that of 111 fast-moving Greenland glaciers studied, 81 were thinning at twice the rate of the slow-moving ice beside them.
This indicates that the glaciers are accelerating and taking more ice into the surrounding sea.
Melting on the ice sheet's surface acts as a feedback mechanism, Dr van den Broeke explained, because the liquid water absorbs more and reflects less of the incoming solar radiation - resulting in a heating of the ice.
"Over the last 10 years, it's quite simple; warming over Greenland has caused the melting to increase, and that's set off this albedo feedback process," he told BBC News.
"Quite likely the oceans have also warmed, and it's likely that explains the [acceleration of] outlet glaciers because they're warmed from below."
Data provided over just the last few years by the Grace satellite mission - used in this study - is giving researchers a closer view of regional variations across the territory.
Grace's twin satellites map gravity at the Earth's surface in unprecedented detail; and it is now possible to tease out from the data that most of the mass is being lost in the southeast, southwest and northwest at low elevations where the air will generally be warmer than at high altitudes.
Professor Barry cautioned that the Grace mission, which has produced valuable data about Antarctica as well as Greenland, has only a further two years to run, and that no replacement is currently scheduled.
