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Greenland's `Niagara' Helps Speed Up Ice-Sheet Flow, Study Says
By Alex Morales
<!-- WARNING: #foreach: $wnstory.ATTS: null at /bb/data/web/templates/webmacro_en/washingtonstory.wm:266.2 --> <!-- WARNING: #foreach: $wnstory.ATTS: null at /bb/data/web/templates/webmacro_en/washingtonstory.wm:280.19 --> April 17 (Bloomberg) -- Water from melting in Greenland can bore through a kilometer (0.6 mile) of ice, rapidly draining lakes and lubricating the ice sheet's path toward the sea, U.S.- based researchers found.
A 5.6 square-kilometer (2.2 square-mile) lake on the surface of the island's ice sheet, which took a month to fill with melt- water, drained in fewer than 18 hours, with peak flow exceeding that of Niagara Falls, the glaciologists at the University of Washington in Seattle and the Woods Hole Oceanographic Institution in Massachusetts said today in the journal Science.
Following the drainage, the first to be directly observed, the ice sheet temporarily sped up toward the sea. The data will help predict Greenland's global warming-related contribution to sea-level rise, said Ian Joughin, a co-author of the paper. Still, the findings show that melt-water isn't the principal reason for the acceleration of the island's glaciers, he said.
``There are large changes occurring in Greenland, with many of the glaciers doubling in speed over the last 10 years,'' Joughin, a researcher at Woods Hole, said by e-mail. While ``our results suggest melt-water lubrication is a real and significant effect, it is not the main cause of these speedups.''
Greenland's ice contains enough water to raise global sea levels by about 7 meters. The United Nations Intergovernmental Panel on Climate Change (IPCC) last year said sea levels will rise by 18 to 59 centimeters by 2100, and acknowledged uncertainties related to ice melt in Greenland and Antarctica.
Accelerating Glaciers
The ice loss on Greenland, a semi-autonomous territory of Denmark, helped raise sea levels by about 0.21 of a millimeter per year from 1993 to 2003, according to the IPCC. A study in Science in February 2006, published after the UN panel had finished gathering data, suggested the island's annual contribution to sea-level rise was almost triple that, due to the acceleration of glaciers.
Joughin and colleagues used seismic instruments, radar and global positioning system sensors to monitor two lakes and the ice sheet in the summers of 2006 and 2007. In July 2006, the level of one lake dropped slowly for 16 hours before a rapid drainage that took 1.4 hours, with a flow of 8,700 cubic meters of water per second -- more than that of Niagara Falls, the scientists said.
The observation confirms a hypothesis that water could bore through the ice sheet to get to its base, Joughin said.
Thousand Lakes
``If you fill a crack (e.g., a crevasse in the ice) with water, the pressure at the bottom of the crack will be greater than at the sides, because of the water's weight,'' Joughin said. ``As the crack propagates, if you keep the crack full to the brim, it can fracture through more than a kilometer of ice. This theory was worked out years ago, but some were skeptical about if it would really work in practice.''
Joughin said that with more than 1,000 lakes in Greenland, the drainage ``was probably one of the bigger events, but not uncommon.'' At the same time as the drainage, the surrounding ice sheet was observed to lift by about 1.2 meters and accelerate briefly, before settling back to its old pace.
``In total, the event resulted in a net westerly surface displacement of 0.5 meters in excess of the average daily displacement of 0.25 meters,'' the scientists wrote. ``The short- lived dynamic response suggests an efficient drainage system dispersed the melt-water sub-glacially.''
In a second paper in Science, the researchers examined the relative seasonal speed-up of outlet glaciers, which discharge ice into the ocean, and the ice sheet. They found that while seasonal melt caused 50 to 100 percent of the increase in the speed of the ice sheet, it accounted for only up to 15 percent of the acceleration of glaciers.
``Our results suggests that this particular process is of less concern that previously thought, though there are many other causes for concern,'' Joughin said, citing the breaking off into the sea of ice tongues at the tips of the island's glaciers. ``The observational evidence thus far suggests greater ice loss as temperatures have warmed over the last several years.''
To contact the reporter on this story: Alex Morales in London at amorales2@bloomberg.net .
Last Updated: April 17, 2008 14:00 EDT
By Alex Morales
<!-- WARNING: #foreach: $wnstory.ATTS: null at /bb/data/web/templates/webmacro_en/washingtonstory.wm:266.2 --> <!-- WARNING: #foreach: $wnstory.ATTS: null at /bb/data/web/templates/webmacro_en/washingtonstory.wm:280.19 --> April 17 (Bloomberg) -- Water from melting in Greenland can bore through a kilometer (0.6 mile) of ice, rapidly draining lakes and lubricating the ice sheet's path toward the sea, U.S.- based researchers found.
A 5.6 square-kilometer (2.2 square-mile) lake on the surface of the island's ice sheet, which took a month to fill with melt- water, drained in fewer than 18 hours, with peak flow exceeding that of Niagara Falls, the glaciologists at the University of Washington in Seattle and the Woods Hole Oceanographic Institution in Massachusetts said today in the journal Science.
Following the drainage, the first to be directly observed, the ice sheet temporarily sped up toward the sea. The data will help predict Greenland's global warming-related contribution to sea-level rise, said Ian Joughin, a co-author of the paper. Still, the findings show that melt-water isn't the principal reason for the acceleration of the island's glaciers, he said.
``There are large changes occurring in Greenland, with many of the glaciers doubling in speed over the last 10 years,'' Joughin, a researcher at Woods Hole, said by e-mail. While ``our results suggest melt-water lubrication is a real and significant effect, it is not the main cause of these speedups.''
Greenland's ice contains enough water to raise global sea levels by about 7 meters. The United Nations Intergovernmental Panel on Climate Change (IPCC) last year said sea levels will rise by 18 to 59 centimeters by 2100, and acknowledged uncertainties related to ice melt in Greenland and Antarctica.
Accelerating Glaciers
The ice loss on Greenland, a semi-autonomous territory of Denmark, helped raise sea levels by about 0.21 of a millimeter per year from 1993 to 2003, according to the IPCC. A study in Science in February 2006, published after the UN panel had finished gathering data, suggested the island's annual contribution to sea-level rise was almost triple that, due to the acceleration of glaciers.
Joughin and colleagues used seismic instruments, radar and global positioning system sensors to monitor two lakes and the ice sheet in the summers of 2006 and 2007. In July 2006, the level of one lake dropped slowly for 16 hours before a rapid drainage that took 1.4 hours, with a flow of 8,700 cubic meters of water per second -- more than that of Niagara Falls, the scientists said.
The observation confirms a hypothesis that water could bore through the ice sheet to get to its base, Joughin said.
Thousand Lakes
``If you fill a crack (e.g., a crevasse in the ice) with water, the pressure at the bottom of the crack will be greater than at the sides, because of the water's weight,'' Joughin said. ``As the crack propagates, if you keep the crack full to the brim, it can fracture through more than a kilometer of ice. This theory was worked out years ago, but some were skeptical about if it would really work in practice.''
Joughin said that with more than 1,000 lakes in Greenland, the drainage ``was probably one of the bigger events, but not uncommon.'' At the same time as the drainage, the surrounding ice sheet was observed to lift by about 1.2 meters and accelerate briefly, before settling back to its old pace.
``In total, the event resulted in a net westerly surface displacement of 0.5 meters in excess of the average daily displacement of 0.25 meters,'' the scientists wrote. ``The short- lived dynamic response suggests an efficient drainage system dispersed the melt-water sub-glacially.''
In a second paper in Science, the researchers examined the relative seasonal speed-up of outlet glaciers, which discharge ice into the ocean, and the ice sheet. They found that while seasonal melt caused 50 to 100 percent of the increase in the speed of the ice sheet, it accounted for only up to 15 percent of the acceleration of glaciers.
``Our results suggests that this particular process is of less concern that previously thought, though there are many other causes for concern,'' Joughin said, citing the breaking off into the sea of ice tongues at the tips of the island's glaciers. ``The observational evidence thus far suggests greater ice loss as temperatures have warmed over the last several years.''
To contact the reporter on this story: Alex Morales in London at amorales2@bloomberg.net .
Last Updated: April 17, 2008 14:00 EDT