environmentalresearchweb.org, November 17, 2010
Meltwater flowing through ice sheets can carry warmth to the interior via crevasses, fractures, and large drains called moulins. A new modeling study shows that such warming can greatly accelerate the thermal response of an ice sheet to climate change: An ice sheet can respond on the order of decades, rather than the centuries projected by conventional thermal models. Ice flows more readily as it warms, so a warming climate can increase ice flows much faster than previously thought.
"We are finding that once such water flow is initiated through a new section of ice sheet, it can warm rather significantly and quickly, sometimes in just 10 years, " said lead author Thomas Phillips, a research scientist with the University of Colorado at Boulder and NOAA's Cooperative Institute for Research in Environmental Sciences (CIRES). "We've termed this process cryo-hydrologic warming."
Phillips, University of Colorado engineering professor Harihar Rajaram, and CIRES Director Konrad Steffen described their model results in a paper published online this week in Geophysical Research Letters.
Conventional thermal models of ice sheets do not consider the presence of water as a warming agent within the ice sheet – those models primarily consider heating by warmer air on the ice sheet surface.  In water's absence, ice warms slowly in response to the increased surface temperatures from climate change, often requiring centuries to millennia to happen. "This would be correct if water only flowed on the ice surface without the opportunity to go into it," said Phillips.
However, the Greenland ice sheet is not one solid, smooth mass of ice. As the ice flows towards the coast, grating against bedrock, crevasses and new fractures form in the upper 100 feet of the ice sheet. Meltwater flowing through these openings can grow "ice caves" and networks of pipes that can carry water through the ice, spreading warmth.
To quantify the influence of that meltwater, the scientists modeled what would happen to the ice sheet temperature if water flowed through it for just eight weeks every summer, about the length of the active melt season.
"The key difference between our model and previous models is that we include heat exchange between water flowing through the ice sheet and the ice," said Rajaram.
The result was a significantly faster-than-expected increase in ice sheet warming, the research team reported. The warming could take place on the order of years to decades, depending on the spacing of crevasses and other "pipes" bringing warmer water into the ice sheet in summer. Several factors contributed to the warming and resultant acceleration of ice flow:
  1. Slower cooling: Some of the water that flows into the ice sheet can stay liquid even through the winter, slowing seasonal cooldown.
  2. Basal lubrication: A warmer ice sheet is more susceptible to increases in flow by long-understood mechanisms, such as basal lubrication of the ice.
  3. A downward spiral of damage: Water cascading into ice can warm the surrounding ice and also re-freeze, creating further cracks in the more vulnerable, warm ice.
Taken together, these interactions between water, temperature, and ice velocity spell even more rapid changes to ice sheets in a changing climate than currently anticipated, the authors concluded.
In fact, the authors compared observed temperature profiles from Greenland with  model results. Unless the cryo-hydrologic warming was accounted for, they could not explain observations.
"The fact that the ice temperatures warm rather quickly is really the key piece that's been overlooked in models currently being used to determine how Greenland responds to climate warming," Steffen said. "However, this process is not the 'death knell' for the ice sheet. It'll still take thousands of years, if not a multiple thereof, for the ice sheet to disappear."
Collaborators on this project include the Aerospace Engineering and Sciences, Geography, and Civil, Environmental, and Architectural Engineering departments at the University of Colorado at Boulder. This study was supported by NASA Cryosphere Science Program grants and is published online in the journal Geophysical Research Letters. Download a copy of the paper here.