Team advances understanding of the Greenland Ice Sheet’s meltwater channels

An international team of researchers deployed to western Greenland to study the melt rates of the Greenland Ice Sheet. -  Matt Hoffman, Los Alamos National Laboratory
An international team of researchers deployed to western Greenland to study the melt rates of the Greenland Ice Sheet. – Matt Hoffman, Los Alamos National Laboratory

An international research team’s field work, drilling and measuring melt rates and ice sheet movement in Greenland is showing that things are, in fact, more complicated than we thought.

“Although the Greenland Ice Sheet initially speeds up each summer in its slow-motion race to the sea, the network of meltwater channels beneath the sheet is not necessarily forming the slushy racetrack that had been previously considered,” said Matthew Hoffman, a Los Alamos National Laboratory scientist on the project.

A high-profile paper appearing in Nature this week notes that observations of moulins (vertical conduits connecting water on top of the glacier down to the bed of the ice sheet) and boreholes in Greenland show that subglacial channels ameliorate the speedup caused by water delivery to the base of the ice sheet in the short term. By mid summer, however, the channels stabilize and are unable to grow any larger. In a previous paper appearing in Science, researchers had posited that the undersheet channels were not even a consideration in Greenland, but as happens in the science world, more data fills in the complex mosaic of facts and clarifies the evolution of the meltwater flow rates over the seasons.

In reality, these two papers are not inconsistent – they are studying different places at different times – and they both are consistent in that channelization is less important than previously assumed, said Hoffman.

The Greenland Ice Sheet’s movement speeds up each summer as melt from the surface penetrates kilometer-thick ice through moulins, lubricating the bed of the ice sheet. Greater melt is predicted for Greenland in the future, but its impact on ice sheet flux and associated sea level rise is uncertain: direct observations of the subglacial drainage system are lacking and its evolution over the melt season is poorly understood.

“Everyone wants to know what’s happening under Greenland as it experiences more and more melt,” said study coauthor Ginny Catania, a research scientist at the institute and an associate professor in the University of Texas at Austin’s Jackson School of Geosciences. “This subglacial plumbing may or may not be critical for sea level rise in the next 100 years, but we don’t really know until we fully understand it.”

To resolve these unknowns, the research team drilled and instrumented 13 boreholes through 700-meter thick ice in west Greenland. There they performed the first combined analysis of Greenland ice velocity and water pressure in moulins and boreholes, and they determined that moulin water pressure does not lower over the latter half of the melt season, indicating a limited role of high-efficiency channels in subglacial drainage.

Instead they found that boreholes monitor a hydraulically isolated region of the bed, but decreasing water pressure seen in some boreholes can explain the decreasing ice velocity seen over the melt season.

“Like loosening the seal of a bathtub drain, the hydrologic changes that occur each summer may cause isolated pockets of pressurized water to slowly drain out from under the ice sheet, resulting in more friction,” said Hoffman.

Their observations identify a previously unrecognized role of changes in hydraulically isolated regions of the bed in controlling evolution of subglacial drainage over summer. Understanding this process will be crucial for predicting the effect of increasing melt on summer speedup and associated autumn slowdown of the ice sheet into the future.

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The research letter is published in this week’s Nature magazine as “Direct observations of evolving subglacial drainage beneath the Greenland Ice Sheet.” The project was an international collaboration between the University of Texas at Austin, Los Alamos National Laboratory, NASA Goddard Space Flight Center, Michigan Technological University, University of Zurich, the Swiss Federal Institute of Technology and Dartmouth College.

This project was supported by United States National Science Foundation, the Swiss National Science Foundation and the National Geographic Society. The work at Los Alamos was supported by NASA Cryospheric Sciences, and through climate modeling programs within the US Department of Energy, Office of Science.

Los Alamos National Laboratory, a multidisciplinary research institution engaged in strategic science on behalf of national security, is operated by Los Alamos National Security, LLC, a team composed of Bechtel National, the University of California, The Babcock & Wilcox Company, and URS for the Department of Energy’s National Nuclear Security Administration.

Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health, and global security concerns.

Meltzone 2011: CCNY expedition to track life and death of supraglacial lake

How do you observe signs of climate change in real time? Dr. Marco Tedesco, associate professor of earth and atmospheric sciences at The City College of New York, plans to be the first to catch sight of one dramatic indicator of a warming world on the Greenland ice sheet this summer, and through social media, people will be able to track his progress.

Professor Tedesco arrived in Greenland earlier this month to attempt to witness – for the first time – the entire lifecycle of a supra-glacial lake – from earliest formation to its catastrophic draining. These huge bodies of water form each year atop melting glaciers. They commonly measure a kilometer or more across, but can drain suddenly within a matter of hours.

Professor Tedesco plans to use data he gathers on his expedition to answer lingering questions about these mysterious pools, including: What causes them to drain? Where does the water go? How does this affect the glaciers’ inevitable flow toward the sea?

“This rapid draining is roughly equivalent to emptying a thousand Olympic-sized swimming pools at a rate of a dozen pools per minute,” notes Nick Steiner, a graduate student in Professor Tedesco’s Cryospheric Processes Lab. Mr. Steiner conducted research with Professor Tedesco in Greenland last year.

Professor Tedesco and his team will hike across the Jakobshavn Isbræ glacier in search of a lake to monitor and eventually make camp on the ice in the midst of an unstable landscape of embryonic lakes, streams and sub-glacier drainage.

Rounding out the expedition party are graduate student Patrick Alexander of the CUNY Graduate Center, biologist Christine Foreman of Montana State University, glaciologist Ian Willis and graduate student Alison Banwell of the Scott Polar Research Institute at the University of Cambridge, UK.

With such a large volume of water flowing out of the lakes – at the surface, under the glacier or thundering into deep holes called “moulins” – the surrounding ice is subject to movements that can cause ground-shaking ‘ice quakes’. The team will drill an array of monitoring equipment into the ice to track ice movement as the lakes drain and better understand the drainage. The monitors use DGPS, a high-precision global positioning system that uses satellite and ground stations to give greater positioning accuracy than a car or cellphone GPS.

A radio-controlled mini helicopter fitted with a camera will help the expedition party estimate the size and depth of the lake. Professor Tedesco used a similar craft on a past expedition in Antarctica.

The team will also study the scattered, dark deposits of “cryoconite,” extremely fine wind-borne particles carried to Greenland from around the world. Cryoconite begins life as soot, dust, soil and pollution and collects in depressions on the glacier. It absorbs more solar energy than the surrounding snow and ice, in turn, causing the snow to heat, melt and form small water-filled holes in the ice. Bacteria appear in these microhabitats, a phenomenon Dr. Foreman will study.


Meanwhile, Dr. Tedesco will measure the albedo, or reflected light, of the cryoconite versus clean snow and ice. Getting a baseline albedo for each type of ground coverage will allow them to calibrate satellite data and map coverage types on wide swaths of glacier.


Professor Tedesco will tweet his progress throughout the expedition for an interactive experience, updating his followers and Facebook page with photos, observations and their locations across the glacier. In addition, he will be reachable via satellite phone, and followers will have the chance to name a newborn lake on the glacier. Links to Twitter and Facebook feeds appear below.