maandag 1 januari 2018

As Greenland Melts, Where’s the Water Going?

As Greenland Melts,
Where’s the Water Going?

In the summer of 2015, two New York Times journalists joined a team of researchers in Greenland that was conducting a unique experiment: directly measuring a river of meltwater runoff on the top of the ice.
Now, the scientists have published the results of that work. A key finding — that not as much meltwater flows immediately through the ice sheet and drains to the ocean as previously estimated — may have implications for sea-level rise, one of the major effects of climate change.
The scientists say it appears that some of the meltwater is retained in porous ice instead of flowing to the bottom of the ice sheet and out to sea.
“It’s always treated as a parking lot, water runs straight off,” said Laurence C. Smith, a geographer at the University of California, Los Angeles who led the field work in 2015. “What we found is that it appears there is water retention.”
“It’s plausible that this is quite an important process, which could render sea-level projections too high,” he added.

There’s still much that remains unknown about the ice sheet, which at roughly 650,000 square miles is more than twice the size of Texas. The sheet, up to two miles thick, contains enough ice that, if it all melted, would raise oceans around the world by 24 feet. Precisely how the ice melts — half or more by warming on the surface, the rest by ice sheet movement toward the sea, where it melts or calves off as icebergs — can greatly affect how much and how fast the seas rise.
Greenland is currently losing an average of about 260 billion tons of ice per year; at this rate, it would contribute about two inches to sea level rise by the end of the century. This ice loss is estimated through gravity measurements by satellites, but computer models that simulate physical processes are used to estimate the surface runoff. The field study was meant to improve those models by providing on-the-ground data on the flow of meltwater.

Change in Greenland Ice Sheet mass
since 2002, in billions of metric tons
No change
−1,000 billion
metric tons
Source: The European Space Agency Climate Change Initiative
The work involved setting up camp near a glacial river that drains meltwater from a surrounding catchment area — in this case about 24 square miles of the ice sheet — into a moulin, a hole that drains to the base of the ice sheet. The researchers suspended a device in the river that uses acoustic signals to gauge the flow, and used satellite and drone images to precisely calculate the catchment area.
The flow data, collected over 72 hours, showed that current models are overestimating the amount of runoff by 20 percent to nearly 60 percent. Were the models wrong? Or were they right about the total amount of melting, but some of the water was not running off?
When he first sent the results to modelers, Dr. Smith said, “they couldn’t believe it.” After months of back-and-forth, Dr. Smith and his colleagues concluded that the model estimates were accurate, but there was something else going on with some of the meltwater. “What is missing,” he said, “is a physical process that is not currently considered by the models — water retention in ice.”
Sunlight hitting the ice sheet melts the surface, Dr. Smith said, but some of the light reaches deeper into the ice and causes some melting there. The ice develops a rotted, porous texture -- and can, the findings suggest, hold on to some of the meltwater.

Area shown
at top of this article
The New York Times | Source: Satellite imagery by DigitalGlobe via Laurence Smith at the University of California, Los Angeles
Marco Tedesco, a co-author of a paper on the work in The Proceedings of the National Academy of Sciences who is a modeler at Lamont-Doherty Earth Observatory, part of Columbia University, said when comparing models with actual field measurements, “you always look at the energy involved.”
“If there’s a mismatch between observation and model,” Dr. Tedesco said, “that means the model is moving the mass in one way or another and not respecting the way things happen in the real world.”
Further work on the ice sheet will try to directly measure how much water is retained in the ice. More direct flow measurements are needed as well, as conditions can vary greatly over the huge icecap. (As the original Times article revealed, gauging stream flow on an ice sheet is far more complicated and dangerous than doing it on land.)
But surface runoff is only one area where much remains to be learned. What happens at the bottom of the ice when meltwater reaches it is understood largely through modeling, and those processes can affect how fast the ice sheet moves — and thus how much ice calves off into the ocean.
Among other aspects that scientists are studying is bioalbedo, which is how the growth of microorganisms in the ice can darken the surface and affect how fast it melts.
Thomas P. Wagner, a program scientist at NASA who directs the agency’s efforts to study polar regions, said that studies like Dr. Smith’s — and research on the far larger Antarctic ice sheet — are critical to understanding how climate change will affect the globe.
“Sea level rising is 3 millimeters a year right now,” Dr. Wagner said. “But in a hundred years, we could have one to five feet of rise.”
“Everyday we are improving our understanding of how ice is getting into the ocean,” he added. “The overall context is, we’re on this.”

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