Melting ice sheets already seen driving sea-level rise

In the Earth's frozen extremes, it appears, the future is now.

For some time, it's been thought in climate science that the glaciers and ice caps crowning mountain ranges from the Andes to the Himalayas are the canaries of the cryosphere. Seen making dramatic retreats in the face of human-caused global warming, melting glaciers have been flagged as the most likely frozen source for the planet's rising seas.

Ultimately, melting of the massive ice sheets in Antarctica and Greenland would become more influential than glaciers in pushing up sea levels, of course. But that was some time away.

Or not.

As a satellite study released today by Nature shows, the Antarctic and Greenland ice sheets have already begun contributing more to the planet's rising seas than the melting of ice caps and glaciers. It is a trend that overthrows conventional understanding of how melting is partitioned, while reaffirming that sea levels are rising as expected.

Overall, the world's ice-covered regions raised the sea level of the planet's oceans 1.48 millimeters a year from 2003 to 2010. Mountain glaciers and ice caps contributed only 0.41 millimeters a year to this total, with the rest covered by the ice sheets and their peripheral glaciers. Notably, even when these peripheral glaciers are included with their mountain peers, the ice sheets still provided the majority of sea-level rise over the past eight years.

"It's the best snapshot of what's going on right now," said John Wahr, a physicist at the University of Colorado, Boulder, and a coauthor of the Nature study. "In the extent that's useful for the future, time will tell."

The study is in no way predictive, and its time span is short by the climate's standards. It is possible that glaciers could come back to drive sea levels, or ice sheets could continue their lead. Prediction is difficult. But what the study does show is that scientists are now empowered to continuously monitor mass changes in most of the world's frozen places, a stark difference from the data-impoverished days of a decade ago.

"We are very slowly accumulating the tools and the data to look more precisely at what global glaciers and ice caps are doing," said Tad Pfeffer, a longtime glaciologist at Boulder and another of the study's co-authors. "What's the contribution of small glaciers and ice caps? We've had these tremendous data gaps and uncertainties."

Independent scientists generally applauded the study. The team's satellite work better explains and matches the actual global sea-level rise seen from new water sources over the past few years -- 1.3 millimeters a year -- than past work, said C.K. Shum, a geoscientist at Ohio State University.

"It's clear that the ice sheets are ahead of the game here," added Eric Rignot, a senior research scientist at NASA's Jet Propulsion Laboratory who has used the same satellite tools to gauge the Arctic regions. "They're losing a lot more mass than the glaciers."

The study lends credence to the notion that scientists have, in the past, overestimated the rate of loss from glaciers, said Graham Cogley, one of the world's pre-eminent glaciologists and a professor of geography at Trent University.

"On the other hand," Cogley added, "it still needs work."

Limits of estimating glacier melt

These overestimates are as recent as several years ago, when a landmark 2007 paper in Science found melting glaciers to be dominating sea-level rise now and in the future. Pfeffer was a co-author on the study, and he admits that glaciologists were perhaps too strident in neglecting the uncertainty in their estimates. It is far from an easy task.

Monitoring glaciers is a long-standing challenge for scientists. The traditional method for assessing glacier changes has relied on observing a couple of representative glaciers in a region, and then, as carefully as possible, expanding these findings to every ice flow in an area. But scientists remain uncertain of even how many glaciers there are in the world, with some estimates sitting at 300,000. For inaccessible regions like the Himalayas, these inventories are far from complete.

That uncertainty has made estimating glacier melt tricky, a limit exposed by the new study.

The paper's most controversial result is its finding that glaciers and ice caps in the Himalayas have experienced little in the way of net melting over eight years, shedding 4 gigatons of ice a year, far less than traditional estimates, which saw the mountainous region losing some 55 gigatons a year. The melting is much less even than changes detected in the region by Koji Matsuo, a scientist at Hokkaido University in Japan, who used the same satellite tools as Wahr and his collaborators.

Rather than detecting melting glaciers, Matsuo likely observed groundwater depletion from farming and rapid growth in nearby India and Bangladesh, the new study suggests.

The ability to confuse groundwater depletion with melting is a hallmark of GRACE, twin satellites launched to measure Earth's gravity in 2002. The satellites travel in tandem 300 miles above the ground, one after the other, carefully measuring the distance between themselves, down to microns. Changes in gravity pull the first satellite slightly ahead, then tug on the second satellite to close the distance. Incredibly, this tug of war allows gravity changes to be documented down to resolutions useful for science.

Matsuo was complimentary of the Nature study and said the groups' estimates stemmed largely from their differing models for separating out groundwater loss in India -- Wahr's group had a rate of 35 gigatons a year, while Matsuo had 10 gigatons a year -- and the time difference of their estimates, since 2010 saw a large increase in the Himalayas' frozen areas, possibly due to a record-breaking Arctic Oscillation.

"Considering these two effects, the estimated glacial losses might be around 10 gigatons a year or less in [High Mountain] Asia as a whole," Matsuo said, revising his estimate down.

Really, it is natural that the ice sheets would provide a majority of the sea-level rise, Rignot added. Even grouped collectively, glaciers are small by comparison, he said.

Still, over the next decades, glaciers will continue to supply a large amount of meltwater to the ocean in unpredictable, variable waves. For example, many glaciers do not drain directly into the ocean, and their meltwater is often trapped high in glacial lakes, said Don Chambers, a physical oceanographer at the University of South Florida.

But that trapped water will ultimately make its way to the ocean, Chambers added.

"It's more than likely that glaciers cannot maintain water in these regions," he said.

'Really important' local problems

Even if glaciers are not the premier driver of rising seas, however, their continued melting can cause untold disruptions.

In the Himalayas and Andes, glaciers are a crucial water source. In the Pacific Northwest, they essentially regulate hydropower generation. In the Gulf of Alaska, fresh water released by the glaciers escapes into the Arctic Ocean, altering its salinity and disrupting an already fragile system.

"Those are really important problems locally," Pfeffer said. Yet, he added, "It can be quite difficult to write a grant proposal on salinity. You tend to get the response, 'What's it got to do with sea-level rise?'"

Indeed, there is some sensitivity among scientists who study glaciers that research on the ice sheets receives far more financial support than glaciers and ice caps. If scientists are expected to accurately predict sea-level rise over the next few decades -- the period most relevant for society -- then ice sheets and glaciers both need attention, Pfeffer said.

"We can't do it by doing a dynamite [job] on the biggest variable," he said.

Improvements need to be made in applying GRACE to the glaciers, too, Cogley said. Wahr is able to cleverly account for the many gravitational influences that may be felt in attempts to survey mountainous regions, but the satellites still cannot detect many regions with low amounts of glacier ice, he said.

"So, for example, it would be nice to know whether those lesser regions would make much of a difference," Cogley said. "With rare exceptions, it is not plausible that any of them are actually gaining mass in the form of glacier ice."

For his part, Wahr plans to continue using GRACE to monitor the ice sheets and glaciers, refining his methods as the United Nations' next climate assessment approaches. Whether GRACE will hold up is uncertain, though, as the satellites have been operating for 10 years and have recently had battery problems, causing instruments to be shut down. A replacement mission is planned for later this decade, largely to continue monitoring efforts.

"If GRACE can hang on for a few years, [it'll be] pretty exciting," Wahr said. "We'll keep doing this. We've got all the formulas set up. We'll keep doing this and we'll just see how it develops."

There is a tinge of irony to how effective GRACE has become for monitoring the ice sheets and, now, mountain glaciers and ice caps. The satellite tandem can only measure ice by detecting changes in the planet's gravity, Chambers said.

"Fact is," Chambers said, "if the ice sheets were not losing mass at the large rate that they are, we wouldn't be able to see this."

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