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Study Gives New Benchmark for How Much Ice is Melting

The vast ice sheets that cover Greenland and Antarctica have begun melting and sliding into the ocean as heat-trapping greenhouse gases continue to build in the atmosphere. How much and how fast the ice is disappearing, however, has been poorly understood, because the satellites that measure it haven’t always agreed. But a report published Thursday in Science has cleared up much of the uncertainty.

A team of no fewer than 47 scientists from 36 laboratories, looking at data from 10 different satellites, has come up with numbers everyone is on board with: between 1992 and 2011, Greenland has lost an average of 152 billion metric tons of ice per year, while Antarctica has shed 71 billion, contributing 11 millimeters to the rise in sea level over that period — about a fifth of the total (the rest has come from from seawater expanding as it warms and from melting mountain glaciers).

View of Sheldon Glacier from Ryder Bay near Rothera Research Station, Antarctica.
Credit: NASA.  

“The new estimates,” lead author Andrew Shepherd, of the University of Leeds, said in a press conference, “are the most reliable to date. They end 20 years of uncertainty.”

The estimates don't change projections about what’s likely to happen for the rest of this century. Scientists’ best estimate is that that the ocean, which has risen an average of 8 inches since 1900, should rise another 3 feet or so by 2100. But without a truly accurate record of how much melting has already occurred, it’s hard to know how good that projection is. It will also be hard to be certain if the melting starts to speed, or slow.

“This [study] provides a nice baseline. It’s really the start of the reliable record,” said co-author Ben Smith, of the University of Washington, at the press conference.

One problem with using information from individual satellites is that they’ve been in orbit at different times over the decades since satellite observations began. Another is that they use entirely different techniques for measuring ice loss — looking at changes in gravitational pull as the mass of ice changes, or bouncing radar beams off the surface to see if the distance between satellite and ice gets bigger as the ice shrinks away. It’s like comparing apples to oranges, with the added confusion that the apples and oranges aren’t on the table at the same time.

“What we found,” Shepherd said in an interview, “was that there was a sweet spot in the mid-2000s when 10 satellites were flying at once.”

Scientists working on data from all 10 came together, and compared results from exactly the same time periods for exactly the same regions within Antarctica and Greenland. It was somewhat analogous to photographing a scene not with a single camera, but with 10 different cameras looking from 10 different perspectives: you get a much richer picture of what’s going on. 

In particular, Shepherd said, scientists can now start to pinpoint changes in ice region by region, and figure out what’s happening in each place.

For example, the ice sheet in East Antarctica, which holds by far the most ice, is actually growing slightly, because of an increase in snowfall. “That’s consistent with climate warming because you expect more evaporation and therefore more rain and snow,” Shepherd said.

By contrast, the smaller ice sheets in West Antarctic and the Antarctic Peninsula are losing ice more quickly. “In general,” said co-author Ian Joughin of the University of Washington in an interview, “this is due to heat from the ocean. We’re doing lots of work trying to understand the melting going on beneath ice shelves.”

In fact, Joughin also served as lead author on a second paper in the same issue of Science that tackles exactly this question.

“It’s not as simple as just saying that the ocean warms up,” he said. “The main effect in Antarctica seems to be warmer water from offshore moving up under the ice shelves [that ring the continent] as changing winds blow colder surface water away.”

Joughlin said there’s another big concern. “The warm water is affecting only a few ice shelves so far. If winds bring in warm water more extensively, it could cause a big ice loss.”

In Greenland, meanwhile, where the ice losses have been most dramatic, the situation is much more complicated. Unlike in more frigid Antarctica, temperatures rise high enough in summer that ice melts on the top surface of the ice sheet. Some of the meltwater percolates down to lubricate the base of seaward-flowing glaciers, which may make them move faster. Down where those glaciers meet the sea, warmer ocean water has led to faster calving of ice chunks into the sea — but nobody knows exactly how it all works.

“There are lots of processes in play and it remains unclear whether the ice loss will level out or slow down or speed up,” Joughin said. “We thought we understood ice sheets, but it’s clear we don’t. We need a lot more observations and a lot better modeling.” 

Related Content
Sea Level Rise Accelerating Faster than Initial Projections
Ice is Flowing Slower on Greenland than Many Feared
The Bad News Continues to Flow About Antarctica's Ice


By James Cook (Encinitas, California 92024)
on November 30th, 2012

I don’t understand why they wouldn’t use median low sea ice extent in the video. Every September the ice is at its lowest, why compare the summer ice with the median for the whole year unless they are trying to deceive the viewer into thinking that the median ice is the norm. The north pole looses ice cover every year, not just in record years.

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By Ben Pranulis (Higganum, CT 06441)
on November 30th, 2012

If I am not mistaken, you have misquoted the article and greatly under-represented the amount of ice being lost.  Here is the actual quote from the article abstract:

“Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by –142 ± 49, +14 ± 43, –65 ± 26, and –20 ± 14 gigatonnes year−1, respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 ± 0.20 millimeter year−1 to the rate of global sea-level rise. “

Those numbers are all in gigatonnes PER YEAR.

So I read the abstract as saying that Greenland is losing 142 billion metric tons PER YEAR, plus or minus 49 billion metric tons:  and Antarctica as a whole is losing 71 billion metric tons PER YEAR, plus or minus ~43 billion metric tons per year.

Please make the corrections. Thanks.

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By Mike Lemonick (Princeton, NJ 08542)
on November 30th, 2012

Quite right! Correction made

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By MARGARET BENNETT (Logan,Ohio, 43138)
on December 6th, 2012

my comment is a question….is anyone doing anything about this..and is the government still denying it exists??..also..what can we do, as individuals with no power…..and not a lot of money….Thanks….

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By James Newberry (New Haven)
on December 10th, 2012

This is a fine report as far as it goes and is a necessary baseline. What is left unsaid here is that melt rate is in fact accelerating, perhaps as high as doubling per decade (J. Hansen at NASA). An averaged rate from twenty years past therefore understates current conditions appreciably, especially for that at the beginning of what appears to be an exponentially increasing curve. Further, it is fact that global heating, and therefore ice melt, from radiative forcing is both time lagged and increasing.

Shall we continue “lighting boilers on the Titanic” with icebergs dead ahead (and lifeboats non-existent on Good Ship Earth)? Annual expenditures for fossil “exploration and development” equal global public subsidies. This is not economics.

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By Clark Davis (Toronto,Ontario. m6n1w1)
on March 25th, 2013

With all this melting ice should this not slow the acidifaction of the oceans?

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