International study provides more solid measure of melting in polar ice sheets

November 29, 2012, University of Washington
This is a view down the Ilulissat Fjord toward the terminus where Jakobshavn Isbrae rapidly discharges ice to the ocean. This fjord is frequently clogged with icebergs along its entire 60-km (37 mile) length. Credit: Ian Joughin, Univ. of Washington

Climatologists have reconciled their measurements of ice loss in Antarctica and Greenland over the past two decades. A second article looks at how to monitor and understand accelerating losses from the planet's two largest continental ice sheets.

The planet's two largest ice sheets have been losing ice faster during the past decade, causing widespread confusion and concern. A new international study provides a firmer read on the state of continental ice sheets and how much they are contributing to sea-level rise.

Dozens of have reconciled their measurements of ice sheet changes in Antarctica and over the past two decades. The results, published Nov. 29 in the journal Science, roughly halve the uncertainty and discard some conflicting observations.

"We are just beginning an observational record for ice," said co-author Ian Joughin, a in the University of Washington's Laboratory who is lead author on an accompanying review article. "This creates a new long-term data set that will increase in importance as new measurements are made."

The paper examined three methods that had been used by separate groups and established common places and times, allowing researchers to discard some outlying observations and showing that the results agree to within the uncertainties of the methods.

"It provides a simpler picture," said co-author Benjamin Smith, a research scientist at the UW's Applied . "In the 1990s, not very much was happening. Sometime around 1999, the ice sheets started losing more mass, and probably have been losing mass more rapidly over time since then."

The effort, led by Andrew Shepherd at the University of Leeds in the UK, reconciles three existing ways to measure this loss. The first method takes an accounting approach, combining and observations to tally up the ice gain or loss. Two other methods use special satellites to precisely measure the height and gravitational pull of the ice sheets to calculate how much ice is present.

Monthly changes in Antarctic ice mass, in gigatones, as measured by NASA's Gravity Recovery and Climate Experiment (GRACE) satellites from 2003 to 2011. The data illustrate the continuing loss of ice from the continent. The plots here depict results from five different IMBIE team members using different methods. The data have been adjusted to reflect new models of post-glacial rebound. Image credit: NASA-JPL/Caltech; NASA GSFC; CU-Boulder; Technical University of Munich; Technical University of Denmark; Delft University of Technology, Aerospace Engineering, Netherlands; Durham University, UK; Leeds University, UK

Each method has strengths and weaknesses. Until now scientists using each method released estimates independent from the others. This is the first time they have all compared their methods for the same times and locations.

"It brought everyone together," Joughin said. "It's comparing apples to apples."

Since 1998, scientists have published at least 29 different estimates of how much ice sheets have contributed to sea-level rise, ranging from 1.9 mm (0.075 inches) a year to 0.2 mm (0.0079 inches) drop per year. The new, combined estimate is that ice sheets have since 1992 contributed on average 0.59 mm (0.023 inches) to sea-level rise per year, with an uncertainty of 0.2 mm per year. Overall sea levels have risen by about 3.3 mm per year during that time period, much of which is due to expansion of warmer ocean waters.

"Establishing more consistent estimates for the contribution from ice sheets should reduce confusion, both among the scientific community and among the public," Joughin said.

Credit: ESA

Understanding why the ice sheets have been shedding mass faster in the last decade is an area of intense research. The accelerated ice loss was not predicted by the models, leading the latest International Panel on Climate Change to place no upper limit on its estimate for future ice-sheet loss.

Joughin is lead author of an accompanying article that reviews factors that cause ice sheets to lose more mass. In particular, it looks at what happens when warmer ocean waters reach the underside of large floating Antarctic ice sheets or abut glaciers in Greenland's fjords.

Joughin and his co-authors, Richard Alley of Pennsylvania State University and David Holland of New York University, suggest ways to better monitor and understand those changes: Create finer-grained ocean models that could include narrow fjords, develop more models to study the interaction between ice sheets and ocean water, and improve ice sheet monitoring.

Taking at ice edges is perilous, they write, because skyscraper-sized chunks of ice can topple on floating instruments with no notice, and outgoing glaciers can scour any instruments moored to the ocean floor.

Understanding ice sheets is central to modeling global climate and predicting sea-level rise. Even tiny changes to sea level, when added over an entire ocean, can have substantial effects on storm surges and flooding in coastal and island communities.

The West Antarctic could trigger abrupt changes globally if it were to become unstable, and although Greenland is thought to be more stable, the recent calving of glaciers has led to some alarm.

Joughin believes the recent activity is a reason to pay attention, but not to panic.

"We don't fully understand why it's accelerating," Joughin said. "But the longer-term observations we have, the more solid predictions we will be able to make."

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4.3 / 5 (6) Nov 29, 2012
OMG! The traitors! They got bought off!
3.9 / 5 (7) Nov 29, 2012
The earth isn't warming because it is against the laws of Libertarian Economics. Just ask any Libertarian Economist and they will explain the dollar physics to you.

Tax cuts for the wealthy are clearly the solution to the Global Warming problem even though there is no problem. This is true because tax cuts for the wealthy are the solution to every problem.

Need to compute the square root of pie? Tax cuts for the rich is the answer.

Need a new rocket fuel forumlation? Tax cuts for the rich is the solution.

The cure for cancer? Cut taxes for the rich.

War in the middle east? Caused by high taxes on the rich.

Got a cold sore. You wouldn't have one if there were greater tax cuts for the rich.

5 / 5 (3) Nov 29, 2012
The new, combined estimate is that ice sheets have since 1992 contributed on average 0.59 mm (0.023 inches) to sea-level rise per year, with an uncertainty of 0.2 mm per year.

That must be an under-estimate, because Grace data claims that over 200 gigatons of ice is melting in Greenland alone each year, and if you do the calculations, 0.59mm per year rise from melted ice caps per year is only 240 gigtons of ice total, which would imply that 5 out of every 6 units of global melt is coming from Greenland alone.

Yet without even looking at Antarctica, when you look at data from Patagonia and the Himalayas and add that to the known value for Greenland, it's already more than 240 gigatons per year....before we even look at Antarctica.

I think more than likely the expansion rate of the water is being over-estimated, or else they aren't taking into account the fact that melted water added to the sea pushes down on the ocean floor, which would increase the volume the oceans can hold.
not rated yet Nov 30, 2012
"or else they aren't taking into account the fact that melted water added to the sea pushes down on the ocean floor, which would increase the volume the oceans can hold."

I think that is not the case, just compare the 0.59 mm increase with the average depth of the ocean. The difference in pressure is insignificant. I find much more plausible that one of the estimates (or both) are way off the real values. (0.2 mm /0.6 mm gives us an uncertainty of 33%)
not rated yet Nov 30, 2012
"That must be an under-estimate..."

I think the main problem might be that it is presenting to us a flat-rate projection developed from an historical base.

If glacial melt is accelerating and subject to tipping points, as I suspect it may be, then over time, it will be increasingly difficult to reconcile a flat-rate projection against the latest measurement data.

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