Ice Sheet Loss At Both Poles Is Clearly Increasing
November 30, 2012

Clearest Evidence To Date That Polar Ice Loss Is Increasing

April Flowers for - Your Universe Online

In a new study that ends 20 years of uncertainty, an international team of satellite experts have produced the most accurate assessment of ice losses from Antarctica and Greenland to date.

Published in the journal Science, the landmark study shows that the melting of the Antarctic and Greenland ice sheets has contributed 11.1 millimeters to global sea levels since 1992, amounting to one fifth of all sea level rise over the survey period.

Greenland accounted for two-thirds of the ice loss, with Antarctica contributing the remaining one-third.

The ice sheet losses fall within the range reported in 2007 by the Intergovernmental Panel on Climate Change (IPCC). The spread of the IPCC estimate, however, was so broad it was unclear whether Antarctica was growing or shrinking. The estimates provided by the new study are more than twice as accurate. This is thanks to the inclusion of more satellite data, which confirms that both Antarctica and Greenland are losing ice.

"We are just beginning an observational record for ice," said Ian Joughin, a glaciologist in the University of Washington's Applied Physics Laboratory. "This creates a new long-term data set that will increase in importance as new measurements are made."

Led by Professor Andrew Shepherd at the University of Leeds and Dr Erik Ivins at NASA's Jet Propulsion Laboratory (JPL), the study reveals that the combined rate of ice sheet melting has increased over time. Greenland and Antarctica are losing more than three times as much ice - equivalent to 0.95 mm of sea level rise per year — as they were in the 1990s - equivalent to 0.27 mm of sea level rise per year.

"It provides a simpler picture," said Benjamin Smith, a research scientist at the UW's Applied Physics Laboratory. "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 research, called the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE), is comprised of 47 specialists from 26 laboratories, with support from both the European Space Agency (ESA) and NASA. IMBIE combines observations from 10 distinct satellite missions to develop the first consistent measurement of polar ice sheet changes. The study also reconciles three existing ways to measure this loss; the first an accounting approach, combining climate models and observations to tally up the ice gain or loss, while the second and third methods use special satellites to precisely measure the height and gravitational pull of the ice sheets to calculate how much ice is present.

Each method had merits, but until now, scientists using each one released estimates independent from the others. This new study is the first time they have compared their methods for the same times and locations.

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

According to glaciologist Alexander Robinson of the Complutense University of Madrid, "We've had a good idea of what the ice sheets are doing, but it seems this study really brings it all together in one data set that gives a much clearer picture."

"It's one more piece of supporting evidence that shows there are some dramatic changes happening, and we know that's being driven mainly by a warmer climate and warmer ocean–but there's still a lot we don't know about these regions and how they're changing," Robinson told Christine Dell'Amore of National Geographic News.

Researchers have published no less than 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, since 1988. The IMBIE 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. Due in a large part to expansion of warmer ocean waters, overall sea levels have risen by about 3.3 mm per year during that time.

Professor Shepherd said in a press statement, "The success of this venture is due to the cooperation of the international scientific community, and due to the provision of precise satellite sensors by our space agencies. Without these efforts, we would not be in a position to tell people with confidence how the Earth's ice sheets have changed, and to end the uncertainty that has existed for many years."

Differences were also found in the pace of change at each pole.

"The rate of ice loss from Greenland has increased almost five-fold since the mid-1990s. In contrast, while the regional changes in Antarctic ice over time are sometimes quite striking, the overall balance has remained fairly constant - at least within the certainty of the satellite measurements we have to hand," explained Dr. Ivins.

The accelerated ice loss of the past decade was not predicted by current models, leading the latest International Panel on Climate Change report to place no upper limit on its estimate for future ice-sheet loss. The devastation that rising sea levels could inflict — catastrophic flooding, widespread erosion, contamination of aquifers and crops, and harm to marine life - make understanding this accelerated loss an area of intense research.

"This project is a spectacular achievement," said Professor Richard Alley, a climate scientist at Penn State University, in a statement. "The data will support essential testing of predictive models, and will lead to a better understanding of how sea-level change may depend on the human decisions that influence global temperatures." Alley underscores the importance by saying that if all the polar ice sheets somehow melted – something that would take centuries – global sea levels would jump by more than 200 feet.

"Understanding how and why the ice sheets are changing today better equips us for understanding and predicting how much and in what ways they will change in the future," Waleed Abdalati of NASA told Seth Borenstein of the Associated Press (AP).

Alley and Joughin, along with David Holland of New York University, authored an accompanying article that reviews factors that cause ice sheets to lose more mass, particularly what happens when warmer ocean waters reach the underside of large floating Antarctic ice sheets or abut glaciers in Greenland's fjords.

They suggest better ways to monitor and understand ice sheet changes, including creating 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.

Skyscraper-sized chunks of ice can topple on floating instruments with no notice, making it perilous to take measurements at ice edges. In addition, outgoing glaciers can scour any instruments moored to the ocean floor.

The recent accelerated activity is a reason to pay attention, but not to panic, Joughin asserts.

"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."

Another study, published in Environmental Research Letters, revealed that sea levels are rising at a rate of 3.2 millimeters a year, 60 percent faster than the latest IPCC estimate of 2 millimeters a year.

"These results should be a major concern for politicians and climate talks in Doha, as they show that global warming is real and having major consequences that will only get bigger over time," Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research, told National Geographic News.

Rising sea levels could also supercharge storms. Higher seas may boost storm surges that can strip away everything in their path and create damaging floods when a storm like Hurricane Sandy make landfall, for example.

Ivins says the half inch of sea level rise we have already experienced probably added to the flooding that Hurricane Sandy caused, saying the extra weight of water gives each wave a little more energy.

"The more energy there is in a wave, the further the water can get inland," Ivins told Borenstein.

For the most part, scientists lay the blame for the ice melt at the feet of man-made global warming. Greenhouse gases, emitted by the burning of fossil fuels, trap heat and warm the atmosphere and oceans. Over time, that erodes the ice sheets from top and bottom. Snowfall replenishes the sheets, but hasn´t been able to keep pace with the melt.

Walt Meier, of the National Snow and Ice Data Center, says such a reconciled data set has been sorely needed.

"You have this huge range of estimates of ice mass loss from Antarctica and Greenland–they're such a large range that you get to the point of you don't know what to trust," making the new study "a much more manageable range, and provides much better guidance in terms of future projections," Meier told National Geographic News.

Even more importantly, Meier says the IMBIE might usher in a stronger model of another kind — scientific cooperation.

The study's authors "came together and sat down–at least figuratively–and came to a consensus for the best estimate that they can," he said. "It's a great example," he said, "that in climate science and science in general, you can't do these kind of big things on your own anymore."

Image 2 (below): Over the course of several years, turbulent water overflow from a large melt lake carved this 60-foot-deep (18.3 meter-deep) canyon (note people near left edge for scale). Image credit: Ian Joughin, University of Washington