Speed of sea level rise outpaces earlier projections
Polar Ice Sheets Shrinking Worldwide, Study Confirms
Rapid loss already contributing to sea level rise
NationalGeographic.com, Nov. 29, 2012
The polar ice sheets are indeed shrinking—and fast, according to a comprehensive new study on climate change.
And the effects, according to an international team, are equally clear—sea levels are rising faster than predicted, which could bring about disastrous effects for people and wildlife.
Rising seas would increase the risk of catastrophic flooding like that caused by Hurricane Sandy last month in New York and New Jersey. Environmental damage may include widespread erosion, contamination of aquifers and crops, and harm to marine life. And in the long term, rising seas may force hundreds of millions of people who live along the coast to abandon their homes.
By reconciling nearly two decades of often conflicting satellite data into one format—in other words, comparing apples to apples—the new study, published in the journal Science, made a more confident estimate of what's called ice sheet mass balance.
That refers to how much snow is deposited on an ice sheet versus how much is lost, either due to surface melting or ice breaking off glaciers.
Between 1992—when polar satellite measurements began—and 2011, the results show that all of the polar regions except for East Antarctica are losing ice, said study leader Andrew Shepherd, a professor of earth observation at the University of Leeds in the U.K.
In that 20-year span, Greenland lost 152 billion tons a year of ice, West Antarctica lost 65 billion tons a year, the Antarctic Peninsula lost 20 billion tons a year, and East Antarctica gained 14 billion tons a year.
"When we did the experiments properly using the same time periods and same maps, the riddles did all agree," Shepherd said.
According to glaciologist Alexander Robinson, "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," said Robinson, of the Complutense University of Madrid in Spain, who was not involved in the research.
Shrinking Ice Consistent With Warming
For the study, Shepherd and his team took data from three fields of satellite research: Altimetry, which measures the shapes of ice sheets and how they change over time; interferometry, which tracks the speed of ice sheets; and gravimetry, which calculates the weight of ice sheets by measuring Earth's gravitational field.
"Up until now there have been more than 30 studies that have each produced their own estimates of changes in ice sheets," Shepherd said.
"What we did was try to take the strengths of each approach and combined all the satellite technology together to get a better estimate of how ice sheets are changing," he said.
The results are also consistent with observations of climate change at the poles, Shepherd noted.
For instance, Greenland is shedding five times as much ice as 20 years ago, which fits with a trend of rising air temperatures in the Arctic.
In West Antarctica, glacier loss is accelerating in an area where the ocean is getting warmer. East Antarctica is experiencing a slight increase in the amount of ice stored there, but that dovetails with higher rates of snowfall expected with climate change.
However, the growth isn't enough to compensate for the larger losses in the rest of Antarctica, the researchers say.
"The fact that Antarctica is definitely losing ice is a novel conclusion when we compare it to the last IPCC report in 2007, when scientists weren't sure if Antarctica was growing or shrinking," Shepherd said.
"Our data are now two to three times as accurate as those that were available at the time of the last IPCC report." The IPCC, or Intergovernmental Panel on Climate Change, is the leading international body for the study of climate change.
Kevin Trenberth, a senior scientist at the Boulder, Colorado-based National Center for Atmospheric Research, who was not involved in the study, said the new study's "evidence is very compelling that global warming is playing a role in massive ice losses on land that contribute to sea level rise."
Overall, polar ice loss has contributed about 11.1 millimeters to global sea level since 1992—roughly 20 percent of the total global sea level rise during that period, according to the study.
What's more, a study published earlier this week in Environmental Research Letters shows that sea levels are rising at a rate of 3.2 millimeters a year. That's 60 percent faster than the latest estimate of 2 millimeters a year projected by the IPCC.
"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," Trenberth said by email.
As the World Meteorological Organization put it in a report released during this week's UN climate change talks in Doha, Qatar, "climate change is taking place before our eyes."
In addition to displacing millions, sea level rise may also supercharge large storms. For example, when a storm like Hurricane Sandy makes landfall, higher seas may boost storm surges that can strip away everything in their path and create damaging floods.
Sandy left at least 157 people dead and caused up to $80 billion in damage in hard-hit New York and New Jersey alone.
Predicting Future Climate Change
Study leader Shepherd hopes that climate modelers will be able to use this new data to better predict these consequences.
Until now, a modeler had to "choose an estimate of sea level rise from a pot of 40 ones with some uncertainty," he said.
Such a reconciled data set has been sorely needed, agreed Walt Meier, of the National Snow and Ice Data Center in Boulder, Colorado.
"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," said Meier, who was not involved in the new study.
The new study is in "a much more manageable range, and provides much better guidance in terms of future projections."
What's more, the study may even usher in a stronger model of another kind—scientific cooperation, Meier noted.
Instead of myriad groups working quasi-independently, the new study's co-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."