Climate: Study quantifies sea level rise from melting glaciers

‘A little bucket with a huge hole …’

Alpine glaciers like the Dachstein have been melting quickly. A new study shows that ice melt from glacial regions outside Greenland and Antarctica contribute significantly to sea level rise. Bob Berwyn photo.

By Summit Voice

FRISCO — The world’s major ice sheets — on Greenland and Antarctica — haven’t really started a major meltdown yet. But the rest of the world’s glacial regions have been losing ice at a rate of about 260 billion metric tons annually, raising sea level by about 0.03 inches per year — about a third of the observed sea level rise.

The biggest ice losses are happening in Arctic Canada, Alaska, coastal Greenland, the southern Andes and the Himalaya. Combined, the areas contribute as much to sea level rise as melting from the major ice sheets, which lock up about 90 percent of the Earth’s land ice, according to a a new study led by Clark University and involving the University Colorado Boulder.

“Because the global glacier ice mass is relatively small in comparison with the huge ice sheets covering Greenland and Antarctica, people tend to not worry about it,” said CU-Boulder Professor Tad Pfeffer, a study co-author. “But it’s like a little bucket with a huge hole in the bottom: it may not last for very long, just a century or two, but while there’s ice in those glaciers, it’s a major contributor to sea level rise,” said Pfeffer, a glaciologist at CU-Boulder’s Institute of Arctic and Alpine Research.

“For the first time, we’ve been able to very precisely constrain how much these glaciers as a whole are contributing to sea rise,” said lead author Alex Gardner, an assistant professor of geography at Clark University in Worcester, Mass. “These smaller ice bodies are currently losing about as much mass as the ice sheets.”

Previous estimates on the contribution of glaciers have differed substantially, partly due to the weaknesses of the individual satellite methods.

“However, in highly glacierized regions the results obtained using the two different methods agree well. With the mix of methods that have now been tested and applied, we have come a major step closer to determining glacier mass loss with higher precision,” said University of Zurich geographer Tobias Bolch.

The study compared traditional ground measurements to satellite data from NASA’s Ice, Cloud and Land Elevation Satellite, or ICESat, and the Gravity Recovery and Climate Experiment, or GRACE, missions to estimate ice loss for glaciers in all regions of the planet.

ICESat, which ceased operations in 2009, measured glacier changes using laser altimetry, which bounces laser pulses off the ice surface to determine changes in the height of ice cover. The GRACE satellite system, still operational, detects variations in Earth’s gravity field resulting from changes in the planet’s mass distribution, including ice displacements.

GRACE does not have a fine enough resolution and ICESat does not have sufficient sampling density to study small glaciers, but mass change estimates by the two satellite systems for large glaciated regions agree well, the scientists concluded.

“Because the two satellite techniques, ICESat and GRACE, are subject to completely different types of errors, the fact that their results are in such good agreement gives us increased confidence in those results,” said CU-Boulder physics Professor John Wahr, a study co-author and fellow at the university’s Cooperative Institute for Research in Environmental Sciences.

Ground-based estimates of glacier mass changes include measurements along a line from a glacier’s summit to its edge, which are extrapolated over a glacier’s entire area. Such measurements, while fairly accurate for individual glaciers, tend to cause scientists to overestimate ice loss when extrapolated over larger regions, including individual mountain ranges, according to the team.

Current estimates predict if all the glaciers in the world were to melt, they would raise sea level by about two feet. In contrast, an entire Greenland ice sheet melt would raise sea levels by about 20 feet, while if Antarctica lost its ice cover, sea levels would rise nearly 200 feet.

The study involved 16 researchers from 10 countries. In addition to Clark University and CU-Boulder, major research contributions came from the University of Michigan, the Scripps Institution of Oceanography in San Diego, Trent University in Ontario, Canada, and the University of Alaska Fairbanks.

Built by Ball Aerospace & Technologies in Boulder, NASA’s ICESat satellite was successfully operated from the CU-Boulder campus by a team made up primarily of undergraduates from its launch in 2003 to its demise in 2009 when the science payload failed. The students participated in the unusual decommissioning of a functioning satellite in 2010, bringing the craft into Earth re-entry to burn up. ICESat’s successor, ICESat-2, is slated for launch in 2016 by NASA.

A paper on the subject is being published in the May 17 issue of the journal Science.


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