May 17, 2013
A Third Of Global Sea Rise Caused By Melting Glaciers
April Flowers for redOrbit.com - Your Universe Online
Scientists have known for some time that melting glaciers are contributing to the global sea-level rise. However, the amount being contributed by each region of the planet has never before been calculated with the accuracy of a new study led by researchers at the University of Colorado at Boulder and Clark University.
Ninety-nine percent of Earth´s land ice is locked up in the Greenland and Antarctica ice sheets. The study, published in the journal Science, reveals that the other one percent of glaciers around the world contributed just as much to sea rise as these two massive ice sheets combined from 2003 to 2009.
Although all glacial regions lost mass during the study time period, the biggest ice losses came from Arctic Canada, Alaska, coastal Greenland, the southern Andes and the Himalayas. Outside of Greenland and Antarctica, the glaciers lost an average of roughly 260 billion metric tons of ice annually, causing oceans to rise by 0.03 inches per year.
To estimate ice loss for glaciers in all regions of the planet, the international team of researchers compared ground measurements to satellite data from NASA´s Ice, Cloud and Land Elevation Satellite (ICESat) and the Gravity Recovery and Climate Experiment (GRACE) missions.
"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 geography professor Alex Gardner of Clark University. "These smaller ice bodies are currently losing about as much mass as the ice sheets."
"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 glaciologist at CU-Boulder's Institute of Arctic and Alpine Research. "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.”
ICESat, which measured glacier changes using laser altimetry — basically, bouncing laser pulses off the ice surface to determine changes in the height of ice cover — ceased operations in 2009. Still operational, the GRACE mission detects variations in Earth's gravitational field resulting from changes in the planet's mass distribution, including ice displacements.
The satellite data has limitations, however. ICESat did not have sufficient sampling density to study small glaciers, while GRACE does not have a fine enough resolution for measuring small glaciers. The mass change estimates created by the two satellites for large glaciated regions agree well, according to the research team, which consisted of 16 scientists from 10 countries.
"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 fellow at the university's Cooperative Institute for Research in Environmental Sciences.
Ground-based estimates of glacier mass changes are based on measurements from a glacier´s summit to its edge. These measurements are extrapolated over a glacier´s entire area. The research team says that while such measurements are fairly accurate for individual glaciers, they tend to cause scientists to overestimate ice loss when extrapolated over larger regions, including individual mountain ranges.
Tobias Bolch of the University of Zurich adds, “We are well aware of 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.”
If all the glaciers in the world were to melt, current estimates predict that they would raise sea level by about two feet. In contrast, an entire Greenland ice sheet melt would raise sea levels by approximately 20 feet. If Antarctica lost its ice cover, sea levels would rise a catastrophic 200 feet.