Biggest Melt Comes From Smallest Glaciers

Andrea Thompson, LiveScience.com Yahoo News 16 Sep 08;

The big glaciers of Greenland get most of the attention in terms of global warming's impact on melting and rising sea levels, but it's actually the little glaciers that count the most, a new study finds.

Satellite observations of the Greenland Ice Sheet indicate that nearly 75 percent of the ice lost there actually comes from the island's small coastal glaciers.

The study's authors say that this finding means small glaciers should be better-observed than they currently are in order to get a better handle on potential contributions to sea level rise.

The team's measurements of melt agree more with the lower end of the range of predictions, on the order of 100 Gigatons of ice melting, versus 200 Gigatons. Study team member Ian Howat of Ohio State University's Byrd Polar Research Center said the higher estimates were made with methods that are still not completely reliable.

Large vs. small

Outside of Antarctica, Greenland has more ice than anywhere else on Earth. Its ice cap covers four-fifths of the island's surface and is 1,491 miles (2,400 kilometers) long, 683 miles (1,100 km) wide, and can reach almost 2 miles (3 km) in thickness at its thickest point.

As global temperatures rise, coastal glaciers flow more quickly out from the ice sheet and into the sea, with massive chunks breaking off and forming icebergs in the ocean. While some of the largest of these glaciers, such as Jakobshavn and Petermann (which just experienced a large breakup) are closely monitored, smaller glaciers are not.

"The coastline is just dotted with [small] glaciers," Howat said.

Howat and his colleagues used observations of the southeastern region of Greenland from two ground-observing satellites to estimate the contribution to total ice loss from both big and small glaciers. While the two largest glaciers in that area, Kangerdlugssuaq and Helheim, contribute more to the total ice loss than any other single glaciers, the 30 or so smaller glaciers in the area accounted for about 72 percent of the total ice lost.

"What we found is the entire strip of ice over the southeast margin, all of these glaciers, accelerated, and they are just pulling the entire ice sheet with it," Howat said.

Better observations needed

Howat says that these findings on the current state of ice melt, funded by NASA and detailed in the Sept. 9 issue of the journal Geophysical Research Letters, suggest that scientists can't just concentrate on monitoring the big glaciers.

"We need to be observing the whole ice sheet," he told LiveScience.

Scientists also need to take more frequent observations of melt Howat said; now, they rely more on satellite pictures of large glacier breakups.