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Jay Zwally on melting at Greenland’s largest outlet glacier

NASA glaciologist Jay Zwally talked to EarthSky about the progress of melting in a large glacier at the western edge of Greenland.

Jay Zwally, a glaciologist at NASA’s Goddard Space Flight Center, talked to EarthSky about the progress of melting in a large glacier at the western edge of Greenland.

Jay Zwally: Jakobshavn Glacier is interesting because of what’s happened to it, and also because it’s the largest outlet glacier in Greenland. It discharges about 5 percent of the ice that leaves the Greenland ice sheet.

Zwally said part of this glacier used to extend into the sea and float on sea water.

Jay Zwally: This is called an ice tongue, and at the end of the ice tongue, the ice caps off into very large icebergs. It’s believed that maybe the iceberg that sank the Titanic came from this glacier.

He said the ocean has gotten warmer in recent years.

Jay Zwally: And as the temperatures were warming, the floating part started to thin. As it got thinner, it could no longer stay in place there. And it disappeared over a period about eight years. Now, the importance of this is that when this floating part went away, it allowed the ice behind it, from the ice sheet, to start flowing faster. And the speed doubled in a period of about five years or so. The glacier is also melting on its surface, because air temperatures have gotten warmer as well.

Zwally said Greenland’s melting ice is likely to contribute to a significant amount to sea level rise over the next 100 years. Zwally explained how melting happens on Greenland’s ice sheet.

Jay Zwally: About 80 percent of Greenland is covered with ice. The ice is very thick, up to two miles thick in the center. And it grows in the center from snow coming in over the whole ice sheet and then the ice slowly moves towards the edges. Some of the ice moves out into the ocean through what are called outlet glaciers. At other parts, it terminates on the land and melts, and then the water runs off into lakes and streams and then into the ocean.

Zwally said that it’s believed Greenland’s ice sheet has been in balance, until recently. Ice flows from the center of the ice sheet toward the oceans all year long. Every summer, ice melts over the lower 13 % of the ice sheet surface. The ice lost to melting and icebergs is approximately replaced by new snow falling all over the ice sheet, even at high elevations in summer

Jay Zwally: If there’s an imbalance between the amount of snow that’s coming in and staying on the ice sheet (called “accumulation”) – and how much is flowing out and melting – then the difference contributes to sea level rise or fall. Greenland is now contributing to sea level rise. So it’s very interesting because as climate changes, the amount of melting changes and the amount of precipitation changes the amount of accumulation. And as a result, some of the glaciers change their speed.

EarthSky asked Dr. Zwally how scientists know that climate change is affecting the glaciers.

Jay Zwally: Well, we think it’s climate change because we know that the temperatures have been increasing in Greenland. The air temperatures have been increasing by about 1.7 degrees Centigrade in about 10 years. So it’s getting warmer, and this increases the melting. And we also know that the ocean temperatures, particularly around the Jakobshavn Glacier, have been getting warmer. And warmer water’s coming up along the southwest coast of Greenland, and so that warmer water, in addition to the air temperatures, has been attacking the floating parts of the glaciers.

Deborah Byrd

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