In this week’s issue of Nature: Ice sheets thinning

Phil Camill — Thu, 15 Oct 2009 18:35:18 +0000

Most people are aware that the loss of ice from land masses like Greenland and Antarctica have the potential to raise sea level. One of the current concerns about polar ice loss is that as sea level rises, ice loss is accelerating.How does this work? Much of the ice along the margins of these land masses is what we call “grounded” –it physically stuck on the rough land lying under the coastal ocean. Imagine a block of ice sitting on a dinner plate, which, in turn, is sitting on something rough like a carpet. If you tilt up one end of the plate just a bit, gravity causes the ice to slide off until it hits the carpet, where it gets stuck. Most of the ice block remains on the plate but a bit is wedged (grounded) into the carpet. This is what basically happens to many of the glaciers flowing off the Greenland and Antarctic land masses.

Here’s the bad news:

As sea levels rise, the oceans are beginning to lift up the ice along the margin from underneath (because ice floats in water). When this happens, these large glaciers become ungrounded, and they slide off the land masses into the ocean (what scientists call “accelerated flow”). Back to our ice block: Imagine lifting it from the carpet side…what happens? It slides completely off the plate.

Some of these moving glaciers can lose between 100 m – 1 km of ice per year. With all of this flow out to the oceans, eventually, the mass of ice on the land mass begins to thin (what scientists call “dynamic thinning”).

The news coming out of Greenland and Antarctic confirms this is happening more than once thought. In this week’s issue of Nature, Pritchard et al.¹ use laser altimeters abord NASA satellites to map changes in thickness of polar ice to more accurately assess the rate of dynamic thinning.

Bottom line: In Greenland the glaciers studied thinned about 84 cm per year. In Antarctica, the glaciers studied thinned at a whopping 4-9 m per year, and the thinning looks to be penetrating more than 100 km into the continental interiors. The magnitude and extent of thinning is more than was previously known.

How this translates to instability of these ice sheets is still unknown. The West Antarctic Ice Sheet and Greenland each have enough ice to cause sea levels to rise ~ 6 m. Stay tuned…

¹Pritchard, H.D., et al (2009) Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets. Nature 461:971-975.

photo credit: http://www.flickr.com/photos/rietje/ / CC BY-NC 2.0

Global Change » climate warming

In this week’s issue of Nature: Ice sheets thinning