THEGREENGROK    Planetary Watch

On Thinner Ice

by Bill Chameides | September 29th, 2009
posted by Erica Rowell (Editor)

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The melting of Antarctica’s ice sheet could cause sea levels to rise 186 feet. So how fast is it thawing? (NASA)

New satellite, new instrument, same result: glaciers are thinning.

As we all know, when temperatures rise, ice melts, and so it’s not surprising that global warming has led to widespread shrinking of ice globally. (See here and here for numbers.) Of particular concern is the melting of grounded glaciers, because when they melt, their water flows to the oceans and adds to sea level rise. Of even greater concern is the melting of the Greenland and Antarctic ice sheets, the two largest reservoirs of frozen freshwater on the planet — collectively, they contain about 99 percent of all freshwater ice on Earth.

  • The melting of Greenland‘s ice would cause sea levels to rise about 20 feet;
  • The melting of the Antarctic’s ice would cause sea levels to rise about 186 feet.

So among the “$64,000 questions” in trying to suss out how much time we have to cut greenhouse gas emissions before climate change bites us in the butt are the following.

  • How fast are the Greenland and Antarctic ice sheets melting?
  • Will that pace increase significantly in the future?

When It Comes to Glacial Thaw, Time Is Not on Our Side

Earlier concepts of glacial dynamics suggested that ice-sheet melting would play itself out over many centuries if not millennia and all the melting would occur along the glacier surface. In summer, warm atmospheric temperatures cause meltwater to form on the glacier’s top where some of the meltwater evaporates while the rest refreezes when summer ends. So the loss of glacier mass is limited by the rate at which the summertime meltwaters evaporate. It is a slow process.

Greenland ice flow.

We now know that there are likely to be much more rapid pathways. Fissures and cracks on the glacial surface — called moulins (see New York Times graphic) — allow meltwaters to flow to the glacier’s base and then into the ocean (the meltwater also acts  as a lubricant allowing the whole glacier to slip more rapidly toward the ocean). And warm seawater coming in contact with the glacier along its edge can melt the glacier from the bottom up. Both processes can cause an accelerated loss of ice as a result of warm-weather events, generally referred to as dynamic thinning.

Three Main Ways We Monitor Ice Sheets and Their Shortcomings

How important are these other processes? Might they undermine the integrity of the entire glacier and cause its precipitous slide into the ocean? We don’t really have the answers yet, but we need them and for that reason scientists have been busy over the past decade or so carefully monitoring the Greenland and Antarctic ice sheets. Thus far three methods have been used:

  1. Flux imbalance measurements: The total change in the glacier mass is estimated from the difference between measurements of snow accumulation in the glacier’s interior and the flow of ice and water at its edges into the ocean. (See here, here and here.)
  2. Satellite gravity measurements: Glacier mass change is inferred from satellite measurements of the gravitational”pull” of the glacier relative to the background “pull” of the Earth. (See here and here.)
  3. Ice sheet elevation: Changes in mass are estimated from the changes in the height of the ice sheet using radar measurements.

Each method has its advantages and disadvantages. One of main issues preventing scientists from getting a complete picture of how ice sheets behave is the spatial resolution of the data. However, with new instrumentation flying on new satellites the data have been getting better and better.

New Satellite Monitor Shows Ice Cover Strikingly Thin in Places

This image of Greenland shows the changes in elevation over the Greenland ice sheet between 2003 and 2006. Gray areas indicate no change in elevation. The white regions indicate a slight thickening, while the blue and purple shades indicate a thinning of the ice sheet. (NASA)

In a new paper published in Nature, Hamish Pritchard of the British Antarctic Survey and colleagues report on the latest hi-tech method for following ice elevation using the Geoscience Laser Altimeter System (GLAS) on board NASA’s Ice, Cloud and land Elevation Satellite (ICESat).

While flying on high on ICESat, GLAS aims a laser at the Earth and measures the time it takes for the light to bounce off the surface and return to the satellite. By very, very accurately measuring that time, the distance between the Earth’s surface and satellite can be calculated. Repeating these measurements as the satellite flies over the glacier allows the glacier’s topography to be mapped. Repeat that process from year to year, and a picture of the glacier’s elevation changes over time emerges.

The elevation changes for Greenland and Antarctica reported by Pritchard and co-authors are complex with some regions over both ice sheets showing modest elevation increases (i.e., less than a foot per year). But there were a number of regions with striking thinning, especially near the ice sheet margins. Between 2003 and 2007, faster flowing glaciers on Greenland were found to be thinning at an average rate of almost three feet per year, and in one area of Antarctica (near the Amundsen Sea) glaciers thinned at more than 30 feet per year.

The largest rates of thinning were found along the ocean margins of the glaciers, suggesting that contact with warm ocean waters are in fact accelerating glacier melting by undermining them from the bottom.

Reading this paper produces a kind of “golly gee” reaction. “Golly,” we can bounce light from a satellite-mounted laser to the Earth and back and measure variations in surface heights below to an accuracy of up to one inch. And “gee,” Antarctica is thinning in some places at tens of feet per year. Yeah, gee.

More on Glaciers

NASA’s At the Edge: Monitoring Glaciers to Watch Global Change

filed under: Antarctica, climate change, faculty, global warming, Planetary Watch, temperatures, water
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