Blog Archive

Monday, July 14, 2008

Roderik S. W. van de Wal: Greenland ice sheet melt not slowing down

Greenland Losing Ice, With or Without Lubrication

by Andrew C. Revkin, New York Times, Dot Earth, July 14, 2008

Melting in summers on the Greenland ice sheet sends gushing torrents into natural drainpipes leading down toward bedrock. (Alberto Behar/NASA-JPL)

After the journal Science published a paper earlier this month concluding that summertime gushers of meltwater percolating to the base of Greenland’s ice sheet didn’t appear to speed the seaward flow of ice, one result was a burst of excited comments from bloggers and others asserting that the impacts of global warming have been hyped.

Greenland’s ice sheet exists through a balance of snow falling inland and ice melting or sliding into the sea around the edges. Here, meltwater glimmers. (Andrew C. Revkin/The New York Times)

Roderik S. W. van de Wal, the lead author of the Science paper, sent me a comment he prepared after the hubbub that he said is aimed at correcting many misinterpretations of the research (whether willful or not) — one of the most important being that Greenland is still losing much more ice than is being added through snowfall, and more losses will come in a warming world.

The note, reproduced with permission, is below. This post was held up by the flood of climate news last week out of the “major emitters” meetings in Japan and Washington:

What about the Greenland ice sheet?

R.S.W. van de Wal (IMAU, Utrecht University, the Netherlands)

A paper in Science (July 4, 2008) caused some rumor about the role of the Greenland ice sheet in the climate change debate. In the paper the authors argue that there is no evidence for a speed up of the ice marginal zone due to enhanced ablation rates, which by some people was explained as if Greenland was not contributing to sea level change any more. In order to understand what it really means in terms of sea level or climate change in general we have to go back to how an ice sheet works.

The Greenland ice sheet gains mass via snowfall and losses mass via the production of icebergs and by melt of ice in the ice marginal zone. If you add snowfall, melt and ice berg production over the entire ice sheet you know whether the ice sheet in total losses mass or gains mass. So an ice sheet can loose mass either by increased iceberg production, increased melt or decreased snowfall. Current estimates from satellites show that the ice sheet is losing mass, and it is predicted by the IPCC that Greenland will contribute modestly to sea level rise by about 10 cm over this century (just under 4 inches).

[Above: a nice New York Times graphic showing several mechanisms for Greenland’s ice shedding.]

There are however a few mechanisms, which might considerably increase this number and those are subject of intense scientific debate. First of all there is the interaction between the ocean and the ice sheet. During the beginning of this century several outlet glaciers, which are the glaciers producing the icebergs, retreated unexpectedly. This is still poorly understood and scientists monitor those glaciers with increased attention since then. Secondly, more recently we were surprised with the retreat of the sea ice in the Arctic Ocean during summer time. Disappearance of the sea ice likely leads to more absorption of sunlight and hence warming of the area. How this affects the ice sheet is yet unclear. Thirdly, we have the recent paper addressing the ice marginal zones of the ice sheet.

As about 50% of the mass loss occurs via the ice marginal zones it is important to study them. What the current paper shows is that the positive feedback between melt and velocities is not so important as expected over a period of 15 years. What is this feedback and why is it important at all? Ice moves from high and cold regions towards low and warm regions. There are indications that this movement is affected by the melt at the surface. It is shown in the paper and a few other studies that during strong melt events in summer the ice moves faster. The reason for this is that the melt water percolates through the ice to the bottom and lubricates the ice so that the friction reduces and the sliding of the ice increases. So, the positive feedback between melt and velocities implies that more melt leads to higher velocities, which bring in more ice from cold regions to warm regions which increases the melt and hence the velocity etc, with as a final result a rapid loss of ice and hence an enhanced increased sea level.

The Science study shows that it doesn’t work like that. Over a period of 17 years the ice sheet is not speeding up in the ice marginal zone, probably because the ice sheet gets more efficient in removing the water near the bottom if the amount of water at the surface increases. It acts as a sink where the drainage pipes widen as soon as you open the tap.

This study does not show that the melt is decreasing, to the contrary it shows a small increase in ablation which is fully consistent with IPCC predictions concerning melt of the ice sheet. So, no new alarm bells this time from the glaciologists, but the uncertainties concerning outlet glaciers and the effects of sea ice retreat are still in the air and imply that sea level rise estimates might need to be reconsidered.

It would all be so nice, in a way, if the science were simple. But it isn’t. That means society is going to have to make up its mind about climate policies and related energy choices without certainty on the level of threat posed by business as usual. And it means more time must be spent on those ice sheets, both in the melt zones and the places where accumulation of snow still dominates — including Swiss Camp, which I visited on the flanks of the ice sheet in 2004:

Swiss Camp, a research station maintained high on the flanks of Greenland’s giant ice sheet. (Credit: Andrew C. Revkin/The New York Times)

Link to post on the Dot Earth blog of the New York Times:

No comments: