Another Greenlandic Saga: Are we getting a break on global warming?

A story on the fate of Greenland's ice sheet published last week in The Guardian attracted the expected level of interest from those who uncritically repeat any scientific tidbit that reminds us we still don't know everything we need to know about climate change. This was because the story, as written, implied the ice sheet isn't as sensitive to global warming as is popularly thought.

Something about the story didn't seem exactly right to me, though. For one thing, it was based not on a peer-reviewed paper but a presentation to the recent scientific meeting on climate change in Copenhagen. So I asked the climatologist behind the report for his take-home message, rather than the one that passed through the filter of reporter David Adam. As I feared, the news isn't quite as reassuring as Adam's story would have us believe.

Here's how The Guardian story begins:

Greenland ice tipping point 'further off than thought'

The giant Greenland ice sheet may be more resistant to temperature rise than experts realised. The finding gives hope that the worst impacts of global warming, such as the devastating floods depicted in Al Gore's film An Inconvenient Truth, could yet be avoided.

Which is semantically correct, as is the rest of the story. But it doesn't quite capture the nuances of the study in question. The basic idea, according to Johnathan Bamber, who is currently at the University of Colorado, is that there is still plenty of uncertainty about what will happen to Greenland's ice sheet if the world warms by 2 or 3 °C on average. The sheet could survive, or it could collapse, depending on a several factors. That's not the interesting part. What Bamber's studies suggest is that the temperature increase at which that uncertainty drops off and a big melt become much closer to certain -- the point at which the "surface mass balance" goes negative and initiates a positive feedback loop of melting -- is about 3° higher than we thought.

Until now, many climatologists who talk about the consequences of a melting Greenland, typically shared the view expressed in a 2004 paper in Nature that came with this scary abstract:

The Greenland ice-sheet would melt faster in a warmer climate and is likely to be eliminated -- except for residual glaciers in the mountains -- if the annual average temperature in Greenland increases by more than about 3 °C. This could raise the global average sea-level by 7 metres over a period of 1,000 years or more. We show here that concentrations of greenhouse gases will probably have reached levels before the year 2100 that are sufficient to raise the temperature past this warming threshold.

Bamber and his colleagues have been examining those conclusions for a few years now. For example, in a 2007 paper in Geophysical Review Letters, he and colleagues in the U.K. and the Netherlands compared methods of predicting what might happen to the GIS under warming scenarios. They concluded that

... large uncertainties in estimates of the future surface mass balance response of the ice sheet remain. Substantial increases in ice velocity have recently been observed in Greenland.... We suggest, therefore, that our ability to predict the future behavior of the GIS is constrained not only by uncertainties in modeling ice dynamics but equally by our ability to adequately model the surface mass balance.

Bamber says new modeling now suggests that that threshold, the global average temperature increase beyond which "the ice sheet is unsustainable... [where] nothing can can keep it there" is more like 6 °C, which means about 8 °C locally.

If Bamber is right -- and remember, he hasn't published a paper on this yet -- then at least it's not bad news. "You could argue that this particular critical threshold ... [is] not quite as bad as we thought," he says. On the other hand, "this threshold is a little bit artificial. The ice sheet could disappear below that threshold."

Either way, Bamber notes, "It's an urgent thing to investigate." After all, the basics of planetary thermodynamics remain: "You warm up the planet, and ice melts. That's the bottom line."

Oliver Morton at Nature's Climate Feedback blog offers this relatively worthy summary:

On the face of it that's a bit of a reprieve: it would seem to suggest that there's more time to act before the world gets committed to a big, big sea level rise than had been thought. But there are lots of caveats. Ice dynamics or some other factor could mean that there's a point of no return before the point at which the mass balance goes negative. And though this model may be better than the previous one (and there may well be people who would doubt that) that doesn't make it definitive. You can look at the best science around -- but there's always going to be doubt as to whether its good enough.

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