Like Olive, I’m at the Climate Change Congress in Copenhagen. My first impression is that it’s like a scaled up version of the Avoiding Dangerous Climate Change meeting that took place in Exeter in 2004: a venue for up-to-date (ie more recent than the most recent IPCC) science to be presented in a polciy relevant atmosphere. And by odd coincidence the first paper I heard presented was on a topic that in my mind is quite strongly linked to that Exeter meeting.
Jonathan Bamber of Bristol University was talking about the stability of the Greenland ice sheet at a session on tipping points. There was a widely cited Brief Communication in Nature that came out just after the Exeter meeting and was discussed there which suggested that if global temperatures rose above 3 degrees the ice sheet was effectively doomed. Bamber and his colleagues have looked at the issue again, treating the point at which the surface mass balance goes negative as the defintive oops-we-lost-Greenland point. Surface mass balance is snowfall minus the run offs. Even with a positive surface mass balance it’s possible for the ice sheet to shrink, because ice is lost through the calving of icebergs as well as runoff; once the surface mass balance goes negative, though, shrinkage is taken to be certain and to feed on itself.
Bamber says he and his colleagues looked at the future of the ice sheet in a warming world using two different types of mass balance model: a positive-degree-day model, which counts days over freezing, as was used in the earlier work, and a more complex energy balance model. Their positive-degree-day model showed the mass balance going negative with four degrees of warming locally, which because warming in the arctic is amplified beyond the global mean fits with the earlier figure of 3 degrees warming worldwide. The energy balance model, though, doesn’t see the mass balance go negative until there’s 8 degrees of warming. Bamber’s clear that there’s a lot of uncertainty in that — but it fits with the palaeoclimate finding that in the previous interglacial period, when temperatures were higher than they are now, significant chunks of the Greenland ice sheet remained un-melted.
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.