A few weeks ago, in a posting on sea level rise, Eli pointed out that sea level rise lags warming by a considerable extent. This accounts for a 500 year delay between when you stop emitting CO2 and the final sea level.
(click to enlarge)
To repeat the top figure is the prediction for increasing CO2 by 1% a year for 70 years and then holding it constant, the bottom is for increasing CO2 by 1% a year for 140 years and then holding it constant. But, as was pointed out, there is a caveat to this prediction, that glaciers respond slowly to external forcings.
A recent feature article in The Earth Observer, v. 18(3), p. 8, 2006 casts strong doubt on that article of fate. The authors, Leigh Stearns and Gordon Hamilton report on measurements by others on two glaciers in southeast Greenland, Kangerdlugssuaq and Helheim. The flow has speeded up greatly, the calving front has rapidly retreated, and the ice surfaces are 100 m lower.
The 11-14 km/yr flow speed cannot be accounted for by internal pressure on the ice and the associated deformation.
The scientists speculate that warmer summer temperatures observed in this part of Greenland are melting increasing amounts of water that is subsequently stored in surface ponds. This melt water eventually reaches the glacier bed, lubricating the ice-rock interface, leading to acceleration. Kangerdlugssuaq and Helheim glaciers are not the only glaciers in Greenland known to be undergoing recent rapid changes. Jakobshavn Isbræ, a tidewater glacier at a comparable latitude (69º N) in west Greenland accelerated 30% between 2000 and 2003 [Joughin et al., 2004] and retreated more than 3 km over the same period [Thomas et al., 2003]. The picture emerging from these observations is one in which large changes in ice dynamics can occur on short timescales of a few years. If the mechanism triggering the changes of Kangerdlugssuaq, Helheim and Jakobshavn affects other Greenland outlet glaciers, which currently show stable ice dynamics [Stearns et al., 2005], the mass balance of the ice sheet will become increasingly negative, unless balanced by an equal increase in snow accumulation, and rates of sea-level rise will increase much faster than current models predict.Oh well.
Update: If you have not already read Jim Hansen's article in the New York Review of Books do so. The base of his argument is the catastropic scenerio discussed in this posting.
Update II: The LA Times publishes a front page article on Greenland glaciers in a perfect storm of articles in the yellow press. And, for the calmer heads we have an earlier post from Real Climate. ThLA Times article was picked up by Political Animal, and other blogs (follow the trackback)
Update III: And to close the circle, Real Climate answers Eli's request for more within the hour. Good going guys:)