So says Howat et al. in Science (why hasn’t this made it into the blogosphere before? Or did I miss it?). Interestingly, though the most recent change is a decrease: Using satellite-derived surface elevation and velocity data, we find major short-term variations in recent ice discharge and mass-loss at two of Greenland’s largest outlet glaciers. Their combined rate of mass-loss doubled in less than a year in 2004 and then decreased in 2006 to near the previous rates, likely due to fast re-equilibration of calving front geometry following retreat. Total mass-loss is a fraction of concurrent gravity-derived estimates, pointing to an alternative source of loss and the need for high-resolution observations of outlet dynamics and glacier geometry for sea-level rise predictions. This is the first evidence I’ve seen that some of the recent increases in glacier flow may just be fluctuations, though it was always a possibility.
And there appear to be some issues in resolving this with GRACE, or perhaps rather suggestions that these glaciers weren’t producing the loss GRACE saw: Other GRACE observations suggest a 450 Gt ice-loss from south Greenland between May 2004 and April 2006 that the authors mostly attribute to increased discharge from HH and KL (20). While the timing of the increased loss agrees well with the KL/HH acceleration, our results suggest that the combined loss from these glaciers over this period can only account for 13% of this loss. Absent an extensive, but unobserved, acceleration elsewhere, measurements for other south Greenland glaciers suggest a 2000 to 2005 loss increase of roughly 23 Gt/yr (1). This suggests that despite large dynamic changes, much of the 2004-2006 loss estimated from GRACE may be related to surface balance anomalies or other causes.
Incidentally… although the title is fair enough, I wondered if it was interesting that it was titled “changes” rather than “decreases”. But then I discover that the Rignot article on increases last year was titled “Changes in the Velocity Structure of the Greenland Ice Sheet” so its fair enough.
[Update: thanks to MW for ref to http://tierneylab.blogs.nytimes.com/2007/02/08/greenlands-glaciers-take-a-breather/ -W]