Also in this week's Science are two articles that further strengthen the case that ice sheets are quite sensitive to warming climate. A paper by Göran Ekström et al. shows that the increased speed of Greenland glaciers occurs in distinct lurches (observed as micro "ice-quakes") that are strongly seasonal, with the greatest number occuring in late summer. This provides evidence that meltwater plays an important role in the acceleration of Greenland's glaciers. Essentially, the idea is that surface melting that occurs in the summer can make its way quickly down to the glacier bed, lubricating the bed and allowing the glaciers to slide more rapidly. The "ice quakes" occur because the rough bedrock surface causes the glaciers to stick; they only accelerate when enough hydraulic pressure has built up to help float the glacier over the bumps. This is strong evidence that climate, not merely "internal ice
sheet dynamics", has contributed to the recent increases in Greenland's glaciers. Indeed, a doubling of the rate of quakes has occurred over the past five years, just as the aerial extent of surface melting has increased.
So, on top of everything else we have "lurching glaciers".