One of the stranger aspects of Greenland's rapid ice loss has been explained and it's as close to good news as this issue gets. The circumstances that lead to surface lakes suddenly disappearing, triggering more rapid ice melting, have been identified. While such disappearances are not going to stop, it looks like they may not spread as far as had been feared.
The rate of melting of glaciers in Greenland and parts of Antarctica has exceeded predictions, partly because early models did not account for the way that meltwater lubricates the bottom of glaciers, hastening their slide into the sea.
In 2008, one of the contributing factors to this was revealed when it was discovered that the ice beneath the lakes on the top of the Greenland ice sheet sometimes cracks suddenly, allowing entire lakes to pour a thousand meters (3000 feet) to bedrock in a few hours. Only large lakes do this, as smaller ones lack the volume of water to keep the cracks open. However, the process that produces these enormous cracks was unknown, as was the reason why about 13% of lakes vanish in this way, while many large lakes drain slowly if at all.
Laura Stevens, a graduate student at MIT, observed ice quakes where ice rose and moved sideways 6-12 hours before a large western Greenland lake vanished each summer from 2011–2013. She is first author of a Nature paper attributing these movements to water draining through smaller cracks. Once the water reached the bottom of the glacier it lifted the ice above it.
"Ordinarily, pressure at the ice sheet surface is directed into the lake basin, compressing the ice together.” Stevens says. “But, essentially, if you push up on the ice sheet and create a dome instead of a bowl, you get tension that stretches the ice surface apart. You change the stress state of the surface ice from compressional to tensional, which promotes crack formation."
Pressure from the water at the surface helps to widen the crack. In an example, Stevens observed that 40 billion liters (11 billion gallons) of water drained the 980 meters (3125 feet) from North Lake to the bedrock in 90 minutes. Stevens and her colleagues estimate that for a time the rate of flow exceeded that over Niagara Falls.
"You need both conditions – tension to initiate the crack and the large volume of water to amplify it – for hydrofractures to form," Stevens says. Without one or the other lakes can persist on the surface of the ice throughout the summer, freezing again in winter.
The good news is that Greenland's inland ice is flatter and has fewer crevices that could allow the small initial flows that initiate the bulging, suggesting that the rapid melting we are seeing around Greenland's periphery may not apply to the places where the ice is thickest. This in turn should mean that sea level rise will not accelerate as much as the most pessimistic projections suggest. “It's essentially a check on the inner ice starting to move along this fast conveyor belt,” Stevens says.