Scientists project that sea level is likely to rise a foot or two this century as a result of climate change. But there's one important asterisk here: How will the Greenland ice sheet respond to global warming? This week, scientists report a bit of reassuring news.
When you fly over Greenland, it seems like one huge chunk of ice, frozen solid.
But if you're lucky enough to set foot on that ice during summer, the picture is very different. Some of Greenland gets warm enough for the surface ice to melt, creating streams across it. Last summer, Sarah Das followed one of those streams down to where it spilled into a lake.
Das is from the Woods Hole Oceanographic Institution in Massachusetts. She flew up to a seasonal lake as part of a project to learn the fate of melted ice. If this meltwater ends up beneath the ice, it can lubricate the ice sheet, making it flow faster to the coast and into the sea. Ultimately, this raises sea level.
A 3,000-Foot Waterfall
At the lake's edge, Das and her research partner, Ian Joughin of the University of Washington, find an impressive torrent of water. It's flowing out of the lake through a canyon it has carved through the ice.
"It's amazing. This feature was probably not here two years ago, and now there's a 60-foot channel ... with a roaring waterfall," Joughin says, adding that the water probably plunges 3,000 feet to the bedrock below.
The raging river comes to a wall of ice. The water takes a nose dive, straight down. This is called a moulin.
"It is quite an exciting discovery. It's amazing to be standing here right at the edge of it," Das says. "You really can actually feel the energy of the water diving down underneath you. It's amazing."
How Fast Does the Ice Flow?
What the researchers really want to know is how all this meltwater affects the flow of the broader ice sheet once it gets to the bedrock below.
"It's kind of hard to say what the impact one particular lake would have on the velocity of a large portion of the ice sheet," Das says. "But in general, it would seem that the ice sheet is speeding up quite a bit."
To figure out just how much faster the ice is flowing, Das and Joughin set up instruments to measure the ice's movement. Now, nine months later, they're getting some answers. The results are published online in Science Magazine. One of the most dramatic findings is that these temporary lakes can be really temporary.
"One of the lakes we were monitoring was about two miles wide and 40 feet deep, and it drained in the space of about 90 minutes, with flow greater than the flow over Niagara Falls," says Joughin.
So much water flowed under the ice, it actually lifted the glacier by three feet and sent it slip-sliding a bit toward the coast.
"We saw a brief speedup, but then a couple of days later really things didn't look much different than, say, they had the week before," says Joughin.
Satellite measurements showed the same thing was happening over hundreds of miles of western Greenland. So the bottom line is that all that meltwater does lubricate the ice, but the effect on global sea level is less than a quarter of an inch per decade. Joughin says this rate is likely to increase a bit as Greenland heats up.
"But it's not going to be a catastrophic effect, I don't think," Joughin says. "It's not likely to have major impacts over the next century. It may have a little impact but not a lot."
Not Worry-Free Yet
There are still big unknowns about Greenland's ice. Most notably, scientists don't know just how fast the big glaciers on the coast will tumble into the sea. So, Joughin and Das are feeling a bit more comfortable about the fate of the Greenland ice sheet in the coming century. That's not to say they are worry-free.
"Even a foot or two of sea level rise has huge economic consequences, and it's definitely something we need to have a much better understanding of," Joughin says.
Joughin and Das plan to head back to Greenland this summer to learn more about the melting ice as it slips toward the sea.