Tracking an Ocean of Ice Atop Greenland
A conversation with geologist Sarah Das
Sarah Das calls herself a “frozen oceanographer.” Most people look at Greenland and see a vast ice sheet covering Earth’s largest island. But Das sees a huge reservoir of water—temporarily removed from the ocean and solidified on land. The Greenland Ice Sheet is the second-largest concentration (after Antarctica) of fresh water on Earth; if it melted completely, global sea levels could rise up to 7 meters (23 feet).
In recent years, evidence has mounted that global warming may be accelerating the melting of land ice and sea ice in the Arctic. That melting would amplify warming even further, because while the white surfaces of ice reflect most incoming solar radiation back to space, darker ocean and land surfaces absorb it. In addition, enhanced ice melting could contribute a big slug of fresh water to the North Atlantic Ocean, which could potentially alter its circulation and cause dramatic regional and global climate change.
The surprise is that these changes in warming and melting may not be happening gradually, but quickly. The ice sheet may be responding to climate change in ways that scientists hadn’t anticipated. The reason, Das explains, is that ice sheets behave less like slowly thawing ice cubes, and more like pancake batter.
When you pour pancake batter in a griddle, it spreads out under its own weight. Ice does that, too. It flows downhill—eventually to the ice sheet’s outer margins, where it drains into the sea.
We’ve thought that the Greenland Ice Sheet, for the most part, moves and reacts slowly to climate changes. But from 1997 to 2003, the speed of Jakobshavn Glacier on Greenland’s west coast has doubled. Jakobshavn is the largest outlet glacier in Greenland and drains 6.5 percent of the entire ice sheet. On the east coast, the Kangerdlugssuaq Glacier, which drains 4 percent of the ice sheet, has also sped up in the last decade, from 5 kilometers (2.25 miles) to 14 kilometers (6.4 miles) per year.
Things are happening much faster than we ever thought possible. It shows that we don’t completely understand how ice sheets work and how they are responding to climate change.
What is making the ice sheets less monolithic and more mobile?
One thing that really controls the speed of a glacier is what is happening at the bed—where the land meets the bottom of the glacier. Conventional wisdom tells us that any warming near the ice sheet’s surface should take thousands of years to penetrate 1 to 2 kilometers of cold ice. Temperatures are near the freezing point at the bed, so the ice is colder there, and less water is available for lubrication. That creates friction. Anything that reduces friction between the ice and the ground, such as adding water, makes the ice sheet flow faster.
Does that mean the glacial beds are getting lubricated?
Our hypothesis is that there may be a connection between more melting on the surface and more water reaching the bed—controlled by the behavior of features called supraglacial lakes. Each summer around the margins of the Greenland Ice Sheet, surface ice melts and pools in low spots to form lakes, some many kilometers across. The lakes drain throughout the season through cracks in the ice, and when there is more melting, the glaciers appear to speed up. The lakes may concentrate water so that it causes the ice to fracture and water to be injected directly to the bed. That could really change the dynamics of the entire ice sheet.
How can water create such large cracks?
That is one of the big unknowns. I’ve been collaborating with another scientist in the WHOI Geology and Geophysics Department, Mark Behn, who studies rock fractures and magma propagation in the seafloor. We just received a WHOI interdisciplinary study award to see if the physics of his models for the seafloor can apply to ice sheets—to see, for example, what happens if you have a large enough pool of water that you drive down a small crack under the pressure and temperature conditions seen on the ice sheet. Will the ice fracture down to the bed?
Are more supraglacial lakes forming because of global warming?
We don’t know yet. We’ve just been funded by the National Science Foundation to look at several decades of high-resolution satellite data that provide snapshots of what’s happening on the ice sheet surface. We can look at how the distribution of lakes changes from year to year and correlate that with climate records from weather stations to determine whether warmer temperatures created more, or bigger, lakes. We can also look at how the lakes drained to understand when and why some lakes drain and some don’t.
We’re also going to several representative lakes next summer. From the Greenland coast, a helicopter will take our team, tents, emergency equipment, and scientific gear about 100 kilometers up the ice, drop us off for a few weeks to work, and move us between field sites. We will put out instruments to measure how fast the ice is moving and to record lake levels and temperatures. We’ll also deploy some seismometers that can record seismic waves caused by any ice fracturing under the lake.
What is life on the ice like?
Being on an ice sheet sort of feels like being at sea, only everywhere you look it’s ice instead of ocean. There is sunlight 24 hours a day. The sun doesn’t really go up or down, it just goes around and around and around. There are subtle variations in how high and how warm it is at different times of day, but you really have to pay attention to notice that. And you are living in the cold, day in and day out. Many times, there isn’t a place you can go to warm up.
You are really isolated and depend on your co-workers. Everyone does everything that needs to be done. You don’t have the hierarchy you have in the rest of your life.
How did you get into this line of work?
I spent summers with my grandparents on Cape Cod. It was the best time of year for me. I fell in love with oceans and said I was going to be an oceanographer. I majored in geology in college and did a semester at sea aboard SSV Westward, Sea Education Association’s ship, on a research cruise out of Woods Hole.
Then someone told me about a summer field course in Alaska. We went up there with our backpacks, our skis, and our senses of humor and traversed the Juneau Ice Field, stopping at huts along the way, doing science, learning how to rescue ourselves from crevasses, climbing mountains, telemark skiing. Man, that was even more exciting than oceanography.
My first job after college was as a field technician with a Cal Tech research group. They needed someone willing to leave routine life for three months and live out of a tent in Antarctica. We drilled boreholes to collect samples and study conditions beneath the ice.
Beyond the adventure, I became amazed and impressed by how important ice sheets are. They are critical to the climate system and sea level. And they aren’t just big blocks of ice; they are also archives of Earth’s climate history.
Near their summits, where the ice sheets don’t melt much, every year’s snowfall is stacked up and preserved—like annual tree rings. You can drill a core through the ice, analyze ice samples, and get a record of climate back through the ice ages and the warm periods between ice ages. In November, a new study, using gas bubbles trapped in ice 3 kilometers below Antarctica’s surface, reported that current levels of greenhouse gases in Earth’s atmosphere are higher now than at any time in the past 650,000 years.
What do you do when you’re not doing scientific work?
My husband and I bought a sailboat cheap and fixed it up, and we sail in Boston Harbor. This summer I raced in the first annual Boston Harbor Islands Regatta in a special class for female skippers, and I took second place. I do a lot of hiking, biking, skiing, kayaking, gardening—anything that gets me outdoors. For the last few years, I have also enjoyed doing triathlons.
Does that training come in handy on the job?
Absolutely. When I take my lunch break and go for a run I think, “Well, this is for work.” When you’re dropped off by a plane at 10,000 feet, and you have to set up camp, drill an ice core, and make dinner in three hours, you need to be in great shape.