When you see the Greenland Ice Sheet for the first time, it’s very difficult to understand, intuitively, that it’s undergoing significant changes. It’s enormous, and very still. The nearest simile I can come up with is that it’s like looking at the ocean in slow motion. With a closer look, however, you start to notice all the strange and subtle things that are occurring. At the ice edges, the landscape is like a construction site, as the ice bulldozes rocks into haphazard piles. Moving inland, you notice rivers, not unlike the ones perhaps meandering through your town, but carving their channels through ice and flowing impossibly fast due to their steepness and smoothness (sometimes they just disappear into frightening voids in the ice). Moving even further up, you’ll find yourself in a snow-swamp or on a lakeshore, both of which mark the beginning of the portion of the glacier that never melts. Even higher towards the top of the ice cap, if you spend enough time there, you may find that you start to notice subtle hills and valleys that are barely noticeable amidst the expansive, blank, white plain. The cycle of snow turning to ice, ice melting into water, and water running off the surface and down into the depths of the glacier are on display here, and each part of this cycle is connected with its other parts. The trick is knowing where to look for these connections.
It’s important to understand, however, the difference between observing all of the (understated) drama at the surface of an ice sheet, and understanding how they are all connected in a way where you could start to make predictions. It’s even more difficult to extrapolate these observations to things that you can’t see, like where does all of that water flowing off of the surface, and into holes in the ice, go? Does it reach the bottom of the ice sheet and flow into the ground, or does it get squeezed out the sides of the ice sheet between the glacier and the bedrock? And do these questions even matter for humans, particularly in the context of a changing climate? As it turns out, the answer to this last question is very likely yes. What happens to water at the surface is very important in determining how much of the Greenland Ice Sheet turns into water that goes into the ocean, even beyond the amount that melted in the first place. The answer to the question of where does the water flowing into holes go, is that it mostly gets squeezed out the sides. We know this because there are very large rivers that flow out from under the ice sheet at its edges.
Thus, we can watch water go in, and we can watch water go out, but what happens in between? This is a very difficult problem. How do you observe the bed of an ice sheet? One thing that we can do is to drill holes through the ice and measure the pressure of the water, and what the bed of the glacier looks like. These are powerful measurements, and they can tell us many things, like how fast the water is flowing, how long it’s been since the water flowed in from the surface (if that’s where it came from), and how hard it’s pushing back on the ice above it. This last bit is important because ice can, in some cases, when the water pushes back hard enough, float. When this happens, the ice starts to slide, and the ice sheet can start to deliver more ice to low elevations (where it’s warm), or directly into the ocean, which is what drives sea level rise. So why not just drill holes everywhere? Drilling a hole through an ice sheet is incredibly expensive, and we couldn’t possibly drill enough to say something about what the water is doing everywhere on an ice sheet bed.
The alternative, and what I do in my research, is to come up with models that simulate the movement of water under an ice sheet, as well as the ice itself (because ice is fluid when it gets deep enough). This sounds complicated, and in a practical sense, it is. However intuitively, a model ice sheet operates much like a model airplane; the parts are simplified and scaled down into a machine that we can work with at home, in the office, on a computer. This allows us to make educated guesses about what might be happening down there (hypotheses, if you will), and to test them without having to go to the ice every time. We determine how good our hypotheses are by comparing the results of our models with the data that we collect in the field (like the pressures that we measure in the few holes in the ice that we have the resource to drill). More often than not, we get it wrong; but this is a great opportunity to rethink our beliefs about the factors that influence glacier movement. Sometimes this means that we learn about some key piece of data that we didn’t know that we needed, and this educates what we do in the field next time. This process of trial and error is ongoing, and makes for an exciting research environment, where we continually test and reform the theory about ice sheet movement, made more exciting by the importance of ice sheets to climate change. And it’s very satisfying when you find that your snippet of computer code accurately simulates some component of the natural world.
– Doug, University of Alaska Fairbanks, USA
In the first picture of your post there are dark lines in the ice. What causes these dark lines?
Hi Mary,
Note that this is only speculation, but we would interpret this as glacier stratigraphy. Snow falls in the accumulation zone of a glacier, then dirt and rocks fall on it and accumulate dark material later in the season (after snow stops falling). This structure gets preserved as it flows through the glacier, and these patterns are the expression of these layers emerging much lower, in the ablation zone of the glacier. These lines might provide insight into the paths that ice takes through the glacier, and also about the rate of snow fall before measurements of such things began. But to our knowledge no one has investigated this yet. Yet another example of the open scientific avenues remaining in glaciology!