Two weeks ago on Climate Monday, I highlighted some different visualizations of sea ice loss in the Arctic. Monitoring the sea ice regime is important for knowing the limits of human navigation, resource extraction, and other activities in the Arctic, but the subsequent decline in land ice has a much broader impact on humans because melting land ice leads to sea level rise. You may have seen time lapses of retreating glaciers before, like this time lapse of Columbia Glacier in Alaska. That is dramatic and provocative, but in the long term, the two most important sources of ice melt are Greenland and Antarctica.
Artistic depiction of the GRACE satellites from the NASA Earth Observatory.
One of the main ways we monitor the loss of mass from these ice sheets is the GRACE satellites. GRACE (Gravity Recovery and Climate Experiment) is a pair of satellites launched in 2002 that follow each other around the world about 120 km (90 miles) apart. What they actually measure are very slight variations in that distance between them, and this is indirectly but accurately measures the regional gravitational pull of the Earth. The stronger Earth’s gravitational pull, the faster the satellites will orbit. Since they’re 90 miles apart, when the first satellite passes over an area with greater mass (and therefore a stronger gravitational pull), it goes a little faster and the distance between the two satellites expands. Then, when the second one passes over the same spot, it catches back up and shrinks the distance. That variation in distance tells NASA scientists how much mass comprises various regions of the Earth. It can’t detect small changes like constructing a new building or cutting down a stand of trees. But it can detect large changes like long-term groundwater withdrawal or melting ice sheets.
NASA has put together two animations that show this system at work in Greenland and Antarctica. The beauty of these animations is that they pair a time series of mass loss with a map of the decline in the height of the ice sheet. (Be careful; the change in “height” of the ice sheet is measure in “water equivalent”, which means they’re reporting the loss as liquid water, not ice. This is done because the density of water is less variable than the density of ice. Using water makes it easier to compare different areas.) In Greenland, you can see the seasonal cycle of accumulation in winter and melt in summer, but the overall decline is also obvious. Most of the ice sheet has lost mass, but the greatest loss has been at a few really large glacial outlets. Overall, there’s about 0.8 mm (0.03 inches) per year of sea level rise coming from Greenland right now. That’s not huge, but combined with mountain glaciers, Antarctica, and thermal expansion, it’s been around 3 mm (0.12 inches) each year overall since GRACE was launched.
Although not very important right now, Antarctica is the most important mass of ice for the long haul. If the entire Antarctic ice sheet melted, it would add roughly 9 times as much water to the oceans as Greenland would (roughly 60 m versus 7 m, respectively). That isn’t going to happen under current projections — but by 2100 we could very well see a meter. The animation starts in 2002 and shows how much mass loss occurred through 2016. The average loss is 125 gigatons per year, which sounds like a lot. It is, to be sure, but it’s only a small amount of sea level rise — about 0.35 mm (0.014 inches) per year. So right now, Greenland is still the bigger contributor. The really cool thing about the animation is that you can see that current mass loss from Antarctica is restricted to just a few places. The Antarctica Peninsula is one place, which makes sense; it’s the farthest north and warmest area of Antarctica. But another is in “West Antarctica” (on the left of the map). This area is losing mass fast, especially Thwaites Glacier and Pine Island Glacier. But overall, Antarctica is contributing only very a small about of melt to the oceans compared to its potential.
