Just like their smaller counterparts, the huge ice sheets of Greenland and Antarctica are also shrinking. Given the remoteness and difficulty associated with studying these ice sheets, we only have good data on their size for the last decade, thanks to the advent of satellite systems that can monitor these glaciers. In particular, the GRACE satellite system has provided some very important data on the changes occurring in Greenland and Antarctica. This ingenious satellite system consists of a pair of satellites that are chasing each other in the same orbit around the Earth. The distance between the satellites changes according to subtle changes in gravity on the surface of Earth. As the lead satellite approaches a region of stronger gravity (due to more mass near the surface), it pulls away from the trailing satellite and then slows down as it passes the region of excess mass. In this way, the satellites can measure the subtle changes in gravity, and since the satellites pass over the same area every few days, they can detect changes in the gravity of a certain spot over time. If a big ice sheet loses mass due to melting, its gravitational effect on the satellites diminishes, and in this way, the satellites can detect the changes in the mass of these ice sheets — they are effectively “weighing” these glaciers, which is an extraordinary achievement. The results can be seen in the figure below.
These figures show the time series of Greenland and Antarctic ice mass changes from GRACE satellite data; raw data in blue, seasonal cycle removed in red, best-fit line in green. Note that the vertical axis has units of Gt, gigatons, or a billion metric tons — that is 1e9 tons or 1e12 kg. This is a lot of ice! For reference, a Gt of water fills a cube that is 1000 meters on a side. Spread over the world’s oceans, the total melting gives about 8 mm of sea level change in just 6 years.
Other satellites passing over these ice sheets can measure the areas where surface melting produces small ponds of melt during the melting season. Much of the meltwater freezes back into the ice, but in some places near the edges of the ice sheets, the water sinks down into crevasses and travels all the way to the bottom of the ice, where it can lubricate the base and help accelerate the flow of the ice.
The above image shows the Greenland melt anomaly, measured as the difference between the number of days on which melting occurred in 2007 compared to the average annual melting days from 1988-2006. The areas with the highest amounts of additional melt days appear in red, and areas with below-average melt days appear in blue. Although faint streaks of blue appear along the coastlines, namely in northwestern and southeastern Greenland, red and orange predominate, especially in the south.