Over the last 20 years, radar from satellites has created a continuous snapshot of the ice sheet in Western Antarctica — and shown that the ice might be thinning faster than previously thought.

Glaciers are enormous flows of ice that move downhill and crash into the sea. The Pine Island Glacier, seen in the GIF above, is the largest glacier on the West Antarctic Ice Sheet. As much as 10 percent of the ice sheet flows into the ocean through this glacier.

The ice sheet protrudes out over the sea, and the point where it leaves land and starts floating is known as the grounding line. Figuring out exactly where the grounding line is located is an important part of understanding the total mass of the ice. Scientists can drill through the ice to figure out where it rests over land but it is not ideal to send people to this region all the time. Overhead satellites, like the European Space Agency's ERS missions, can provide better long-term monitoring of the ice.

The part of Pine Island Glacier that extends over water bobs up and down on the surface, bending and stretching parts just above the grounding line and leaving a signature that can be seen with radar. As warm water eats away at the ice sheet, it thins the ice and consequently drives the grounding line backwards. A recent study looked at the position of the grounding line since 1991, showing that it had retreated by 12.5 miles on average while the ice had thinned by more than 650 feet. The thinning shows no sign of slowing down, suggesting that Pine Island could contribute more to sea-level rise than previously projected.