Satellites map melting ice sheets

Ice sheet losses from Greenland and Antarctica have outpaced snow accumulation and contributed around 14 millimetres to sea level rise over the past 16 years, a new analysis of data from NASA’s laser-shooting satellites has revealed. 

Scientists say that by combining information from ICESat and the next-generation ICESat-2, they were able to account for even subtle changes in ice masses, which have been previously overlooked, resulting in small yet significant biases on ice mass changes. 

“If you watch a glacier or ice sheet for a month, or a year, you’re not going to learn much about what the climate is doing to it,” says Benjamin Smith, a glaciologist at the University of Washington, US, and lead author of a paper in the journal Science.

“We now have a 16-year span between ICESat and ICESat-2 and can be much more confident that the changes we’re seeing in the ice have to do with the long-term changes in the climate.”

Previous studies of ice loss or gain have tended to analyse data from multiple satellites and airborne missions. The new study used just a single type of measurement: height as measured by an instrument that bounces laser pulses off the ice surface.

Smith and colleagues took tracks of ICESat measurements and overlaid the denser tracks of ICESat-2 measurements from 2019. Where the two data sets intersected – tens of millions of sites – they ran the data through computer programs that accounted for the snow density and other factors, then calculated the mass of ice lost or gained.

“It was amazing to see how good the ICESat-2 data looked, right out of the gate,” says ICESat-2 project scientist Tom Neumann. “These first results looking at land ice confirm the consensus from other research groups, but they also let us look at the details of change in individual glaciers and ice shelves at the same time.”

Smith says the results show a significant amount of thinning of coastal glaciers in Greenland. The Kangerdulgssuaq and Jakobshavn glaciers have lost four to six metres of elevation per year, for example, and warmer summer temperatures have melted ice from the surface of glaciers and ice sheets.

In Antarctica, the ice sheet is getting thicker in parts of the interior – likely as a result of increased snowfall, Smith suggests – but the loss of ice from the continent’s margins, especially in West Antarctica and the Antarctic Peninsula, far outweighs any gains in the interior. In those places, the ocean is also likely to blame.

“There are ice shelves at the downstream end of those glaciers, floating on water, and those ice shelves are thinning, letting more ice flow out into the ocean as the warmer water erodes the ice,” says Smith

Ice that melts from ice shelves doesn’t raise sea levels, since it’s already floating, but ice shelves provide stability for the glaciers and ice sheets behind them.

“The ice shelves hold the ice sheet up,” says co-author Helen Amanda Fricker, a glaciologist at Scripps Institution of Oceanography. 

“If you take away the ice shelves, or even if you thin them, you’re reducing that buttressing force, so the grounded ice can flow faster.”