12 May 2020
NASA compares new data with that measured by original ICESat mission 16 years before.NASA has ever flown in space, scientists have made precise, detailed measurements of how the elevation of the Greenland and Antarctic ice sheets have changed over 16 years.
The results provide insights into how the polar ice sheets are changing, demonstrating definitively that small gains of ice in East Antarctica are dwarfed by massive losses in West Antarctica. The scientists found the net loss of ice from Antarctica, along with Greenland's shrinking ice sheet, has been responsible for 14 mm (0.55 in) of sea level rise between 2003 and 2019 – slightly less than a third of the total amount of sea level rise observed in the world's oceans.
The findings come from NASA's Ice, Cloud and land Elevation Satellite 2 (ICESat-2), which was launched in 2018, to gather detailed global elevation measurements, including over Earth's frozen regions. By comparing the recent data with measurements taken by the original ICESat between 2003 and 2009, researchers have generated a comprehensive portrait of the complexities of ice sheet change and insights about the future of Greenland and Antarctica.
The study found that Greenland's ice sheet lost an average of 200 gigatons (200 x 109) of ice per year, and Antarctica's ice sheet lost an average of 118 gigatons of ice per year. Climate change is reshaping ice sheets in the Arctic and Antarctic, and NASA's ICESat and ICESat-2 satellites help researchers determine what those alterations look like.
"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," said Ben Smith, a glaciologist at the University of Washington and lead author of the new paper, published in Science, on April 30. "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."
ICESat-2's instrument is a laser altimeter, which transmits 10,000 pulses of light per second down onto Earth's surface and measures how long it takes to return to the satellite with an accuracy of 10-9 seconds. The instrument's pulse rate enables the generation of a dense map of measurement over the ice sheet, accurate to 25mm.
The researchers took tracks of earlier ICESat measurements and overlaid the tracks of ICESat-2 measurements from 2019, and took data from the tens of millions of sites where the two data sets intersected. That gave them the elevation change, but to derive to how much ice has been lost, the researchers developed a new model to convert volume change to mass change.
"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," commented Tom Neumann, ICESat-2 project scientist at NASA Goddard.
In Antarctica, for example, the detailed measurements showed that the ice sheet is getting thicker in parts of the continent's interior as a result of increased snowfall. But the loss of ice from the continent's margins far outweighs any gains in the interior. In those places, the loss is due to warming from the ocean.
In Greenland, there is significant thinning of coastal glaciers, Smith said. The Kangerdulgssuaq and Jakobshavn glaciers, for example, have lost 4 to 6 meters (14 to 20 feet) of elevation per year – the glacial basins have lost 16 gigatons per year and 22 gigatons per year, respectively.
"The new analysis reveals the ice sheets' response to changes in climate with unprecedented detail, revealing clues as to why and how the ice sheets are reacting the way they are," said Alex Gardner, a glaciologist at NASA's Jet Propulsion Laboratory in Southern California, and co-author on the Science paper.
Laser altimetry measures ice shelf loss
The study also examined ice shelves – the floating masses of ice at the downstream end of glaciers. These ice shelves, which rise and fall with the tides, can be difficult to measure, said Helen Amanda Fricker, a glaciologist at Scripps Institution of Oceanography at the University of California San Diego, and co-author on the Science paper. Some of them have rough surfaces, with crevasses and ridges, but the precision and high resolution of ICESat-2 allows researchers to measure overall changes.
This is one of the first times that researchers have used laser altimetry to measure loss of the floating ice shelves around Antarctica simultaneously with loss of the continent's ice sheet. The researchers found ice shelves are losing mass in West Antarctica, where many of the continent's fastest-moving glaciers are located as well. Patterns of thinning over the ice shelves in West Antarctica show that Thwaites and Crosson ice shelves have thinned the most, an average of about 5 meters (16 feet) and3 meters (10 feet) of ice per year, respectively.
The following NASA video describes the mission and some of its key observations: