Using the most advanced Earth-observing laser instrument NASA has ever flown in space, a team of scientists including glaciologists from Scripps Institution of Oceanography at the University of California San Diego have made precise measurements of how the Greenland and Antarctic ice sheets have changed over a 16-year period.
In a new study published in the journal Science on April 30, scientists found that net loss of ice from Antarctica, along with Greenland’s shrinking ice sheet, has been responsible for 14 millimeters (0.55 inches) of sea-level rise to the global ocean between 2003 and 2019 – slightly less than a third of the total amount of sea level rise observed. In Antarctica, sea-level rise is being driven by the loss of floating ice shelves melting in a warming ocean. The ice shelves help hold back the flow of land-based ice that contributes to sea-level rise into the ocean.
The findings come from the Ice, Cloud and land Elevation Satellite 2 (ICESat-2), which was launched into orbit in Fall 2018 and began taking detailed global elevation measurements, including over Earth’s frozen regions. By comparing the new data with measurements taken by the original ICESat from 2003 to 2009, researchers have generated a comprehensive portrait of the complexities of ice sheet change and insights into the future of Greenland and Antarctica.
“Antarctica and Greenland are so vast, and so inaccessible that you can’t make a dent in measurements from observations on the ground,” said Helen Amanda Fricker, a glaciologist at Scripps Oceanography who specializes in studying ice shelves, and a co-author of the study. “The vantage point from space gives the big picture perspective of what is happening, and is now providing us an uninterrupted record of measurements.”
This is one of the first times that researchers have measured loss of floating ice shelves around Antarctica simultaneously with loss of the continent’s ice sheet, allowing them to better understand the interconnectedness and mechanisms driving change.
The ICESat-2 laser works by sending 10,000 laser pulses a second to Earth’s surface that measure the height of ice sheets, glaciers, sea ice, and vegetation by calculating the time it takes a handful of those pulses to return to the satellite. Each photon has a time tag, and these tags can combine with the GPS location to pinpoint its exact place and height on the ground. Scientists compared these new measurements with those from the original ICESat mission to calculate the mass of ice lost or gained.
“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. “We now have a 16-year span between ICESat and ICESat-2 and can be much more confident that changes we’re seeing in the ice have to do with the long-term changes in the climate.”
The study found that Greenland’s ice sheet, or grounded ice, lost an average of 200 gigatons of ice per year over the last 16 years, and Antarctica’s ice sheet lost an average of 118 gigatons of ice per year. In total, 5,088 gigatons of ice were lost from Greenland and Antarctica’s ice sheets from 2003 to 2019. One gigaton of ice is enough to fill 400,000 Olympic-sized swimming pools.
“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 Pasadena, California, and co-author on the paper.
In many key regions of Antarctica, the contribution to sea-level rise is being driven by the loss of floating ice shelves, which normally help hold back the flow of additional ice into the ocean from land. Of the global sea-level rise that resulted from ice sheet meltwater and iceberg calving, about two-thirds of it came from Greenland, the other third from Antarctica.
Warming ocean water is likely to blame for eroding the floating ice shelves, said the researchers. Ice that melts from ice shelves doesn’t raise sea levels since it’s already floating – just like an ice cube in a full cup of water doesn’t overflow the glass when it melts. But the ice shelves provide stability for the glaciers and ice sheets behind them.
“It’s like an architectural buttress that holds up a cathedral,” Fricker said. “The ice shelves hold the ice sheet up. 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.”
Ice shelves can be particularly difficult to measure, said Fricker, because of their rough surfaces, with crevasses and ridges. The precision and high resolution of ICESat-2 allowed researchers to measure overall, big picture changes. They found that ice shelves in West Antarctica, where many of the continent’s fastest-moving glaciers are located, are losing mass. 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 five meters (16 feet) and three meters (10 feet) per year, respectively.
The ICESat-2 measurements also showed that in Antarctica the ice sheet is getting thicker in parts of the continent’s interior, likely as a result of increased snowfall. But the massive loss of ice from the continent’s margins far outweighs any small gains in the interior.
In Greenland, there was a significant amount of thinning of coastal glaciers. The Kangerdulgssuaq and Jakobshavn glaciers, for example, have lost four to six meters (14 to 20 feet) of elevation per year. Warmer summer temperatures have melted ice from the surface of glaciers and ice sheets, and in some places warmer ocean water erodes away the ice at their fronts.
As for understanding the future of Greenland and Antarctica, Fricker says the new insights gained from ICESat-2 will help with future prediction.
“We will now be able to run more accurate models to better predict sea-level rise,” said Fricker. “When it comes to climate change, better predictions and improved understanding can help us as a society work towards bending the sea-level rise curve and adapting to the future.”
For more information on ICESat-2, visit https://nasa.gov/icesat-2 or https://icesat-2.gsfc.nasa.gov