>>Kendra Pierre-Louis, Henry Fountain and Denise Lu, The New York Times
Published: 2020-05-01 16:11:18 BdST
Data from a new satellite provides the most precise picture yet of Antarctica’s ice and where it is accumulating most quickly, in parts of East Antarctica, and disappearing at the fastest rate, in West Antarctica and the Antarctic Peninsula.
The information, in a paper published Thursday in Science, will help researchers better understand the largest driver of ice loss in Antarctica, the thinning of floating ice shelves that allows more ice to flow from the interior to the ocean, and how that will contribute to rising sea levels.
Helen A Fricker, an author of the paper, said that scientists have tried to study the link between thinning shelves and what is called grounded ice, but have been hampered because most observations were of one or the other, and made at different times. “Now we’ve got it all on the same map, which is a really powerful thing,” said Fricker, a glaciologist at the Scripps Institution of Oceanography in La Jolla, California.
The Ice, Cloud and land Elevation Satellite-2, or ICESat-2, was launched in 2018 as part of NASA’s Earth Observing System to replace an earlier satellite that provided data from 2003 to 2009. ICESat-2 uses a laser altimeter, which fires pulses of photons split into six beams toward the Earth’s surface 300 miles below. Of the trillions of photons in each pulse, only a handful of reflected ones are detected back at the satellite. Extremely precise measurement of these photons’ travel times provides surface elevation data that is accurate to within a few inches.
“It’s not like any instrument that we’ve had in space before,” said another of the authors, Alex S Gardner, a glaciologist at NASA’s Jet Propulsion Laboratory in Pasadena, California. The resolution is so high that it can detect rifts and other small features of the ice surface, he said.
The researchers used the elevation measurements from both satellites to determine how Antarctica’s mass balance — the difference between accumulation and loss — changed from 2003 to 2019 for each of its 27 drainage basins. Overall, they reported that the continent lost enough ice to raise sea levels by 6 millimetres, or about one-quarter of an inch, over that time, a finding that is consistent with other studies.
Ice loss was limited to West Antarctica and the Antarctic Peninsula; the bigger East Antarctic sheet gained mass over that time. Why East Antarctica is gaining mass is not completely understood, but precipitation has likely increased relative to some point in the past, Gardner said. Increased precipitation in the form of snow leads to an increase in ice sheet mass because as snow compresses over time it turns to ice.
Floating ice shelves accounted for 30% of the ice loss in West Antarctica, the researchers found. Floating ice is lost in two ways: by calving of icebergs and by melting from underneath by a deep current of warmer water that circulates around the continent.
Floating ice is, by definition, already in the water, so when it calves or melts it does not add to sea level rise. But ice shelves act as buttresses against the grounded ice behind them; when they thin they allow that ice to flow faster. And when the previously grounded ice reaches the water, it adds to rising seas.
“When we see changes in Antarctica, especially in the grounded ice, those are changes due to changes in ice flow,” said Ben Smith, a geophysicist at the University of Washington and another author of the study.
Scientists are increasingly concerned that the loss of floating ice in West Antarctica is causing more rapid flow of grounded ice in the West Antarctic ice sheet, and that a portion of the sheet could collapse over centuries, greatly increasing sea levels.
The study looked at the changes in the Greenland ice sheet as well. Unlike Antarctica, where little ice is lost through surface melting and runoff, as much as two-thirds of Greenland’s ice is lost this way.
Using their elevation data, the researchers found that Greenland is losing on average about 200 billion tons of mass each year, enough to raise sea levels by about 8 millimetres, or a third of an inch, over the study period. The mass loss figure is roughly similar to other recent estimates.
The study is the first to be published using data from ICESat-2, which was designed to have an operating life of at least three years. Many more studies are expected that should add to understanding of Earth’s frozen expanses.
“Where we’re at in ice sheet science is there are still a lot of unknowns,” Gardner said. One advantage of ICESat-2, he said, is its ability to measure changes in some of the smallest ice sheet features. That will help scientists better understand how the changes are occurring and improve forecasts of future impacts as the climate continues to shift.
ICESat-2, he said, “reveals the process of change, and without understanding those processes you have no ability to make predictions.”
“It really just gives us this incredibly crisp, unified picture.”
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