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The information, in an article published Thursday in the journal Science, will help researchers better understand the largest ice loss factor in Antarctica, the thinning of floating ice shelves that allows more ice to flow from the interior to the ocean, and how that can contribute to sea level rise. Researchers have long known that while the continent is losing overall mass as the climate changes, the change is uneven. It is gaining more ice in some areas, such as parts of East Antarctica, and is rapidly losing it in others, in West Antarctica and the Antarctic Peninsula.
Helen A. Fricker, author of the article, said that scientists have attempted to study the link between slimming platforms and what is called crushed ice, but have been hampered because most observations were from one area or another, and were held at different times. “Now we have it all on the same map, which is really powerful,” said Fricker, a glaciologist at the Scripps Institution of Oceanography in San Diego.
The Ice-Cloud and land Elevation Satellite-2, or ICESat-2, was launched in 2018 as part of NASA’s Earth Observation System. It replaced a satellite that had provided data from 2003 to 2009. ICESat-2 uses a laser altimeter, which shoots photon pulses divided into six beams toward the Earth’s surface, 300 miles below. Of the trillions of photons in each pulse, only a handful of the reflected ones are detected on the satellite. The extremely accurate measurement of the travel times of these photons provides surface elevation data with an accuracy of a few inches.
“It’s not like any instrument we’ve had in space before,” said another author, Alex S. Gardner, a glaciologist at NASA’s Jet Propulsion Laboratory in Pasadena, California. The resolution is so high that it can detect cracks and other small features of the ice surface, he said.
The researchers used 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 the sea level by 6 millimeters, about a quarter of an inch, during that time period.
While that finding is consistent with other studies that used data from other instruments, “in many ways, this is a more definitive measure,” said Ben Smith, a glaciologist at the University of Washington and author of the study. “It shows a set of differences that we can really understand in detail and know what they mean for ice sheets.”
Ice loss was limited to West Antarctica and the Antarctic Peninsula; the largest layer in East Antarctica gained mass during that time. The increase in East Antarctica is likely due to increased rainfall, Gardner said. “While we cannot say that these changes are related to contemporary climate change, we can say that these are the patterns of change that we hope to see in a warming world,” he said.
Increased precipitation in the form of snow leads to an increase in the mass of the ice sheet because, as the snow compresses over time, it turns into ice.
Floating ice is lost in two ways: by calving icebergs and melting from below by a deep stream of warmer water flowing through the continent.
Floating ice is, by definition, already in water, so when it breeds or melts it does not increase sea level rise. But the ice shelves act as buttresses against the crushed ice behind them; when they become thin, they allow ice to flow faster. And when previously grounded ice reaches the water, it adds to the rise of the seas.
Scientists are increasingly concerned that the loss of floating ice in West Antarctica is causing a faster flow of ice to land in the West Antarctic Ice Sheet, and that part of the sheet may collapse for centuries, considerably increasing the sea level.
The study also looked at changes in the Greenland ice sheet. Unlike Antarctica, where little ice is lost through surface melt and runoff, up to two-thirds of Greenland’s ice is lost in this way.
Using their elevation data, the researchers discovered that Greenland is losing approximately 200 billion tons of mass each year on average. That is enough to raise the sea level by about 8 millimeters, or a third of an inch, during the study period. The mass loss figure is more or less 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 to contribute to an understanding of the frozen expanses of Earth.
“Where we are in the science of the ice sheet is that there are still many unknowns,” said Gardner. An advantage of ICESat-2, he said, is its ability to measure changes in some of the smallest characteristics of the ice sheet. That will help scientists better understand how changes are occurring and improve predictions of future impacts as the climate continues to change.
ICESat-2, he said, “reveals the process of change, and without understanding those processes you don’t have the ability to make predictions.”
“It really gives us this incredibly sharp, unified image.”
