Hydrology, Cryosphere & Earth Surface News

Shrinking Ice Sheets Lifted Global Sea Level 14 Millimeters

Researchers measure both grounded and floating ice sheets using satellite data spanning a 16-year period.

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Using satellite data stretching back more than a decade, scientists have found that the loss of ice in Antarctica and Greenland accounts for a 14-millimeter elevation in sea levels since 2003. The data provide the most accurate and granular picture of ice loss so far, a crucial element for predictions of how high seas will rise as the climate changes.

Using data from the mission of Ice, Cloud, and Land Elevation Satellite 2 (ICESat-2), the new report looked at only ice surface height to measure ice loss. Reporting in Science, researchers at the University of Washington, NASA, and other centers describe how they used data from the original ICESat, gathered from 2003 to 2009, along with measurements taken by its successor in 2018 and 2019. They believe the long observational time frame supports the idea that ice changes are related to long-term climate changes.

Unprecedented Accuracy and Precision

ICESat-2 is a sophisticated piece of hardware. It fires 10,000 laser pulses a second at ice masses to measure their changing height over time. It carries a single instrument, the Advanced Topographic Laser Altimeter System (ATLAS), that bombards Earth with laser pulses, each consisting of roughly 300 trillion photons. Because of reflection and scattering, only about a dozen of these photons will reflect all the way back up to a telescope aboard ICESat-2 as it orbits 500 kilometers above our world.

“With the unprecedented accuracy and precision of this novel measurement system, we are now able to detect the small signals far into the ice sheet interior, as well as map out the changes over narrow glaciers on high-slope terrain around the ice sheets,” said study coauthor Fernando S. Paolo of NASA’s Jet Propulsion Laboratory. “Previous altimeters struggled with these challenges. Because we used laser measurements over a fairly long time span (about 16 years), we are getting the overall trends of ice sheet mass loss with higher confidence.”

The researchers took data from ICESat and overlaid them with findings from ICESat-2, accounting for snow density and other factors, to determine how much ice had been lost. They also developed a new model for firn (snow that is transitioning to ice on the top of ice sheets) to estimate how ice sheet air content changes over time.

“This is important for knowing how much of the change we observe is due to gain or loss of ice and how much is due to change in the density of the top of the ice sheet,” said Benjamin Smith, a glaciologist at the University of Washington and lead author of the study.

The team determined that Greenland’s ice sheet lost an average of 200 gigatons of ice per year, and Antarctica’s ice sheet lost an average of 118 gigatons of ice per year.

“The amount of ice lost from both ice sheets is equivalent to a layer of water half an inch [1.27 centimeters] thick on top of all the oceans or to fill around 13 billion swimming pools,” Smith and Paolo said. “It’s also about the volume of Lake Michigan, or enough to cover the continental United States to a depth of about 2 feet [about 0.6 meter].”

Interior Ice Gains Offset

The study considered both grounded ice and floating ice for the same time period, using the same data and methods to get estimates of total mass change. Assessing both ice masses enabled the researchers to make links between where the ice shelves are losing mass and where the grounded ice is responding. Although loss of floating ice does not directly contribute to sea level, it does further accelerate the loss of grounded ice.

“This is because ice shelves constitute a natural ‘barrier’ or ‘buffer’ to grounded ice flowing into the ocean via the glaciers, holding it back—this is a process we refer to as ‘buttressing’ because it is like an architectural buttress that holds up a cathedral,” said study coauthor Helen Amanda Fricker, a glaciologist at Scripps Institution of Oceanography. “As the ice shelves thin, they are less able to hold the grounded ice back. The glaciers then flow faster towards the ocean, and they thin.”

Patterns of thinning over the ice shelves in West Antarctica show that the Thwaites and Crosson ice shelves have thinned the most, at an average of about 5 and 3 meters of ice per year, respectively, Fricker added.

Map of Greenland showing loss of the ice sheet
This map shows the amount of ice gained or lost by Greenland between 2003 and 2019. Dark reds and purples show large rates of ice loss near the coasts. Blues show smaller rates of ice gain in the interior of the ice sheet. Credit: Smith et al., 2020, https://doi.org/10.1126/science.aaz5845

The scientists also noted ice height increasing in the interior of ice sheets over the observation period; ice is not flowing out of the interior of the ice sheets as fast as snow is falling. This phenomenon could be due to a relatively small increase in snowfall in some regions as a consequence of global warming changing atmospheric circulation, which would be expected on the basis of atmospheric models, Smith and Paolo said, adding that the interior mass increase is much smaller than the mass lost around the peripheries of the ice sheets.

“This is a very significant study, a big step forward in both the quality of data and the confidence in that observed change occurring on both the grounded and the floating ice for Greenland and Antarctica,” said David Holland, director of the Center for Sea Level Change at New York University Abu Dhabi; he was not involved in the study. “This study brings together the observed changes of both ice types in a single data analysis package and looks to me like the new gold standard for observed change of the great ice sheets. This high-quality data and data analysis are critical to improving projections of future global sea level.”

The group is now looking to map out trends in ice sheet mass using only data from ICESat-2, starting in 2018.

“With more measurements coming from ICESat-2 we will be able to construct time series of ice sheet change with unprecedented quality and to start understanding the details of why these changes are happening,” said Paolo. “We are also mapping out all the other challenging regions around the world, such as the glaciers in Alaska, the Andes, and the Himalayas.”

—Tim Hornyak (@robotopia), Science Writer

Citation: Hornyak, T. (2020), Shrinking ice sheets lifted global sea level 14 millimeters, Eos, 101, https://doi.org/10.1029/2020EO144043. Published on 15 May 2020.
Text © 2020. The authors. CC BY-NC-ND 3.0
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