New research shows that while some Antarctic ice shelves have been thinning since at least the 1970s, widespread thinning accelerated in the 1990s.The 1970s were a rough decade for many of the ice shelves around Antarctica. Now, satellite images show that many of the once visible bulges on the ice shelf's surface have smoothed out, meaning the ice shelf has become thinner and less stable.
The role of ice shelves
Ice shelves are extensions of land-based ice, tongues of ice that extend from the coast to the ocean surface. Most of the Earth's ice shelves are located on the edge of Antarctica, where they play an important role in blocking or supporting ice flows from inland and upstream. This supporting effect slows the flow of ice into the ocean and limits sea level rise. Thick, stable ice shelves provide this support most effectively.
A historical perspective on ice shelf thinning
Previously, scientists using satellite altimetry data collected since the 1990s found that ice shelves in parts of West Antarctica, the West Antarctic Peninsula and East Antarctica have become significantly thinner. Now, Bertie Miles and Robert Bingham of the University of Edinburgh have used 50 years of Landsat imagery to go back even further in time to expand our view of this changing continent.
Their study shows that ice shelf thinning between 1973 and 1989 was limited to a small portion of the ice shelf, mainly along the Amundsen Sound and Wilkes Land coastlines in East Antarctica. Then, starting in the 1990s, ice shelf thinning spread rapidly. Their findings were published in the February 22 issue of
The turning point of the 1990s
A look back over time shows that the 1990s were a turning point. "While many previous studies have reported that ice shelves around Antarctica have been thinning since the 1990s, we didn't previously know that much of the ice shelf thinning began around that time," Bingham said.
Satellite altimetry data - measuring the height of land and ice - was not available before the 1990s, so Myers and Bingham turned to optical images to track changes in bulges on the ice. These bumps are surface manifestations of fixed points - places where floating ice shelves are anchored to high points on the ocean floor. Pinpoints are a useful indicator of ice shelf thickness: over time, the bulges become smaller or even completely smooth, indicating that the ice shelf has thinned and may have lost its pinpoints.
"Berti's new method of mapping pin point anchorages using Land Remote Sensing satellites, along with more sophisticated altimetry methods commonly used by the industry, can serve as a proxy for changes in ice shelf thickness," Bingham said.
Pine Island Glacier: Case Study
The set of images at the top of this page shows Pine Island Glacier, an area of the Amundsen Sea embayment where the ice had begun to thin in the 1970s. Some bumpy areas can be seen on the ice in January 1973 (above), but the ice was basically smooth in December 2001 (below). The images were acquired using the MSS (Multispectral Scanner) on Landsat 1 (top) and the ETM+ (Enhanced Thematic Imager) on Landsat 7 (bottom). Note that these images use a grayscale palette to achieve a closer match between different sensors.
"These images show that the ice shelf's anchor points became smaller and smaller over time as warm ocean currents melted the ice shelf, causing it to thin and subsequently break away from its anchor high on the ocean floor," Myers said.
Myers and Bingham's findings confirm that Pine Island Glacier thinned earlier than most Antarctic ice shelves. Of the approximately 600 spikes tracked by the researchers, only 15 percent shrank in area from 1973 to 1989, including the pinpoint on Pine Island Glacier. This number increased to 25% between 1990 and 2000 and to 37% between 2000 and 2022.
The above image, taken with the Land Imager 2 (OLI-2) on Landsat 9, shows the ice shelf of Pine Island Glacier in January 2024. By that time, the smooth, thinning front and northern edge of the ice shelf had lost more ice, and broken ice was clearly visible on the southern edge.
With Pine Island Glacier at or near complete loss of anchorage, its ability to support the ice has been reduced to a minimum. Myers and Bingham note in their paper that "of greater concern" may be other major ice shelves, which are still heavily anchored but have signs that they may soon lose their anchor points.
Reference: "Progressive Unanchoring of the Antarctic Ice Shelves Since 1973" by Bertie W.J. Miles and Robert G. Bingham, February 21, 2024, "Nature" magazine. J. Miles and Robert G. Bingham, February 21, 2024, Nature.
DOI:10.1038/s41586-024-07049-0
Compiled source: ScitechDaily