Short-term changes in Antarctica's ice shelves are key to predicting their long-term fate
Posted by admin on 13th June 2018

Antarctica’s ice sheet contains enough ice to raise global sea levels by around 180 feet if it all melted. But dramatic, eye-catching changes to Antarctica’s floating ice shelves, such as calving icebergs, are often highlighted in the news without a sense of long-term context or a clear connection to what is causing the changes.

Antarctica is losing land ice at an accelerating rate, and current observations suggest it will become the largest contributor to sea level rise by the middle of this century. Understanding variations in the height of Antarctic ice shelves – the floating edges of the continent’s ice sheet – can tell us how and why Antarctica is changing, and what that could mean for future sea levels.

We study changes in Antarctic ice shelves, along with our colleague Laurie Padman at Earth & Space Research, a nonprofit institute in Seattle. One of us, Helen Amanda Fricker, contributed to two articles in a special issue of the journal Nature that brings together current understanding of the state of Antarctica. Here’s what we see happening.

Antarctica’s major geographical features, including the West and East Antarctic ice sheets, the Antarctic Peninsula and some of the larger ice shelves around the continent’s edges.

Ice shelves hold back the grounded ice

Antarctic ice shelves provide mechanical support to hold back the flow of ice from the continent to the ocean, regulating the pace of mass loss from the enormous ice sheet. Scientists call this process “buttressing,” since it works in the same way that an architectural buttress prevents a building from collapsing.

Reducing the mass of an ice shelf does not contribute directly to sea level rise, since this ice is already floating on the ocean, but it promotes faster discharge of grounded ice, which increases sea level. To understand how Antarctic mass loss varies, we need to understand how ice shelves grow and shrink.

Ice shelves gain mass mainly through ice flowing from the continent and local snowfall on their surfaces. They lose mass primarily through melting by the ocean and by iceberg calving.

Antarctica has more than 300 ice shelves, and the net change in their mass is a delicate balance between gains and losses. Determining this balance requires understanding how ice, ocean, and atmosphere interact to drive changes around Antarctica. Climate change will alter the overall balance between gains and losses, and will determine the future of Antarctica’s ice loss.

The critical role of satellites

Antarctica’s small ice shelves are roughly the area of small cities, and its largest is the size of Spain. The total ice-shelf area is around 1.5 million square kilometers (580,000 square miles), about as large as Mongolia. The only viable way to routinely monitor changes in their mass is with satellites.

Since the launch of Landsat 1 in 1972, satellite data have taught us a lot about the ice sheet, including its large-scale structure, surface properties and flow rates. A recent synthesis combined 150 independent estimates of ice-sheet mass loss from satellite data and atmospheric models to show that the ice sheet is losing more mass to the ocean with every passing year. The largest changes have occurred in places where ice shelves have either thinned or collapsed.

Single satellite missions typically only last five to 10 years, but we can stitch together data from consecutive missions to increase the length of the record. This helps us separate long-term trends from natural climate variability and unravel processes that drive changes around the margins of Antarctica.

The European Space Agency (ESA) has launched four ice-observing satellites since 1992, carrying radar altimeters to precisely determine the distance between the satellite and the Earth’s surface beneath it. These data have now provided a continuous time series of variations in ice-shelf surface height since the early 1990s. Combining measured increases and decreases in surface height with the latest generation of climate models to infer how the atmosphere has changed, we can estimate the amount of mass an ice shelf can lose to the ocean.