May 23, 2014

Making Sense of West Antarctic Melting

by Julia Rosen

The Thwaites Glacier in West Antarctica. (Credit: NASA.)

The Thwaites Glacier in West Antarctica. (Credit: NASA.)

 

Two new studies unveiled last week require revising estimates of future sea-level rise upward. Together, the studies — which concern melting of Antarctic ice — suggest that oceans could rise by as much as 11 feet in the coming centuries, and, most importantly, that nothing can be done to stop them.

If true, we have just passed a so-called “tipping point” in Earth’s climate — a moment where the effects of climate change can no longer be undone. The studies suggest we have already committed to an amount of sea-level rise that will radically reshape coastlines around the world, and ironically, the oceans themselves flipped the switch. More on that in a minute.

First, the results: one of the studies assessed the state of a large sector of the West Antarctic Ice Sheet (or WAIS), which sits across the mostly ice-covered Transantarctic Mountains from the colder, higher, and substantially more massive East Antarctic Ice Sheet. A large portion of WAIS flows into the sea via six major outlet glaciers that tumble into the Amundsen Sea.

Credit: NASA

Left: Antarctica actually contains two ice sheets: the East and West Antarctic Ice Sheets. The studies released last week concern the glaciers that flow into the Amundsen Sea, the small embayment below the Antarctic Peninsula in West Antarctica. (Credit: NASA.) Right: The six glaciers in the first study are shown on a close-up map of the Amundsen Sea sector. The second study focuses on one of them, the Thwaites Glacier, which drains a large area of WAIS. (Credit: Eric Rignot.)

 

The authors of the first study, accepted for publication in Geophysical Research Letters, used radar to probe the geometry of the ice and bedrock below the surface of these glaciers. They found that the grounding line of each glacier — the point where the front of the glacier rests on the bedrock below — has retreated 6 to 20 miles over the last two decades. Ice thinning has caused most of the retreat by making it easier for the glaciers to float and lift away from the bed.

Credit: AntarcticGlaciers.org

The studies found that the grounding lines of the glaciers in the Amundsen Sea sector of WAIS have rapidly retreated. Melting from below causes the ice sheet to thin until it is light enough to float. (Credit: Bethan Davies/AntarcticGlaciers.org.)

This spells trouble for WAIS because grounding lines anchor glaciers in place. Once a marine glacier loses contact with the bedrock, it will retreat rapidly until it hits another underwater bump. In West Antarctica, however, scientists say there are no more bumps. In every case, the bedrock behind the ice front plunges ever deeper below sea level into a massive depression that underlies much of West Antarctica and seals its collective fate.

The loss of just these six glaciers would raise sea levels by 4 feet and would likely lead to the eventual disintegration of the rest of the West Antarctic ice sheet, which would contribute 7 feet more, the scientists report.

And at least in the case of one of these glaciers, such retreat is already underway. A second study, published last week in Science, illustrates that the Thwaites Glacier appears to be in the early stages of what glaciologists call “collapse.” Scientists reserve this word for the rare instance when a glacier thins, calves, and melts away in a geologic instant, even though that might mean decades or centuries of human time.

Credit: NASA

Most of the West Antarctic Ice Sheet rests on bedrock that lies below sea level. This map of the land under Antarctica shows these areas in brown while sea level itself is outlined in yellow. Areas above sea level are shown in green. (Credit: NASA/GSFC/SVS.)

The study shows that while the glacier still rests on a ridge of submarine topography 2000 feet below the surface, it has already begun the process of prying itself loose. The scientists demonstrated this by modeling how melting has driven the Thwaites Glacier to its present state and then simulating what will happen to it in the future. The model shows that, given its current health, the glacier’s chances of recovery are slim and it is almost certainly doomed to disappear within 200 to 900 years.

But why did WAIS begin to disintegrate all at once and why right now? It turns out that the main culprit behind the loss of West Antarctica’s ice shelves and glaciers is not melting of the ice sheet’s surface by the sun, but warm seawater eating away at the ice from below.

Credit:

Climate change has increased the temperature gradient between the tropics and Antarctica, causing winds to strengthen. This shows the monthly average surface wind speeds of the Westerly winds that blow around Antarctica. (Credit: Polar Meteorology Group, Ohio State University, polarmet.osu.edu)

The ocean in contact with the ice has warmed in recent years because of climate change, accelerating melting, but probably not for the reasons you might think. Changes in the temperature difference between the warming tropics and cold, isolated Antarctica have caused the westerly winds that swirl around the continent to grow stronger in recent years. This has changed ocean circulation around WAIS, allowing warm deep water to creep onto the continental shelf and nip beneath the ice.

Because of this change in circulation, it will be difficult to stop Antarctic glaciers from melting further. Mountain glaciers and the Greenland Ice Sheet will also continue to lose mass (see here for the results of recent work on Greenland). However, future choices about greenhouse gas emissions could slow the rate at which collapse — and thus, sea-level rise — occurs.

Geologically speaking, sea levels higher than the present are not entirely unprecedented — they have been up to 200 feet higher at times in the very distant geologic past. The problem is that, worldwide, over 100 million people live within 3 feet of current sea levels. If these studies are correct, and WAIS has indeed started down the road to its demise, the important question won’t be how high sea levels will rise, but how fast.

LEARN MORE

Check out our module on The Hydrologic Cycle, which starts by examining how different sea levels were in the recent geologic past.

Watch videos from NASA’s Jet Propulsion Lab about the new results. One introduces you to the glaciers of the Amundsen Sea sector and how they are changing, another shows how grounding line retreat happens.

Dive deeper into the science behind ice sheet collapse at AntarcticGlaciers.org.

Julia Rosen

Written by

Julia Rosen is a freelance science writer and PhD student at Oregon State University. She received a Bachelor’s degree in Geological and Environmental Sciences from Stanford University before beginning her doctoral research on polar ice cores and climate change. In between, she did her “Master's” in backpacking around the world and skiing. Julia is a periodic contributor to Oregon State’s research magazine, Terra, and helps write blog content and develop learning modules for Visionlearning.

The views expressed above do not necessarily represent those of Visionlearning or our funding agencies.