Crucial West Antarctic glaciers are retreating unstoppably / NATURE.

Crucial West Antarctic glaciers are retreating unstoppably

Modelling and radar data from Amundsen Sea suggest current melting will run away.

Alexandra Witze

12 May 2014 / NATURE / http://www.nature.com/news/crucial-west-antarctic-glaciers-are-retreating-unstoppably-1.15202

Several of Antarctica’s most vulnerable glaciers have already begun a runaway meltdown, two studies suggest. The work provides some of the first detailed forecasts of how quickly glaciers are likely to disappear from a region that has long been a concern for scientists.

One paper uses modelling to find that ongoing losses at the Thwaites Glacier have permanently destabilized the ice river, which drains into West Antarctica’s Amundsen Sea1. In the second study, satellite radar observations reveal that Thwaites and four neighbouring glaciers have nothing to hold them back from catastrophic collapse2, so they are more vulnerable than was previously thought.

“Ice is going to retreat from this sector for decades and centuries to come, and we can’t stop it,” says Eric Rignot, a glaciologist at the University of California, Irvine, and co-author of the satellite analysis.

Were they all to melt, the five Amundsen Sea glaciers studied by Rignot’s team contain enough water to raise global sea level by 1.2 metres. That process is likely to unfold slowly: at Thwaites alone, the other team found, melting over the next century will probably cause sea levels to rise by less than quarter of a millimetre per year, or just 2.5 centimetres in total.

But it could speed up to more than 1 millimetre per year within 200–900 years, says Ian Joughin, a glaciologist at the University of Washington in Seattle and co-author of the Thwaites paper, which is published today in Science1. "We are seeing the early stages of the collapse," he says.

Global sea levels are currently rising by about 3 millimetres per year. Much of that comes from the thermal expansion of the warming oceans, but some also comes from melting ice in Greenland and Antarctica.

Thwaites is important because it flows from a broad, deep interior basin into the sea, so it has a vast storehouse of ice to draw on. The nearby Pine Island Glacier is retreating faster than Thwaites, but drains only a very narrow trough.

The Joughin study “is a seminal paper”, says Andrew Shepherd, who studies the cryosphere at the University of Leeds, UK. “It’s the first to really demonstrate what people have suspected, that Thwaites Glacier is a bigger threat to future sea level than Pine Island.”

Poised for speed

Joughin’s team simulated how Thwaites responds to melting along the bottom of its floating ice tongue. They found that the glacier’s grounding line — the border between sections of ice that float on the sea and sections that rest on the bedrock — would retreat in stages as the ice disappears.

The Thwaites grounding line currently sits about 600 metres below sea level. But 60 to 80 kilometres inland, the bedrock topography drops to more than 1.2 kilometres below sea level. When the grounding line reaches that inward-sloping basin, the glacier's retreat will speed up dramatically, Joughin’s team calculates. That could happen in a matter of centuries.

Rignot and his colleagues used radar data from the now-defunct European Remote Sensing satellites to measure how the grounding line is retreating in Amundsen-area glaciers, including Thwaites and Pine Island. The work will be published in Geophysical Research Letters2.

The radar data show how the centre of Pine Island Glacier retreated by 31 kilometres between 1992 and 2011. It retreated fastest between 2005 and 2009, but has since slowed.

Rignot’s team also developed an improved picture of the topography of the bedrock beneath the ice. For each of the glaciers studied, they found no underlying ridge or other obstacle that could potentially slow the retreat.

“These systems, whether Greenland or Antarctica, are changing on faster timescales than we expected. We are kind of rediscovering that every day,” says Rignot.

Researchers have at least one new tool for watching the ice disappear. In April, the European Space Agency’s Sentinel-1 radar satellite began sending back data, which include measurements of ice motion. Shepherd has already used Sentinel-1 to gather fresh data on the Pine Island and Thwaites melting.

NASA may have to wait a little longer for its own dedicated satellite to monitor polar ice loss. Its Icesat-2 mission is not slated to launch until 2018 at the earliest, says Tom Wagner, programme scientist for the cryosphere at NASA in Washington DC. Icesat-2 would carry an altimeter system with six laser beams that would simultaneously measure the height of glaciers, providing better data as the ice melts.