Warm Oceans Wearing Away Antarctica’s Plug

Chris Mooney in the Washington Post:

A hundred years from now, humans may remember 2014 as the year that we first learned that we may have irreversibly destabilized the great ice sheet of West Antarctica, and thus set in motion more than 10 feet of sea level rise.

Meanwhile, 2015 could be the year of the double whammy — when we learned the same about one gigantic glacier of East Antarctica, which could set in motion roughly the same amount all over again. Northern Hemisphere residents and Americans in particular should take note — when the bottom of the world loses vast amounts of ice, those of us living closer to its top get more sea level rise than the rest of the planet, thanks to the law of gravity.

The findings about East Antarctica emerge from a new paper just out in Nature Geoscience by an international team of scientists representing the United States, Britain, France and Australia. They flew a number of research flights over the Totten Glacier of East Antarctica — the fastest-thinning sector of the world’s largest ice sheet — and took a variety of measurements to try to figure out the reasons behind its retreat. And the news wasn’t good: It appears that Totten, too, is losing ice because warm ocean water is getting underneath it.

“The idea of warm ocean water eroding the ice in West Antarctica, what we’re finding is that may well be applicable in East Antarctica as well,” says Martin Siegert, a co-author of the study and who is based at the Grantham Institute at Imperial College London.

University of Texas:

AUSTIN,Texas — Researchers at The University of Texas at Austin’s Institute for Geophysics (UTIG) in the Jackson School of Geosciences have discovered two seafloor gateways that could allow warm ocean water to reach the base of Totten Glacier, East Antarctica’s largest and most rapidly thinning glacier. The discovery, reported in the March 16 edition of the journal Nature Geoscience, probably explains the glacier’s extreme thinning and raises concerns about how it will affect sea level rise.

Totten Glacier is East Antarctica’s largest outlet of ice to the ocean and has been thinning rapidly for many years. Although deep, warm water has been observed seaward of the glacier, until now there was no evidence that it could compromise coastal ice. The result is of global importance because the ice flowing through Totten Glacier alone is sufficient to raise global sea level by at least 11 feet, equivalent to the contribution of the West Antarctic Ice Sheet if it were to completely collapse.

“We now know there are avenues for the warmest waters in East Antarctica to access the most sensitive areas of Totten Glacier,” said lead author Jamin Greenbaum, a UTIG Ph.D. candidate.

The ice loss to the ocean may soon be irreversible unless atmospheric and oceanic conditions change so that snowfall outpaces coastal melting. The potential for irreversible ice loss is due to the broadly deepening shape of Totten Glacier’s catchment, the large collection of ice and snow that flows from a deep interior basin to the coastline.

“The catchment of Totten Glacier is covered by nearly 2½ miles of ice, filling a sub-ice basin reaching depths of at least one mile below sea level,” said UTIG researcher Donald Blankenship.

Greenbaum and Blankenship collaborated with an international team from the United States, Australia, the United Kingdom and France.

Because much of the California-sized interior basin lies below sea level, its overlying thicker ice is susceptible to rapid loss if warm ocean currents sufficiently thin coastal ice. Given that previous work has shown that the basin has drained its ice to the ocean and filled again many times in the past, this study uncovers a means for how that process may be starting again.

“We’ve basically shown that the submarine basins of East Antarctica have similar configurations and coastal vulnerabilities to the submarine basins of West Antarctica that we’re so worried about, and that warm ocean water, which is having a huge impact in West Antarctica, is affecting East Antarctica, as well,” Blankenship said.

The deeper of the two gateways identified in the study is a three-mile-wide seafloor valley extending from the ocean to beneath Totten Glacier in an area not previously known to be floating. Identifying the valley was unexpected because satellite analyses conducted by other teams had indicated the ice above it was resting on solid ground. Special analysis of ice-penetrating radar data shows the bottom of the ice over the valley is smoother and brighter than elsewhere in the area — tell-tale signs that the ice is floating and being eroded by the ocean.

“Now we know the ocean is melting ice in an area of the glacier that we thought was totally cut off before,” Greenbaum said. “Knowing this will improve predictions of ice melt and the timing of future glacier retreat.”

Imperial College London:

“It’s only one glacier, but it’s changing now and it is significant for sea levels globally,” said study co-author Professor Martin Siegert, Co-Director of the Grantham Institute at Imperial College London. “The 3.5 metre rise may take several centuries to complete, but now the process has started it is likely irreversible. This is another example of how human-induced climate change could be triggering major changes with knock-on impacts that will be felt globally.”

The East Antarctic Ice Sheet was previously thought to be surrounded by colder water and so relatively stable compared to the smaller West Antarctic Ice Sheet, which is losing more than 150 cubic kilometres of ice each year. However, satellite data have shown that the Totten Glacier has also been thinning considerably. To investigate why, the team of researchers surveyed the area.

Using radar and other geophysical techniques, they obtained a map of the topographical landscape underlying the glacier where it met the sea. Their results revealed a 5 km-wide valley running underneath the glacier capable of letting warm ocean water reach the ice base.

Thin ice at the margins of ice sheets can float on the ocean, but the ice inland is ‘grounded’ and in contact with the bedrock. The newly-discovered valley allows warm ocean water to flow underneath a region of floating ice, exposing the grounded edge of the ice sheet to the warmth and leading to glacier melting.

Destabilisation of the Totten Glacier could leave more ice inland vulnerable to change, said Professor Siegert: “Once a certain region starts to change, the implications for the connected ice are potentially significant. We are using computer modelling to understand whether changes in Totten Glacier could lead to changes in both adjacent and more distant places in Antarctica. While this work needs to be undertaken, the change at Totten Glacier itself is significant and concerning.”

There are still many unknown landscapes beneath the Antarctic ice, and Professor Siegert and his colleagues are planning to map even more remote regions in an effort to understand the dynamics of the continent. These include two ‘poles of ignorance’ – spots on Antarctica where you can stand and be at least 200km from the nearest data point relating to the continent beneath the ice.