Warming ocean water undercuts Antarctic ice shelves

“Upside-down rivers” of warm ocean water threaten the stability of floating ice shelves in Antarctica, according to a new study led by researchers at the National Snow and Ice Data Center published today in Nature Geoscience. The study highlights how parts of Antarctica’s ice sheet may be weakening due to contact with warm ocean water.

“We found that warm ocean water is carving these ‘upside-down rivers,’ or basal channels, into the undersides of ice shelves all around the Antarctic continent. In at least some cases these channels weaken the ice shelves, making them more vulnerable to disintegration,” said Karen Alley, a graduate research assistant at NSIDC and lead author of the study. Alley is also a Ph.D. student in the University of Colorado Boulder’s Department of Geological Sciences.

Ice shelves are thick floating plates of ice that have flowed off the Antarctic continent and spread out onto the ocean. As ice shelves flow out to sea, they push against islands, peninsulas, and bedrock bumps known as “pinning points.” Contact with these features slows the flow of grounded ice off the continent. While ice shelves take thousands of years to grow, previous work has shown that they can disintegrate in a matter of weeks. If more ice shelves disintegrate in the future, loss of contact with pinning points will allow ice to flow more rapidly into the ocean, increasing the rate of sea level rise.

“Ice shelves are really vulnerable parts of the ice sheet, because climate change hits them from above and below,” said Ted Scambos, NSIDC lead scientist and study co-author. “They are really important in braking the ice flow to the ocean.”

The features form as buoyant plumes of warm and fresh water rise and flow along the underside of an ice shelf, carving channels much like upside-down rivers. The channels can be tens of miles long, and up to 800 feet “deep.”

When a channel is carved into the base of an ice shelf, the top of the ice shelf sags, leaving a visible depression in the relatively smooth ice surface. Alley and her colleagues mapped the locations of these depressions all around the Antarctic continent using satellite imagery, as well as radar data that images the channels through the ice, mapping the shape of the ice-ocean boundary.

The team also used satellite laser altimetry, which measures the height of an ice shelf surface with high accuracy, to document how quickly some of the channels were growing. The data show that growing channels on the rapidly melting Getz Ice Shelf in West Antarctica can bore into the ice shelf base at rates of approximately 10 meters (33 feet) each year.

The mapping shows that basal channels have a tendency to form along the edges of islands and peninsulas, which are already weak areas on ice shelves. The team observed two locations where ice shelves are fracturing along basal channels, clear evidence that basal channel presence can weaken ice shelves to the point of breaking in vulnerable areas.

While no ice shelves have completely disintegrated due to carving by basal channels, the study points to the need for more observation and study of these features, said co-author Helen Amanda Fricker of Scripps Institution of Oceanography at UC San Diego. “It's feasible that as ocean temperatures around Antarctica continue to rise, melting in basal channels could contribute to increased erosion of ice shelves from below.”

The study, “Impacts of warm water on Antarctic ice shelf stability through basal channel formation,” was led by University of Colorado Boulder Ph.D. student Karen Alley, who worked with coauthors Ted Scambos of NSIDC and Matthew Siegfried and Helen Amanda Fricker of Scripps Institution of Oceanography, UC San Diego. Their work was funded in part by NASA and the U.S. Geological Survey. More

Contacts

Jane Beitler,Communications, National Snow and Ice Data Center, press@nsidc.org, +1-303-492-1497
Brittany Hook, Communications Coordinator, Scripps Institution of Oceanography, scrippsnews@ucsd.edu, 858-534-3624

 

Why Greenland’s “Dark Snow” Should Worry You

Jason Box’s research into Greenland’s dark snow raises more concerns about climate change.

Jason Box knows ice. That’s why what’s happened this year concerns him so much. Box just returned from a trip to Greenland. Right now, the ice there is … black:

The ice in Greenland this year isn’t just a little dark—it’s record-setting dark. Box says he’s never seen anything like it. I spoke to Box by phone earlier this month, just days after he returned from his summer field research campaign.

“I was just stunned, really,” Box told me.

The photos he took this summer in Greenland are frightening. But their implications are even more so. Just like black cars are hotter to the touch than white ones on sunny summer days, dark ice melts much more quickly.

As a member of the Geological Survey of Denmark and Greenland, Box travels to Greenland from his home in Copenhagen to track down the source of the soot that’s speeding up the glaciers’ disappearance. He aptly calls his crowdfunded scientific survey Dark Snow.

There are several potential explanations for what’s going on here. The most likely is that some combination of increasingly infrequent summer snowstorms, wind-blown dust, microbial activity, and forest fire soot led to this year’s exceptionally dark ice. A more ominous possibility is that what we’re seeing is the start of a cascading feedback loop tied to global warming. Box mentions this summer’s mysterious Siberian holes and offshore methane bubbles as evidence that the Arctic can quickly change in unpredictable ways.

This year, Greenland’s ice sheet was the darkest Box (or anyone else) has ever measured. Box gives the stunning stats: “In 2014 the ice sheet is precisely 5.6 percent darker, producing an additional absorption of energy equivalent with roughly twice the US annual electricity consumption.”

Perhaps coincidentally, 2014 will also be the year with the highest number of forest fires ever measured in Arctic.

Box ran these numbers exclusively for Slate, and what he found shocked him. Since comprehensive satellite measurements began in 2000, never before have Arctic wildfires been as powerful as this year. In fact, over the last two or three years, Box calculated that Arctic fires have been burning at a rate that’s double that of just a decade ago. Box felt this finding was so important that he didn’t want to wait for peer review, and instead decided to publish first on Slate. He’s planning on submitting these and other recent findings to a formal scientific journal later this year.

Box’s findings are in line with recent research that shows the Arctic is in the midst of dramatic change.

A recent study has found that, as the Arctic warms, forests there are turning to flame at rates unprecedented in the last 10,000 years. This year, those fires produced volumes of smoke and soot that Box says drifted over to Greenland.

In total, more than 3.3 million hectares burned in Canada’s Northwest Territories alone this year—nearly 9 times the long term average—resulting in a charred area bigger than the states of Connecticut and Massachusetts combined. That figure includes the massive Birch Creek Complex, which could end up being the biggest wildfire in modern Canadian history. In July, it spread a smoke plume all the way to Portugal.

In an interview with Canada’s National Post earlier this year, NASA scientist Douglas Morton said, “It’s a major event in the life of the earth system to have a huge set of fires like what you are seeing in Western Canada.”

Box says the real challenge is to rank what fraction of the soot he finds on the Greenland ice is from forest fires, and what is from other sources, like factories. Box says the decline of snow cover in other parts of the Arctic (like Canada) is also exposing more dirt to the air, which can then be more easily transported by the wind. Regardless of their ultimate darkening effect on Greenland, this year’s vast Arctic fires have become a major new source of greenhouse gas emissions from the thawing Arctic. Last year, NASA scientists found “amazing” levels of carbon dioxide and methane emanating from Alaskan permafrost.

Earlier this year, Box made headlines for a strongly worded statement along these lines:

That tweet landed Box in a bit of hot water with his department, which he said now has to approve his media appearances. Still, Box’s sentiment is inspiring millions. His “f’d” quote is serving as the centerpiece of a massive petition (with nearly 2 million signatures at last count) that the activist organization Avaaz will deliver to “national, local, and international leaders” at this month’s global warming rally in New York City on Sept. 21. More