What about that hole in the sea ice?

There’s been discussion about a big opening in sea ice, called a polynya, and if it had anything to do with the Russian expedition ship, Akademik Shokalskiy, getting stuck near the Antarctic coast. The answer is not so straightforward. “In the winter, polynyas can close up really quickly,” said Kevin Arrigo, a professor at Stanford University. When they close, whatever is inside may be trapped.

Polynyas allow access to Antarctica’s shores, and most Antarctic research stations exist where they do because of polynyas. Winds blow, push the ice off the shore, and create holes in the sea ice. Ships want to go into these open spaces. “If the winds stop blowing, polynyas can just freeze over and fill in,” Arrigo said. That is partially what happened with Akademik Shokalskiy, but not the full picture. Also, polynyas are not only an essential component of navigating sea ice; they are vital to polar ecosystems. Here’s why:

On the horizon

Most of the polynyas around Antarctica are wind driven. “In many ways, where those winds blow depends on topography, by the shape of the hills and mountains,” Arrigo said. Strong winds, known as katabatic winds, come off the cold ice sheet, flow downhill and continually blow ice off of the coastal shore.

McMurdo Station

McMurdo Station sits on Ross Island in McMurdo Bay in Antarctica. It is a critical access point for ships to resupply researchers. The station also houses the largest population of about 1200 people on the continent. The Ross Sea polynya is visible in the distance. Credit: Ted Scambos, NSIDC

Many polynyas recur in the same region year after year—a useful manual for navigating the waters. “Having open water reoccur in the same region at certain times of year, makes it possible to get ships in and out,” said Seelye Martin, a professor at the University of Washington. McMurdo Station, a research base on Ross Island in Antarctica, exists because of the Ross Sea polynya, which forms in October and November. It’s also a reason why the base is important to the history of exploration. In the early 1900s, Robert F. Scott and Ernest Shackleton ventured into the continent from this entry point in part because of the polynya, which enabled ships to approach the land.

Polynyas vary dramatically in shape and size, some deceivingly large. When early explorers ventured into open waters, they often mistook the expanse for new sea. Still, others have seen something even more peculiar. “When I used to work in the Arctic,” Martin said, “we would see these cloudy regions, thinking that we were seeing signs of industrial activity.” But really it was a polynya at work, generating low clouds and fog. “We called it sea smoke,” he said. This sea smoke billows, forming a warm microclimate like, for instance, the North Water Polynya, on the northern end of Baffin Bay off the Greenland coast. This polynya provides refuge for big mammals: narwhal, walrus, and various species of whales to feed and rest.

So more happens within these open waters than meets the eye.

The light switch

NASA image of a Ross Sea polynya

The image from 2004 above shows a polynya (the dark opening) in the Ross Sea near Antarctica. The polynya is located just north of the Drygalski Ice Tongue. Giant icebergs float from shore to the right of the image. And in the dark open water, herded ice crystals generate streaks, which move parallel to the wind. Credit: NASA Moderate Resolution Imaging Spectroradiometer (MODIS)

The polar regions see no light in their respective winters. “That’s when polynyas become little ice engines,” Martin said. When winds push the ice pack away, near freezing point water temperatures generate more ice. The wind pushes formed ice crystals with the wave, piling them against the edge of a polynya, like swelling debris on a shore. Long streaks of ice crystals form. “You can look at a satellite image and spot the polynyas because they look like corduroy,” Martin said.

Along with wind, another environmental force forms polynyas: ocean upwelling. Upwelling, or the movement of deep, dense, and nutrient-rich waters toward the ocean surface, causes most open-ocean polynyas—though this process is much less common. “Over a big chunk of the world there is a layer of warm water that sits between 200 meters and 800 meters,” Arrigo said. “Anytime that water makes it up to the surface, it has the ability to melt ice.”

Sometimes wind sweeping and upwelling maintain the same polynya. Then spring comes and something else happens.

Common Eider rests in Arctic

For the Common Eider, a duck species that lingers year-round in the Arctic, polynyas are a matter of survival. It breeds in the Arctic, diving for mollusks and mussels. The male is unmistakable with its black and white plumage and green nape. The female is brown. The image above captures the Common Eider resting on rocks near Kirkcaldy Fife, Scotland. Credit: Gordon Ednie, flickr.com

“Come early spring, polynyas are ice free first,” Arrigo said, “but phytoplankton in these high latitude waters can’t start growing until the light is turned on.” And when light appears, so do the first signs of life. The lack of sea ice in polynyas permits sunlight to reach the upper ocean layers. Sunlight, combined with the warmer temperatures, causes polynyas to brim with phytoplankton. This primary food source turns polynyas into feeding lots for fish, seals, penguins, and other marine mammals. “Polynyas are cool things,” Arrigo said. “They’re basically these oases in areas of the ocean where you would not expect to find much life.”

An undulating system

Sea ice, however, is dynamic. For the Akademik Shokalskiy, it wasn’t merely a shift in the wind pattern, but in part, a past event—the breaking of a glacier—catching up to it. Blocks of the glacier floated in, surrounding the ship. Something similar happened in the Ross Sea in 2000 and 2002 when ice broke off the ice shelf: the Ross Sea Polynya almost didn’t form. “The winds were blowing right but the ice had nowhere to go,” Arrigo said. Gigantic icebergs blocked the flow of ice—and shut down food production. “That was a really bad year for the animals,” he added. “Overall, however, we wouldn’t have nearly as many penguins in Antarctica where it not for polynyas.” And we wouldn’t have nearly as many ships transporting researchers to study them.

Can liquid water persist within an ice sheet?

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Scientists have discovered a large aquifer, the size of Ireland, near the surface of the Greenland Ice Sheet. “This was a big surprise,” said Jason Box, a researcher for the Geological Survey of Denmark and Greenland, “because we were drilling before melt had begun.” So liquid water had to survive since at least the previous year. Such water storage within the ice had not been previously considered, not on this massive scale. How can a giant reservoir of water exist inside a frozen ice sheet?

Scientist Jason Box drills a long hole to extract ice core samples. These enable him to map precipitation and melt levels for the South Eastern tip of Greenland. Credit: Nick Cobbing/Greenpeace.

Scientist Jason Box drills a long hole to extract ice core samples. These enable him to map precipitation and melt levels for the South Eastern tip of Greenland. Credit: Nick Cobbing/Greenpeace.

A new chapter

“We’re adding a chapter to the textbooks and that chapter is not yet written,” Box said. Scientists know how glaciers and ice sheets form. New layers of snow bury old layers, forcing snow to recrystallize into firn, an intermediary stage between snow and ice before finally becoming glacial ice. The compression traps pockets of air, critical in ice coring and analyzing Earth’s past atmosphere. “But now we have a completely different storage mechanism,” said Richard Forster, a researcher at the University of Utah, “where liquid water is stored as a reservoir in the firn year round.” This is new.

The discovery was an accident. Southeast Greenland, known for its high snow accumulation rates, has been understudied; but Box and Forster wanted to know just how much. In drilling for ice cores, they struck water. Traversing the rough terrain with snowmobiles, they moved the operation several kilometers away, and hit water again.

Water drains from an extracted core, pulled out 12 meters (40 feet) below the surface of the Greenland Ice Sheet. Just below the snow, water persists within the firn layer before the summer surface melt, with air temperatures of minus 15 degrees Celsius (5 degrees Fahrenheit). Credit: Ludovic Brucke, NASA

Water drains from an extracted core, pulled out 12 meters (40 feet) below the surface of the Greenland Ice Sheet. Just below the snow, water persists within the firn layer before the summer surface melt, with air temperatures of minus 15 degrees Celsius (5 degrees Fahrenheit). Credit: Ludovic Brucke, NASA

This was not a fluke. It was big. Consulting radar instruments, the team was able to map the aquifer’s size. Radar waves propagate through the ice sheet and as they hit different media—from air to ice or snow to water—the wavelengths reflect. Electrical properties also shift with density, and water is a great conductor. “When you hit a liquid layer, you get a really big reflection,” Forster said. This shimmering layer stretches 70,000 square kilometers (27,000 square miles). The ice sheet here is about 800 meters (2600 feet) thick, while the liquid is between 5 and 50 meters (16 to 160 feet) deep. This aquifer then rests in the upper skin of the ice sheet beneath the snow layer, within the firn. But why doesn’t this water refreeze?

Feathers and air bubbles

“New snow acts like a thermal insulator,” said Forster. “Like a down jacket, the air space between the feathers—that’s what’s keeping you warm.” The air space between snow grains insulates liquid water. But another process is also at work. “Certainly some of that liquid water is feeling the cold atmosphere,” said Forster. As some of the water refreezes, it releases latent heat. Surrounding water absorbs this heat, and along with the snow insulation, provides enough warmth to maintain water in liquid state.

Relying on modeling, scientists determined the aquifer has existed since the 1970s. So far, fieldwork confirms the models. “No matter how sophisticated the satellite, aircraft or computer model is, you always need to gauge its accuracy,” Box said. “Even very low tech observations on ground tell us a lot.” Since April 2011, when the team first identified the aquifer, they have returned twice. Models show the aquifer extent has only marginally increased, but fieldwork is the only way to determine its thickness beneath the surface. For now, only one in situ measurement exists, recording a thickness of 22 meters (72 feet), between 12 and 34 meters (39 to 112 feet) deep.

(Left) Scientist Jason Box extracts an ice core sample, which enables him to map precipitation and melt levels to build a map of weather and climate changes on the south eastern tip of Greenland. (Right) Scientist Jason Box scrutinizes an ice core extract. Credit: Nick Cobbing/Greenpeace

(Left) Scientist Jason Box extracts an ice core sample, which enables him to map precipitation and melt levels to build a map of weather and climate changes on the south eastern tip of Greenland. (Right) Scientist Jason Box scrutinizes an ice core extract. Credit: Nick Cobbing/Greenpeace

The two extremes

How does this water factor into global sea level rise? “Now we have to really figure out how the water is moving through this aquifer system,” said Forster. Glaciers and ice sheets are dynamic. How much water eventually enters the ocean from melting ice sheets has always been a key question in calculating sea level rise. The two extremes of this aquifer system depend on where water goes. It either doesn’t go anywhere or it migrates through the system, connects to crevasses or moulins, vertical pipelines, and gets to the oceans, a process that may take a few years or decades. “In reality,” Forster said, “it’s probably some of each, depending on the location.”

The next step is to do more fieldwork and to date the water using chemical analysis. Figuring out the depth of liquid water is essential to knowing just how much water there is. Only then can contribution to sea level rise be accurately calculated. “You get this aquifer in areas where there is a lot of melting and a lot of snowfall,” Box said. “I think it’s natural, not climate driven, but we need to figure out the climate change response for this and that could have really fundamental implications for the ice properties and ice flow.”

Video

Water drains from a core out of the Greenland perennial firn aquifer. The extraction is lifted from 33 feet below the surface of the ice sheet. 

Reference

Forster, Richard F. et. all. 2013. Extensive liquid meltwater storage in firn within the Greenland ice sheet. Nature Geoscience. doi:10.1038/ngeo2043.

Are scientists conservative about sea ice?

The U.S. Coast Guard Cutter Healy encountered only small patches of sea ice in the Chukchi Sea when this photograph was taken on July 20, 2011. (Courtesy NASA Goddard Space Flight Center)

The U.S. Coast Guard Cutter Healy encountered only small patches of sea ice in the Chukchi Sea when this photograph was taken on July 20, 2011. (Courtesy NASA Goddard Space Flight Center)

Guest post by Walt Meier, NSIDC Scientist

Arctic sea ice set a record minimum extent in September 2012, far below the previous record low in 2007. Summer extents have been far lower than average for the last decade, with several record or near-record years. Looking at the numbers, one is tempted to think that the Arctic Ocean may reach nearly sea ice-free conditions within just a few years. But most expert analyses indicate that we’re likely at least a couple decades away from seeing a blue Arctic Ocean during the summer.

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Are Greenland’s galloping glaciers slowing down?

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One of the three major ice streams studied, the outlet of the Jakobshavn-Isbræ glacier produces about 10 percent of Greenland’s icebergs due to calving. Located in West Greenland, its icebergs float down the fjord, sometimes getting stuck in shallower waters for years. Though the glacier’s acceleration rate has fluctuated through the years, it is still a major contributor to Greenland’s ice loss. Courtesy Spencer Weart, flickr (http://www.flickr.com/photos/weartpix/4077525679/)

For the past decade, Greenland’s ice sheet has been losing its ice more rapidly, raising concerns about its contribution to sea level rise. A recent study, published in Nature, proposes that Greenland could slow its shedding of ice from its massive ice sheet into the ocean. “This doesn’t mean glacial recession and melting will slow,” said Faezeh M. Nick, a glaciologist from the University Centre in Svalbard in Norway. Nick’s study points out that the problem is not so straightforward. Continue reading