Can liquid water persist within an ice sheet?

Featured

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.

Celebrating 35 years of sea ice satellite data

Image of Arctic sea ice derived from SMMR data

This image is derived from the Scanning Multichannel Microwave Radiometer (SMMR), and shows Arctic Ocean sea ice extent in August 1985. Purple and red show greater ice coverage, while greens and blues indicate less ice. The black circle over the pole indicates no data—SMMR took observations very close to, but not directly over, the pole. Image credit: NSIDC

Polar scientists are celebrating an anniversary of sorts. Thirty-five years ago, sea ice research took a great leap forward. On October 26, 1978, the Scanning Multichannel Microwave Radiometer (SMMR) beamed its first data records back down to Earth. The instrument, pronounced simmer, was capable of mapping global sea ice concentration and extent, giving scientists a more comprehensive look at Arctic and Antarctic sea ice. Thanks to SMMR and its successor remote sensing instruments, scientists now have a long and detailed record of sea ice that helps them understand how sea ice works, and how it is changing. Continue reading

Do satellites sometimes see ice where there isn’t any?

Readers often ask us, “Why does your sea ice map show sea ice where there is none?” Sometimes our Daily Sea Ice Extent images show sea ice in a particular area, but when readers who live in those areas look out their windows, they see open water—or they even may see ice where our maps show open water. This occurs most frequently along rivers or near coasts. Why does this happen?

Ups and downs of passive microwave

Photograph of Qaanaaq, a small town on the Greenland coast

The rugged coast near Qaanaaq, Greenland, illustrates the challenge to satellite sensors, which must distinguish between land and ocean signals within the same image. Credit: Andy Mahoney

These discrepancies are most often caused by the resolution of the satellite sensor. NSIDC relies on passive microwave sensors to compile daily sea ice maps. These sensors have the advantage of being able to see through the Arctic’s cloudy weather and capture surface data even during long, dark winters, making them ideal for tracking sea ice. The disadvantage, however, is that passive microwave sensors often have low spatial resolution. The sensors collect data in “footprints” that are up to 50 to 70 kilometers (31 to 44 miles) in diameter. Continue reading

Getting beneath the ice

Researchers can measure ice thickness by drilling holes in the sea ice. But the method is not a practical way to measure thickness over the millions of square miles of Arctic sea ice. Image courtesy of Martin Hartley

NSIDC reports ice extent, a two-dimensional measure of the Arctic Ocean’s ice cover. But sea ice extent tells only part of the story: sea ice is not all flat like a sheet of paper. While freshly formed ice might not be much thicker than a few sheets of paper, the oldest, thickest ice in the Arctic can be more than 15 feet thick—as thick as a one-story house. Scientists want to know not just how far the ice extends, but also how deep and thick it is, because thinner ice is more vulnerable to summer melt. Continue reading

Arctic sea ice before satellites

Last week, a reader of Arctic Sea Ice News & Analysis asked what we know about Arctic sea ice extent before the satellite records began in 1979. Those records show that Arctic sea ice has been declining at an increasing pace since 1979—enough data to see a strong signal of climate change. But scientists also want to know what sea ice was like before satellites were there to observe it. Mark Serreze, NSIDC director and research scientist, said, “The better we understand how the climate system behaved in the past, the better we can understand and place into context what is happening today.”  What do we know about sea ice conditions before 1979, and how do we know that?

Sea ice charts of the Arctic Ocean show that ice extent has declined since at least the 1950s. Credit: NSIDC and the UK Hadley Center

Historical data on sea ice

Scientists have pieced together historical ice conditions to determine that Arctic sea ice could have been much lower in summer as recently as 5,500 years ago. Before then, scientists think it possible that Arctic sea ice cover melted completely during summers about 125,000 years ago, during a warm period between ice ages. Continue reading