Not-So-Perma Frost

Daniel Fortier spends his summers studying the permafrost on Bylot Island, high in the eastern Canadian Arctic. While hiking there early in the 1999 field season, he distinctly heard the sound of running water yet saw no streams nearby. “I thought to myself, ‘Where is this sound coming from?'” says Fortier. “So, like a good researcher, I started to dig.”

Excavating the soil, known as permafrost because its temperature is below 0°C year-round, Fortier tapped into a torrent-filled tunnel a meter or so below the surface. By tracking the water course uphill, he found its source: Large volumes of snowmelt had flowed into open fissures in the ground and had then melted a passage through a network of subterranean ice wedges that had formed over millennia (SN: 5/17/03, p. 314: Patterns from Nowhere).

Eventually, the surprising tunnel grew so wide that its roof caved in, creating a gully that erosion then widened, says Fortier, a geomorphologist at the University of Alaska in Fairbanks. By the end of the summer, that gully was about 250 m long and 4 m wide. During the next 4 years, the network of underground tunnels at the site turned into a 750-m-long system of gullies that drained an area about the size of four soccer fields. Since then, Fortier and his colleagues have observed the same phenomenon at other sites on Bylot Island.

Several teams of scientists had previously described similar networks of gullies at various sites in the Arctic, but those highly eroded features had been deemed as much as several thousand years old. “No one had ever seen one of these things forming,” says Fortier. “We were in the right place at the right time.”

Researchers are observing many new phenomena in the Arctic—most of them related to the world’s changing climate. Globally, 11 of the 12 years from 1995 to 2006 are among the dozen warmest since the mid-1800s, scientists of the Intergovernmental Panel on Climate Change reported last month (SN: 2/10/07, p. 83: Available to subscribers at From Bad to Worse: Earth’s warming to accelerate). Average temperatures worldwide have risen about 0.7°C in the past 100 years, but those in the Arctic have risen even more. In high-latitude portions of Alaska and western Canada, average summer temperatures have increased by about 1.4°C just since 1961 (SN: 11/12/05, p. 312: Available to subscribers at Runaway Heat?).

Those warmer air temperatures are significantly boosting soil temperatures in many regions, new studies show. Because the average annual temperature at many Arctic sites sits at or just below water’s freezing point, even a small increase in local warming can have big consequences. Besides rendering underground ice wedges more susceptible to melting, the hike in temperatures threatens near-surface permafrost that has been in place since the height of the last ice age, about 25,000 years ago. Ecological changes, such as shifts in the patterns and timing of forest fires, further endanger near-surface permafrost. But researchers are still working out whether the permafrost will disappear over decades or millennia.

Permafrost serves as a stable foundation for much of the Arctic’s infrastructure, including pipelines, roads, buildings, and bridges. In many areas, that frozen ground also contains huge amounts of organic material, which could readily decompose and send carbon dioxide, a greenhouse gas, into the atmosphere if the permafrost thaws (SN: 11/12/05, p. 312: Available to subscribers at Runaway Heat?).

Balancing act

When most people think of permafrost, they envision the coldest Arctic landscapes, where layers of ground hundreds of meters thick have remained deep-frozen since the last ice age, maybe even longer. However, permafrost need not be either long-lived or icy. Geologists consider any soil or rock that’s been colder than 0°C for more than 2 years to be permafrost.

Permafrost lies beneath as much as 25 percent of the land area of the Northern Hemisphere. Although much of the frozen ground occurs in high-latitude regions, the rocky summits of many high-altitude peaks in temperate and tropical latitudes also consist of permafrost, says Margareta Johansson, a physical geographer at the Abisko Scientific Research Station in Abisko, Sweden. She and her colleagues have conducted long-term permafrost studies in the region surrounding Abisko, which is about 200 kilometers north of the Arctic Circle. They reviewed their findings in the June 2006 Ambio.

The presence or absence of permafrost at any particular spot depends on the balance between geothermal heat making its way up from Earth’s interior and the average annual air temperature at the site, says Johansson. “The lower a site’s average air temperature is, the more heat the air pulls from the ground,” she notes, leaving the soil colder and the permafrost thicker.

The slope of the terrain has a significant effect as well. South-facing slopes usually receive more direct sunlight and therefore are warmer than flat terrain would be. By contrast, northern slopes spend much of the day in shade, so soil temperatures there are chillier than the region’s average and more conducive to the formation of permafrost.

Although permafrost can form in any climate where the average annual air temperature is below freezing, it doesn’t normally occur or persist widely until temperatures are substantially lower, says Johansson. When an area’s average temperature lies between 0°C and –1.5°C, permafrost is patchy and typically underlies no more than 10 percent of the region. At sites with average air temperatures below –6°C, few spots if any are free of permafrost.

“The amount of snowfall at a site significantly affects the permafrost there, but in a counterintuitive way,” says Johansson. When snow forms a thick blanket that lasts all winter, it insulates the ground from the most frigid air of the year. Near Abisko, which receives only about 30 centimeters of snow each year, the permafrost is about 16 meters thick, the deepest in the region, she notes. At similarly cold sites that receive as little as 1 m of snowfall each winter, permafrost is patchier and only a few meters thick.

In experiments at several sites in the Abisko region, Johansson and her colleagues piled up extra snow at some sites, artificially doubling or tripling the snowfall that the spot would normally receive over a winter. As a result, average ground temperatures rose as much as 2.2°C. That large a change can melt underlying permafrost.

Scientists elsewhere have noted that winter snow cover can keep the average ground temperature as much as 10°C higher than the average air temperature, Johansson notes.

It’s often difficult for scientists to accurately predict how vegetation will affect ground temperatures, says Johansson. Evergreen trees and shrubs cast shadows that cool the ground during the summer. However, the vegetation forms a windbreak that tends to trap snow in winter, creating drifts that warm the soil. Computer simulations suggest that shrubby sites in northern Alaska accumulate as much as 20 percent more snow than bare ones do, and scientists have found that the soil in shrubby areas is about 2°C warmer than soil in shrub free spots nearby.

Fire and ice

The wildfires that intermittently ravage Arctic forests can exact a harsh toll from permafrost. It’s not the heat of the conflagration that does the damage but the changes that take place after the fire dies down.

A severe fire strips away the foliage that shades the forest floor. The resulting increase in sunlight reaching the ground boosts soil temperature, says Eric S. Kasischke, a fire ecologist at the University of Maryland, College Park.

An even greater warming effect stems from the fire’s consumption of the limbs, twigs, needles, and leaves that had fallen to the ground and insulated it. Unlike a blanket of snow, forest litter insulates the ground year-round. It keeps the ground warmer in winter and cooler in summer. On balance, the insulation favors permafrost formation and retention.

Consider what happens in a black spruce forest, the type that makes up more than half of North America’s boreal forests. Scientists have gathered data at more than 200 central-Alaska sites that had recently suffered wildfires. On average, between 50 and 60 percent of the forest-floor litter goes up in smoke during a fire, Kasischke and his colleagues reported at a meeting of the American Geophysical Union in San Francisco last December.

After a fire has destroyed so much litter, a much thicker surface layer of soil thaws each summer, says Kasischke. During the growing season, seedlings quickly become established in that thawed soil. Then, as trees mature, they shade the ground more effectively and drop limbs and needles to reestablish the forest floor’s veneer of insulation.

Computer models suggest that permafrost begins to recover when organic material on the forest floor accumulates to a depth of at least 9 cm. In a region where trees grow slowly, that could take decades.

The interval between wildfires in any particular patch of boreal forest ranges between 30 and 300 years, Kasischke notes. But, the postfire recuperation of a forest’s permafrost isn’t a sure bet. Because today’s climate in a region may be substantially warmer than it was the last time fire swept through, conditions may not be conducive to permafrost recovery.

Hanging on

When the centuries-long cold spell called the Little Ice Age ended about 150 years ago, glaciers and permafrost reached their maximum extent of the past few millennia. Deep remnants of that permafrost will probably persist for millennia to come. However, in a world that’s warming, it’s only a matter of time until much of that ice melts. Most permafrost loss will take place at shallow depths, where it will have the greatest effect on ecosystems and people.

In many regions, permafrost temperatures, like air temperatures, have been climbing steadily for decades, says Sergei Marchenko, a permafrost researcher at the University of Alaska in Fairbanks. Data gathered in field studies since the early 1970s indicate that permafrost temperatures in the Altai region of Mongolia and the Tian Shan mountains of central Asia have risen as much as 0.2°C per decade, he notes. Similar rates of warming have been observed on the Tibetan Plateau since 1985.

In the Tian Shan mountains, the thickness of the seasonally thawed layer has increased 23 percent since the early 1970s. It’s now 5 m thick, says Marchenko. Climate simulations suggest that since the end of the Little Ice Age, the lowest altitude at which permafrost could persist has climbed about 200 m. During that time, about 16 percent of the region’s permafrost would have disappeared, according to the model that Marchenko and his University of Alaska colleague Vladimir Romanovsky described at the American Geophysical Union meeting.

Measurements taken inside three boreholes, each at least 400 m deep, at a mine in the barren terrain of northern Quebec also chronicle modern-day warming, says Christian Chouinard, a paleoclimatologist at McGill University in Montreal. The data suggest that surface soil has heated up about 2.75°C in the past 150 years, he and his colleagues reported at the meeting.

A slight cooling trend in the region from the 1940s to the early 1990s has since been replaced by extremely rapid warming—more than 1°C in the past 15 years or so, the researchers note.

Permafrost can be quick to warm to its melting point but then slow to melt. The energy needed to melt a block of ice at 0°C is about 160 times the amount that’s needed to raise its temperature from –1°C to 0°C, says Sharon L. Smith, a permafrost researcher at the Geological Survey of Canada in Ottawa.

Data gathered throughout Canada show that permafrost in the coldest regions of the country is steadily warming, as are soils in areas free of permafrost. However, in the areas where permafrost sits at its melting point, ground temperatures aren’t changing significantly. Much of the air’s thermal energy goes into melting the permafrost rather than into warming it.

About 42 percent of Canada’s land area, or about 4 million square kilometers, overlies permafrost, says Smith. In about half that area, the permafrost is patchy and thin, with a temperature above –2°C. If many scientists’ climate-warming scenarios come to pass, Smith says, “permafrost in those regions could ultimately disappear.”

When it will disappear is another issue. Research published in 2005 sparked a major debate. In that report, climate scientists David M. Lawrence of the National Center for Atmospheric Research in Boulder, Colo., and Andrew G. Slater of the University of Colorado at Boulder suggested that climate warming will wipe out more than 90 percent of the world’s near-surface permafrost by the year 2100.

That dramatic claim is almost certainly wrong, says Christopher Burn, a permafrost researcher at Carleton University in Ottawa. Burn says that although he doesn’t dispute the predictions of climate warming, he does question Lawrence and Slater’s predictions concerning the pace and extent of the permafrost’s demise.

Burn says that the Colorado scientists’ estimate requires that permafrost melt almost instantaneously. Instead, the time lag between the climate warming and the permafrost melting will probably be hundreds of years, he suggests.

Lawrence agrees that the computer model that he and Slater used for their study had some limitations—for instance, it included only the top 3.4 m of the ground and didn’t account for conditions associated with some soil types. The pair has now modified its model to look 50 m into the ground, says Lawrence. Preliminary results suggest that this deeper permafrost will indeed last longer than they’d previously predicted—but only a couple of decades longer at most—he reports.

Nevertheless, Burn says that the model doesn’t take into account the cooling effect of permafrost that lies deeper. For example, permafrost in Alaska and western Canada extends as much as 600 m into the ground, and in Siberia it’s more than 1.5 km thick. “The persistence of permafrost increases with its thickness,” Burn adds. So, deep soil will stay cold for millennia, thereby putting brakes on the warming of the higher layers.

Whatever the rate of permafrost loss, Earth’s rapidly warming climate will continue to gnaw at the long-frozen soil that serves as the bedrock of the Arctic. The carbon dioxide that will probably be released in the process will only tend to accelerate the permafrost’s disappearance.