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Permafrost erosion

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SCIENCE NEWS

Vol. 171, No. 10

Week of March 10, 2007

Not-So-Perma Frost

Warming climate is taking its toll on subterranean ice

Sid Perkins

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:

http://www.sciencenews.org/articles/20030517/bob10.asp).

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. 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

http://www.sciencenews.org/articles/20051112/bob9.asp).

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

http://www.sciencenews.org/articles/20051112/bob9.asp).

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.

TOP VIEW. Permafrost, depicted in various shades of purple, underlies

about one-fourth of the Northern Hemisphere's land area. The darker

the purple, the greater the percentage of local landscape that

permafrost underlies.

Intl. Permafrost Assn. and P. Rekacewicz/UNEP/GRID-Arendal

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 80

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.

To sign up for the free weekly e-LETTER from Science News, go to

http://www.sciencenews.org/pages/subscribe_form.asp.

References:

Alley, R., et al. 2007. Climate Change 2007: The Physical Science

Basis: Summary for Policymakers. Intergovernmental Panel on Climate

Change. Feb. 2. Full text available at

http://www.ipcc.ch/SPM2feb07.pdf. (Information regarding past and

future IPCC reports can be found at http://www.ipcc.ch).

Burn, C.R., and F.E. Nelson. 2006. Comment on "A projection of severe

near-surface permafrost degradation during the 21st century" by David

M. Lawrence and Andrew G. Slater. Geophysical Research Letters

33(Nov. 16):L21503. Abstract available at

http://dx.doi.org/10.1029/2006GL027077.

Chouinard, C., R. Fortier, and J. Mareschal. 2006. Ground surface

temperature history inferred from a borehole temperature profile

through the permafrost in northern Quebec: Evidence for recent

warming (Presentation C51B-0420). American Geophysical Union meeting.

Dec. 11-15. San Francisco. Abstract.

Fortier, D., Y. Shur, and M. Allard. 2006. Role of underground

erosion of ice wedges in drainage system formation (Presentation

C51B-0408). American Geophysical Union meeting. Dec. 11-15. San

Francisco. Abstract.

Johansson, M., et al. 2006. What determines the current presence or

absence of permafrost in the Torneträsk region, a sub-arctic

landscape in Northern Sweden? Ambio 35(June):190-197. Abstract.

Kasischke, E.S., et al. 2006. Consumption of surface organic layer

carbon during fires in Alaskan black spruce forests (Presentation

B43B-0268). American Geophysical Union meeting. Dec. 11-15. San

Francisco. Abstract.

Lawrence, D.M., and A.G. Slater. 2005. A projection of severe

near-surface permafrost degradation during the 21st century.

Geophysical Research Letters 32(Dec. 28):L24401. Abstract available

at http://dx.doi.org/10.1029/2005GL025080.

Marchenko, S., and V. Romanovsky. 2006. Temporal and spatial changes

of permafrost in the Tien Shan Mountains since the Little Ice Age

(Presentation C51B-0426). American Geophysical Union meeting. Dec.

11-15. San Francisco. Abstract.

Smith, S.L., and M.M. Burgess. 2004. Sensitivity of permafrost to

climate warming in Canada. Bulletin 579. Geological Survey of Canada.

Ottawa.

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Permafrost erosion

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