Not-so-perma frost: warming climate is taking its toll on subterranean ice

Science News,  March 10, 2007  by Sid Perkins

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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[degrees]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).

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 Inter-governmental Panel on Climate Change reported last month (SN: 2/10/07, p. 83). Average temperatures worldwide have risen about 0.7[degrees]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[degrees]C just since 1961 (SN: 11/12/05, p. 312).

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).

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[degrees]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 longterm 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 o 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.