Permafrost

Introduction

Permafrost is any soil that stays frozen for two or more years. The soil above a permafrost layer typically thaws at

the surface during the summer to form a shallow active layer where plants are able to grow and flower. Perma-frost is found at high latitudes and altitudes and cloaks roughly 25% of the landmass of the Northern Hemisphere.

Because cold slows or prevents decay, permafrost can contain large amounts of carbon, mostly in the form of roots, leaves, and other organic materials that have accumulated over thousands of years. Estimates of the amount of carbon stored in permafrost globally range from 350 to 900 billion metric tons (gigatons). For comparison, the atmosphere currently contains about 730 gigatons of carbon, over a third of which has been placed in the atmosphere by people since the beginning of the Industrial Revolution, contributing to a global rise in temperature.

The Arctic (far north), where most permafrost is located, is warming more rapidly than the rest of the planet, with some areas experiencing an average temperature increase of 2.5°F (1.4°C)—more than twice the current global average. Warming of the air and lengthening summer months have led to the increasing warming and melting of permafrost and the deepening of the active layer. This trend will likely open up a substantial amount of organic material to decay, potentially releasing massive amounts of carbon into the atmosphere as carbon dioxide and methane, which would exacerbate global warming.

Historical Background and Scientific Foundations

Permafrost has repeatedly expanded and contracted its range and depth at northern latitudes over the last two million years in close conjunction with the expansion and contraction of continental ice sheets and the cooling and warming of global climate. Glaciers and permafrost reached their most recent maximum at the end of the Little Ice Age about 150 years ago.

Scientists first noticed recent widespread warming and melting of permafrost in the 1960s, when the U.S. Geological Survey began recording rising temperatures in Arctic boreholes. Since then, researchers have documented temperature increases in permafrost around the globe. In the United States, the permafrost below the trans-Alaska oil pipeline warmed 1.08-2.7°F (.6-1.5°C) between 1983 and 2003. The permafrost in eastern Siberia has been warming an average of .05°F (0.03°C) every year between 1960 and 1992. Permafrost temperatures in the Altai region of Mongolia and the Tian Shan Mountains of central Asia have risen by as much as 0.36°F (0.2°C) per decade since the early 1970s. In the Tian Shan Mountains, this has led to a 23% increase in the thickness of the active layer during the summer. The high Qinghai Plateau in Tibet has seen similar changes.

The effect of rising temperatures on permafrost is not identical in all regions. Melting is proceeding fastest in the southerly and lower-altitude regions of permafrost range, where the frozen soil is typically patchy and only a few meters thick. In northern Manitoba, Canada, for example, the permafrost's perimeter is retreating an average of 1 ft (31 cm) a year, leaving behind bogs and drowned forest. Some computer models have shown that significant increases in average temperature could result in the loss of 25-90% of near-surface permafrost over the next century as the active layer deepens. But scientists are uncertain about whether and how quickly thicker permafrost will melt. In the coldest parts of the Arctic, permafrost can be tens to hundreds of meters thick and may take centuries or millennia to disappear completely, if at all. In addition, localized variations in vegetation, hydrology, and terrain, as well as inadequate and highly localized data collection, have complicated efforts to determine global trends in permafrost degradation and make definitive predictions.

Impacts and Issues

Widespread melting of permafrost has major implications for ecosystems, human infrastructure, and climate warming.

WORDS TO KNOW

ALBEDO: A numerical expression describing the ability of an object or planet to reflect light.

BOREAL FOREST: Type of forest covering much of northern Europe, Asia, and North America, composed mostly of coniferous evergreen trees. Although low in biodiversity, boreal forest covers more of Earth's land area than any other biome.

BOREHOLES: Hole or shaft drilled into solid ground or a glacier for the purpose of extracting information rather than some resource (such as petroleum). In climate research, temperature measurements are often made along the height of a borehole: this information can be mathematically processed (deconvolved) to give information about past changes of surface temperature up to several centuries past.

DEFORESTATION: Those practices or processes that result in the change of forested lands to non-forest uses. This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosynthesis are no longer present and contributing to carbon storage.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

HYDROLOGY: The science that deals with global water (both liquid and solid), its properties, circulation, and distribution.

INDUSTRIAL REVOLUTION: The period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.

METHANE: A compound of one hydrogen atom combined with four hydrogen atoms, formula CH₄. It is the simplest hydro-carbon compound. Methane is a burnable gas that is found as a fossil fuel (in natural gas) and is given off by rotting excrement.

SINKHOLE: A natural depression in the surface of the land caused by the collapse of the roof of a cavern or subterranean passage, generally occurring in limestone regions.

STEPPES: Treeless biome dominated by short grasses (as opposed to prairies, which are dominated by long grasses).

In areas with a significant amount of ice in perma-frost and poor drainage, melting can lead to soil collapse, termed “wet thermokarst,” and the drowning of trees and shrubs in newly formed wetlands and lakes. The decay of submerged plant material releases the greenhouse gas methane. In areas on slopes and highlands with good drainage, the melting of permafrost can result in what is termed “dry thermokarst,” the collapse of soil, resulting in increased run-off and erosion and decreased ground water content. In boreal forests, this can eventually lead to the replacement of trees with dry steppes.

Degradation of permafrost can lead to significant damage to human infrastructure in the Arctic and sub-arctic and may eventually force the abandonment of some structures if the cost of repair becomes too great. Pockets of collapsed soil have broken building foundations, buckled highways, and destabilized pipelines in central Alaska, for example, and sinkholes have caused major problems for industry in Siberia.

Most importantly, the widespread melting of permafrost may release massive amounts of carbon into the atmosphere as formerly frozen organic material becomes available for decay, contributing to even greater warming. If even a small fraction of permafrost carbon is released into the atmosphere, it could easily match or surpass the current human rate of carbon emissions— about 9 gigatons annually, mostly from deforestation and the burning of fossil fuels.

However, this feedback mechanism is poorly understood and may be moderated by several negative feedback mechanisms. Warmer temperatures have led to a thicker layer of insulating moss in some areas, for example, helping keep permafrost cold during the warmer months. Advancing forests and shrublands, and other changes in vegetation, may help draw excess carbon back out of the air, though others—such as the shift to bogs—may release more carbon as methane. Also, advancing forests and shrublands decrease albedo (reflectivity), contributing to warming. Meanwhile, the cooling effect of deeper layers of permafrost may stave off extensive melting in surface layers.

See Also Feedback Factors; Melting; Tundra.

BIBLIOGRAPHY

Periodicals

Christensen, Torben R., et al. “Thawing Sub-arctic Permafrost: Effects on Vegetation and Methane Emissions.” Geophysical Research Letters 31 (2004): L04501.

Davidson, Eric A., and Ivan A. Janssens. “Temperature Sensitivity of Soil Carbon Decomposition and Feedbacks to Climate Change.” Nature 440 (2006): 165-173.

French, Hugh M. “Past and Present Permafrost as an Indicator of Climate Change.” Polar Research 18 (1999): 269-274.

Stokstad, Erik. “Defrosting the Carbon Freezer of the North.” Science 304 (2004): 1618-1620.

Walker, Gabrielle. “A World Melting from the Top Down.” Nature 446 (2007): 718-721.

Zimov, Sergey A., et al. “Permafrost and the Global Carbon Budget.” Science 312 (2006): 1612-1613.

Web Sites

Britt, Robert Roy. “Ground Frozen Since Ice Age Thaws and Collapses.” LiveScience, December 20, 2005. <http://www.livescience.com/environment/051220_permafrost.html> (accessed November 5, 2007).

Perkins, Sid. “Not-So-Perma Frost: Warming Climate Is Taking its Toll on Subterranean Ice.” Science News 171 (2007): 154. <http://www.sciencenews.org/articles/20070310/bob10.asp> (accessed November 5, 2007).

Romanovsky, Vladmir E. “How Rapidly Is Permafrost Changing and What are the Impacts of those Changes?” NOAA: Arctic Theme Page. <http://www.arctic.noaa.gov/essay_romanovsky.html> (accessed November 5, 2007).

Sarah Gilman