Tundra

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Tundra

Introduction

Tundra is land that is covered with plants but is treeless because of a cold climate with a short growing season. There are two kinds of tundra, Arctic and alpine. Arctic tundra covers landscapes in the polar regions, especially in the North, while alpine tundra occurs at high altitudes in mountain ranges. Arctic tundra covers about 20% of Earth's whole land surface; alpine tundra covers a much smaller area, only about 3% of the vegetation-covered area. Arctic tundra usually grows on top of permafrost and is found in a roughly ring-shaped area surrounding the North Pole (and in smaller areas of Antarctica, which is mostly ice-covered).

Arctic tundra supports native human peoples and such animals as fox, wolves, caribou, musk-oxen, and ptarmigan (a species of game bird). Alpine tundra occurs as small islands surrounded by lower-altitude, warmer environments. Both kinds of tundra are being strongly affected by global climate change. The Arctic has experienced twice as much warming as the global average, and alpine tundra areas are shrinking as treelines ascend.

Historical Background and Scientific Foundations

Arctic Tundra

In Arctic tundra, most of which is found above the Arctic Circle, the average year-round temperature is 10-20°F (-12-6°C). In summer, when the Earth's tilt keeps the sun above the horizon around the clock, temperatures go above freezing and the top layer, from a few feet to about 10 ft (3 m) deep, melts. This allows plants to grow for 50-60 days per year. Below the roots of the living plants is a thick layer of permafrost, a mixture of rock, soil, and dead plant matter that is typically about 1,475 ft (450 m) deep but in some places, such as Siberia in northern Asia, may be up to three times thicker. All water is frozen year-round in permafrost, hence its name. The great thickness of the permafrost means that it contains a great deal of carbon locked up in ancient plant tissues. Thirty to forty percent of all the carbon stored in soils worldwide is stored in Arctic tundra.

During the last half century, temperatures in Alaska, western Canada, and eastern Russia have gone up by 4-7°F (3-4°C), twice as much as the world has warmed as a whole. In 2007, scientists were forecasting that by the end of this century, Arctic temperatures would have risen by another 7-13°F (4-7 °C). Warming means that not only is the growing season warmer, it is also longer— spring arrives sooner, fall later. Arctic climate change has been seen in increased temperatures, melting of permafrost in the more southerly areas, and the rapid shrinkage of the north-polar ice cap. The number of shrubs (low bushy plants) growing in the Arctic has also increased. Recent studies suggest that 1,544,000 square mi (4,000,000 square km) of tundra could change to shrub-land as a result of global warming.

Alpine Tundra

In alpine or mountain tundra, cold is created not by Earth's tilt but by altitude. In most parts of the world, temperature goes down as altitude goes up. The rate of chilling with increased height, called the environmental lapse rate, is about 3.5°F per every 1,000 ft (6.5°C per every 1,000 m).

In many environments, species that require a colder environment shift northward as climate warms, staying with the colder weather. For example, butterflies and birds in the temperate zones have shifted their ranges toward the poles at about 3.8 mi (6.1 km) per decade since the 1960s. Inhabitants of alpine tundra, however, have nowhere to go: as warmer zones climb the side of the mountain, the area available for cold-loving species simply shrinks. In effect, alpine tundra environments are islands, and treeline (the highest altitude at which trees grow and lowest altitude at which tundra grows), their equivalent of sea level, is rising rapidly. Since 1970, the lowest altitude at which permanent freezing occurs in temperate-zone mountain ranges has climbed 490 ft (150 m) since 1970. In the European Alps, grassland species have crept uphill as much as 13 ft (4 m) per decade over the last century, probably a response to the 1.26°F (.7° C) average warming that has occurred in that region.

WORDS TO KNOW

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

CARBON: Chemical element with atomic number 6. The nucleus of a carbon atom contains 6 protons and from 6 to 8 neutrons. Carbon is present, by definition, in all organic substances; it is essential to life and, in the form of the gaseous compounds CO₂ (carbon dioxide) and CH₄ (methane), the major driver of climate change.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

PERMAFROST: Perennially frozen ground that occurs wherever the temperature remains below 32°F(0°C) for several years.

TEMPERATE ZONE: Band of moderate, seasonal climate located between the tropics and the polar regions. There is one temperate zone in the Northern Hemisphere and one in the Southern Hemisphere.

TREELINE: The highest-altitude (or highest-latitude) line along which trees can grow. As climate warms, treelines advance to higher altitudes and latitudes.

Impacts and Issues

In 2007, Arctic warming set records that surprised scientists. The area covered in summer by floating sea ice shrank to the lowest area ever measured, about 20% less than the area of the previous record year, 2005 (a difference equal to the area of California and Texas combined). The Northwest Passage, the land-hugging ocean route connecting the Pacific and Atlantic over the top of North America, was free of ice for the first time in recorded history. The melting of permafrost and the alteration of tundra vegetation do not occur as fast as sea-ice melting and cannot be so easily watched from space. However, these processes are certainly furthered by the warm summers that have been melting the ice. In the early 2000s, researchers predicted that between 77% and 90% of the Arctic tundra that existed in 1920 will be gone by the end of the twenty-first century.

One effect of tundral warming will be the release of carbon stored in permafrost. As noted earlier, from 30% to 40% of Earth's soil carbon is locked up in permafrost. About 90% of the carbon in a tundra ecosystem is in the soil, far more than in other ecosystems. (In a rainforest, for example, most of the carbon is in living plants.) As permafrost melts, microorganisms will digest the dead plant material that contains this carbon, releasing carbon dioxide (CO₂) to the atmosphere. Since CO₂ is the major greenhouse gas causing the warming that is melting the permafrost to begin with, this is a positive feedback—an effect that strengthens its own cause.

Some scientists had suggested that breakdown of plant material in the permafrost would make nutrients more available to plants, which, along with a longer growing season and the shift to large, shrubby plants, would cause plants to grow faster and remove CO₂ from the atmosphere, actually increasing ecosystem carbon storage and so acting as a brake on global warming. Experiments with tundra fertilization have shown that release of CO₂ from melted soils is greater than uptake by growing plants, so permafrost melting will indeed accelerate, rather than brake, global climate change. However, the exact balance between increased plant growth and CO₂ release is not yet known.

A second positive feedback to climate change from Arctic melting is the shrinkage of snow and ice cover. Snow and ice have high albedo (brightness) and reflect most of the solar energy that strikes them back into outer space. Open water and ground, being much darker, absorb far more solar energy. Heat absorbed by soil or water is eventually transferred, for the most part, to the atmosphere. Therefore, when snow melts to expose open ground or sea-ice melts to expose open water, this speeds up the climate warming that caused the melting.

The shift to shrubs accelerates global warming even apart from the issue of carbon storage. Observations of Alaskan tundra have found that when shrubs grow above a certain small size, their upper twigs remain exposed to sunlight all winter rather than being covered with snow. This greatly increases the amount of solar energy absorbed over any given area, and so adds to warming of the atmosphere.

For alpine tundra, the consequences of warming may be particularly harsh. Not only do alpine tundra biomes have limited area and no way to shift their location, but the isolation of these environmental islands has led, as on ocean islands, to the evolution of many distinct species. Many of these species are threatened with extinction as climate warms and treeline ascends.

BIBLIOGRAPHY

Periodicals

Foley, Jonathan A. “Tipping Points in the Tundra.” Science 310 (2005): 627-628.

Sturm, Matthew, and Tom Douglas. “Changing Snow and Shrub Conditions Affect Albedo with Global Implications.” Journal of Geophysical Research 110 (2005): 546-547.

Sturm, Matthew, et al. “Increasing Shrub Abundance in the Arctic.” Nature 411 (2001): 546-547.

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

Web Sites

Roach, John. “Forests Have Replaced Tundra Due to Warming, Study Finds.” National Geographic News, June 22, 2007. <http://news.nationalgeographic.com/news/2007/03/070309-warming-tundra.html> (accessed October 16, 2007).

Larry Gilman

Climate Change: In Context