Permafrost thaw causes the ground to become unstable as the soil collapses. This can damage building and roads built on permafrost. Cracks also expose the carbon stored within to sunlight, which may speed the release of carbon dioxide, a greenhouse gas.
Photo credit: Dentren (http://upload.wikimedia.org/wikipedia/commons/a/ac/Storflaket.JPG)
As greenhouse gas emissions continue to rise, researchers are looking at a source of even more carbon emissions from underground: thawing permafrost. A warming Arctic may let significant amounts of dead, organic material currently frozen in the ground to pump more carbon into the atmosphere. How exactly does permafrost store carbon? And what are the consequences if the permafrost thaws?
Carbon and permafrost
In the Arctic, soils with temperatures at or below zero degrees Celsius for at least two or more years in a row are called permafrost. Some areas of permafrost absorb enough heat in the summer time to let the topmost layer of soil, called the active layer, to temporarily thaw, allowing plants to grow and animals to find food. Underneath this layer, the soil remains frozen, preventing decay and preserving plant matter and organic material, effectively storing the carbon within. Some carbon has been trapped in permafrost for thousands of years.
If temperatures rise and permafrost thaws, the organic material decays, and the soil becomes wet and marshy. In this moist environment, the carbon rots and gives off methane, a greenhouse gas. When the frozen ground thaws, the soil may also collapse and creates holes in the tundra, exposing the carbon directly to the atmosphere. Under dry and sunny conditions, microbial decay can convert carbon into carbon dioxide, which is another greenhouse gas, nearly 40 percent faster than if the soil were not exposed.
This airport in Greenland was built on formerly frozen ground. As the permafrost thaws, the runways sink and crack as do the roads around it, impairing aircraft travel to the area. In fact, airport managers have already repaired part of a runway that sank into melting permafrost.
Photograph credit: Jamie McIntyre (http://www.flickr.com/photos/obstreperously/3793709435/)
Local consequences
On a local scale, permafrost thaw has direct and immediate impacts for people and animals living in the Arctic. As the ice in the permafrost melts, the ground becomes unstable. Houses and buildings that previously rested on solid frozen soil begin to sink, and structural foundations crack. Thawing permafrost also causes roads to heave and crack, making travel difficult.
When temperatures rise and the active layer becomes thicker, the Arctic is also likely to become greener as more plants are able to survive the terrain and climate. Arctic tundra vegetation and needle-leaf forests are receding in the north, as deciduous shrubs and other nonnative vegetation edge to higher latitudes. The encroaching shrubs replace the tundra vegetation, which feeds caribou, hares, and marmots. In turn, these animals are vital food sources for Arctic predators. Thawing permafrost may lead to a decline in biodiversity for both vegetation and wildlife.
Global impacts
While local effects of thawing permafrost are worrisome, the sheer amount of carbon that could be released in the future concerns everyone. Scientists estimate that Arctic permafrost contains nearly 1,700 billion tons of carbon, about twice the carbon that is currently in the atmosphere. Methane is over twenty-five times more potent at retaining heat than its carbon dioxide counterpart. In addition, carbon dioxide released due to sunlight exposure is a phenomenon only recently discovered and could convert Arctic carbon into carbon dioxide much more quickly than previously thought.
As the Arctic continues to thaw, scientists continue to study its impact on local and global systems, as well as raise awareness about increased greenhouse gases in the atmosphere. The carbon released into the atmosphere from permafrost, either as methane or carbon dioxide, aids in increasing global temperatures, which may thaw more frozen ground. At this rate, permafrost may be not only a consequence of global climate change, but a contributor to it as well.
References
Cory, R. M., R. C. Crump, J. A. Dobkowski, and G. W. Kling. 2013. Surface exposure to sunlight stimulates CO2 release from permafrost soil carbon in the Arctic. Proceedings of the National Academy of Sciences of the United States of America 110(9), doi:10.1073/pnas.1214104110.
Schaefer, K., T. Zhang, L. Bruhwiler, and A. P. Barrett. 2011. Amount and timing of permafrost carbon release in response to climate warming. Tellus 63B: 165-180.
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