Methane in the Arctic and its Role in Global Climate Change
Written by Barbara Travis and Dr. Natalia Shakhova in coordination with principal investigator, Dr. Igor Semiletov
It is estimated that northern ecosystems have accumulated 25-33% of the world's soil carbon. In a warming climate, carbon and methane trapped in permafrost have a high potential for release into the atmosphere through chemical and biological processes such as thawing. When permafrost thaws and higher levels of CO2 and CH4 are released, atmospheric temperature also increases. This can result in a feedback loop and more permafrost thaw.
Methane is a major product of the anaerobic decomposition of organic matter. When organic matter breaks down in the absence of oxygen, methane is released. Like carbon dioxide (CO2 ), methane (CH4 ) effectively traps heat in the atmosphere. Through its infrared spectrum, methane absorbs radiation that would have escaped from earth and therefore is an important greenhouse gas in our climate system. IARC visiting researcher Natalia Shakhova and IARC scientist Dr. Igor Semiletov have been studying methane levels in the Arctic and the long-term effects on global climate change.
Methane levels in the atmosphere have been on the rise and, during the last two centuries, atmospheric concentrations of CH4 have nearly tripled from approximately 0.7 to 2.0 parts per million (ppm). Dr. Shakhova states that, at the current rate of increase, global warming due to CH4 is expected to reach up to 40-50% of the CO2 warming effect.
In addition to atmospheric impacts, permafrost is also affected by the increase of greenhouse gasses and global warming in Arctic environments. Some areas of discontinuous permafrost throughout Alaska and Siberia are currently thawing from the top and bottom, as noted by Dr. V. Romanovsky of the Snow, Ice and Permafrost Group at the Geophysical Institute, UAF.
Water-saturated soils and lake sediments are considered the primary sources of CH4 in Arctic and Subarctic landscapes during the summer. It was found that in wet meadow tundra, a 2°C increase in temperature at a depth of 10 to 20 cm in the soil increases CH4 transport, or flux, to the atmosphere by approximately 120%. In the winter, when soils are frozen, northern lakes are the main source of CH4 release, as their sediments maintain a positive temperature and anaerobic conditions throughout the year (Fig.1). Thawed lakes that have aged a few thousands years might cover a layer of thawed permafrost, or talik, by ~100-200m, or more. Vast reservoirs of ancient organic carbon immobilized in permafrost then become available for anaerobic decomposition as the lakes evolve.
Taliks, which lie under lake sediments, are zones of thawed permafrost and are the places where methane originates in winter. Methane accumulates under the ice and is released through cracks and holes. The role of taliks in the current atmospheric CH4 balance could be significantly underestimated.
As thawed lakes age, the depth of taliks increase, ranging from 100 to 102 m, and can completely penetrate permafrost. Dr. Semiletov estimates that the high-latitude winter accumulation of CH4 in the atmosphere may be generated by northern lakes. Since Alaska has almost 3 million lakes, which cover at least 30,000 km 2 of the state, studying the role of Alaskan lakes is of great importance to global climate research. This initial study has shown that their contribution to methane levels could be very significant.
Thawing of permafrost under lakes, both onshore and offshore, may be the means by which reservoirs of methane hydrate. During the warming process, the methane gas, trapped in ice, is disturbed and rises to the surface of the lake. Some of the permafrost pockets were formed before the Holocene flooding 10,000 years ago. This is a way in which ancient methane enters the modern chemical cycle (Fig.2).
The largest source of natural gases, mostly composed of CH4 , is stored in gas-hydrates beneath permafrost and the onshore permafrost reservoir is roughly estimated to be as much as 32,000 Gt. (1Gt = 109 tons). This is 106, or one million, times as much as the CH4 released in the atmosphere of all northern ecosystems. Dr. Shakhova feels that a very small disturbance of gas hydrates could cause catastrophic consequences within a few decades. Shallow bottom sediment and underlying permafrost have warmed approximately 15°C since the time they originated. The implications of this trend are that shallow off-shore gas hydrate deposits could become vulnerable (Fig.2). She also notes that methane plumes found in the East-Siberian Sea (ESS) during the 1 st and 2nd Russian-U.S. joint cruises during September of 2003 and 2004 may indicate decaying gas hydrates in thawing undersea permafrost.
Semiletov I., Shakhova N., V.Romanovsky , and I. Pipko (2004) Methane Climate Forcing and Methane Observations in the Siberian Artctic Land-Shelf System, World Resource Review, 16 (4), 503-542.