The influence of permafrost on global climate
Large amounts of soil carbon deposited in permafrost may be released into the atmosphere due to deeper seasonal thawing under the climatic conditions projected for the future. An increase in the volume of the available organic material together with the higher ground temperatures may lead to enhanced emission of greenhouse gasses.
Projected changes in the volume of the seasonally thawing peat in Russian permafrost regions under the some climatic scenarios show that the largest relative increase of methane emission, by 50% or more, is expected along the Arctic coast, in Central Siberia and Yakutia. Wetlands are sparse here, whereas in West Siberia the projected flux changes are below 20%. Given that most of the wetlands are located in this region, it is unlikely that by the middle of the century the total methane emission from the Russian permafrost will increase by more than 25%-30%.
Picture 1. Projected changes in the soil temperature averaged over the warm period. GFDL climatic scenario for 2050.
Picture 2. Projected changes in the depth of seasonal thawing for peat, in units of m. Calculations were made for soil moisture content 0.8m/m. GFDL climatic scenario for 2050.
Arctic soils contain approximately 455 Gt C, or 14% of the global soil carbon. The Russian Arctic wetlands occupy almost 0.7 million square km in the circumpolar region, contain about 50 Gt C, and due to the relatively high ground water levels favor the production of methane rather than carbon dioxide in the anaerobic carbon-rich soil layer. The most wetlands region is Western Siberia (see picture).
Picture 3. Fraction of land area occupied by wetlands in Russian permafrost region.
Frozen wetlands put a significant contribution to the current annual net source of about 20 Mt resulting from the balance between the much larger global source (about 550 Mt) and sink (about 530 Mt) of methane.
Picture 4. Balance between the global source and sink of methane.
Contributions from regional sources to the global balance of the atmospheric methane. Numbers indicate million tons of CH4 per year. Projected by the mid-21st-century, an increase of the contribution from the Russian frozen wetlands by 6тАУ10 million tons is comparable with the modern net annual atmospheric gain of methane (20 million tons) and is likely to affect the global radiative balance.
Wetlands ecosystems act as a sink (photosynthetic uptake) and source (release due to soil decomposition) of carbon in the entire Arctic. The carbon turnover in the Arctic is projected to increase under the warmer climate; however, the timing of the processes that determine the status of the Arctic as a net sink or source is different. Increased trace gas emissions due to soil warming is likely to be the short-term response to climate change. In the longer-term warmer climate, more protracted growing periods and northward movement of productive vegetation may increase photosynthetic carbon uptake. The effect that the increase in the rate of soil carbon decomposition in the next few decades may have on the radiative forcing depends on the balance between the amounts of carbon emitted as CO2 and CH4. Methane has more than 20 times stronger greenhouse effect than an equal amount of CO2. A few ecosystems in the Arctic, including wetlands, convert part of carbon that has been photosynthetically captured from the atmosphere as CO2 to methane, which is further released as the product of organic soil decomposition. Because of this, even the areas and ecosystems that have net C-sink status, such as tundra, may enhance the global radiative forcing if a sufficient fraction of carbon is emitted as CH4. Methane in the Arctic soils is produced by bacteria in the anaerobic zone of the active layer (the uppermost layer of permafrost affected by seasonal thawing) underneath the water table. It is then transported to the atmosphere by bubble emission and diffusion through water as well as through the vascular system of plants.
In the case of the sinks being unchanged, this extra emission may shift the current global balance of methane by 50%, ultimately leading to a much faster rise of the atmospheric CH4 concentration and significant enhancement of the radiative forcing.
Increase in biomass and higher ground temperatures provoke the enhancement of greenhouse gases emission. Such emission can be calculated by diffusions-kinetic carbon model.
Picture 5. Diffusion-kinetic carbon model.
Results for the mid-21st century climate indicated up to 50% increase in the emission of methane in the northernmost locations along the Arctic coast, in the discontinuous permafrost zones up to 30% - 50% and a relatively small increase by 10%–15% in West Siberia, where wetlands occupy 50%–80% of the land. The annual emission of methane from Russian permafrost region may increase by 6–8 Mt, i.e. by 20%–30% compared to the current 24–33 Mt.
Picture 6. Projected changes of methane fluxes from the seasonally thawing wetlands in Russian permafrost regions. Climatic scenario for the mid-21st century is based on results from GFDL model. Changes are expressed in percentage from the modern norm.
The question is, what is the potential contribution of the projected enhanced emission of methane to the global radiative forcing due to wetlands thawing in the permafrost regions of Russia.
The average residence time of methane in the atmosphere is 12 years. If other sinks and sources remain unchanged, by the mid-21st century the additional annual 5-10 Mt source due to thawing of permafrost may increase the overall amount of atmospheric methane by approximately 100 Mt, or 0.04 ppm. Given that the sensitivity of the global temperature to 1 ppm of atmospheric methane is approximately 0.3°С/ppm, additional radiative forcing resulting from such an increase may raise the global mean annual air temperature by 0.012°С.