Global Warming and Methane: Capping Methane Emissions
Global Warming and Methane
Reports have established that among the greenhouse gases methane (CH4) is one of the most dangerous and most powerful. Its contribution to global warming is estimated to be about 18 per cent and it has 20 times higher global warming potential than CO2. During the past 200 years, its concentration has increased dramatically because of imbalance between global sources and sinks. According to a report of IPCC the atmospheric concentration of methane has more than doubled in the past 200 years, rising over the past 15 years by an average of yearly 1 per cent every year. The concentration of atmospheric methane has increased steadily during the last few decades due to several reasons like increasing temperature, over precipitation, construction of new dams, etc.
Methane Emission Sources
Methane emission sources are categorized into two groups:
1) Natural sources (70 per cent) e.g. natural wetlands, termites and wild ruminants.
2) Anthropogenic sources (30 per cent) e.g. ponds, dams, paddy fields, cattle, and other domestic ruminants.
Approximately 70 – 80 per cent of the total global emission is biogenic in origin. The most important known source of methane is natural wetlands, fossil fuels like coal and natural gas, enteric fermentation, rice paddies, biomass burning, landfills and animal waste. For instance, nearly 110 Tg (1 Tg = one million tons) methane is emitted every year from wetlands. Among them tropical wetlands contribute 60 per cent. It is well established that the occurrence of high temperature and heavy rainfall in the tropical region are responsible for high emission rates from this region. Methane production is also carried out by anaerobic bacteria known as methanogens and the process is referred to as methanogenesis.
Paddy fields are the most significant contributors of atmospheric methane accounting for 11 – 13 per cent of the world’s total anthropogenic methane emission. In flooded paddy fields, methane is produced by anaerobic bacteria (methanogens) as the terminal step of anaerobic degradation of organic matter.
Many methanogenic bacteria are found in the rumen of cows where they produce methane from the H2 and CO2 released by other anaerobic gut bacteria. Methanogens in the rumen of cow are estimated to release about 50L methane per day. Some examples of methanogens are: methanococcus, methanoculleus, methanosarcina, methanoplanu, methanospirillium etc.
Major Sinks of Methane
Sinks of CH4
In the global CH4 cycle, CH4 is also consumed by chemical and biological processes. The major sinks of CH4 are biological oxidation at or near the sites of production, and photochemical oxidation in the atmosphere. However, oxidation of atmospheric CH4 by aerobic soils also provides a significant additional sink. About 90 per cent chemical oxidation occurs in the troposphere through reaction with free hydroxyl radical – the ‘detergent’ of the atmosphere. The only known biological sink for atmospheric CH4 is its oxidation in aerobic soils (forest and dry land paddy soils) by methanogenic bacteria. This CH4 sink mediated by soil methanotrophs can contribute up to 15 per cent to the total global CH4 destruction.
Several scientific investigations have demonstrated that aerobic forest soils also serve as sinks for atmospheric CH4. The current temperature soil sink is estimated at 20 Tg per year. Most unsaturated soils consume atmospheric methane. The rate of methane oxidation varies with soil water content, land use and ammonium inputs. Consumption of atmospheric CH4 has been demonstrated in coniferous and deciduous forest soils, agricultural soils, grasslands, and tundra soils. The consumption of atmospheric methane by oxic soils has been estimated at 40 to 60 Tg per year.
Because the biological sink for atmospheric CH4 in soil is microbial mediated it is sensitive to environmental factors like moisture or temperature, fertilizer application and disturbances by soil management practices. Atmospheric methane uptake is decreased after fertilization of the soils with nitrogen, conversion of grasslands to croplands, tillage, and clearing of forest lands. The conversion of grassland to cropland results in a reduction in methane consumption in these ecosystems of 1.5 to 7 Tg per year.
The increase of CH4 emission had declined during the past 2 – 3 years, although it is not known whether the decline rate is due to decrease in emission or increase in CH4 oxidation. It has been recently reported that a 10 per cent reduction in emissions of CH4 may stabilize the current concentration of CH4 in the atmosphere. Therefore, it is of much importance to maintain a balance between the sources and sinks of atmospheric methane to mitigate the global warming problem caused by emission of atmospheric methane.