According to the Kyoto Protocol, six greenhouse gases are taken into account: three natural (carbon dioxide, methane and nitrous oxide) and three human-made (halofluorocarbons, perfluorocarbons and sulphur hexafluoride). A brief description of natural and anthropogenic GHG sources and sinks is presented below.
Carbon dioxide (CO2). Every single life form on the Earth contributes to the global carbon cycle. During the vegetation period plants use solar energy to absorb carbon dioxide and transform it into the biomass (the process of photosynthesis). Scientists assume that about 110 billion tons of carbon are absorbed annually in the process of photosynthesis. This amount does exceed the anthropogenic emissions significantly. 60 billion tons of carbon are emitted into the atmosphere as а result of human and animal breath while the decay of land biomass releases the remaining 50 billion tons. A portion of carbon is stored in soil; and water surfaces also absorb and emit large amounts of carbon.
The natural carbon cycle is stable because of the balanced feedback mechanisms. For example, it is proven that а small increase of carbon dioxide concentrations in the atmosphere leads to intensified photosynthesis and, therefore, increases the amount of land biomass. The flows of carbon dioxide between different carbon reservoirs occur in huge proprotions but are sufficiently balanced to maintain the cycle. Human activity disturbs this balance.
The problem is that the amount of flowing carbon is much less than the carbon that accumulated in sedimentary rocks over thousands of millions of years. Anthropogenic combustion of solid, liquid, and gaseous fossil fuel leads to carbon dioxide emissions which are extra in the atmosphere. Stocks of coal, oil and natural gas are only about 0.5 % of all carbon stored in sedimentary rocks, but the period of their production and use is the past one or two centuries and is by far shorter than geological cycles. Thus, the relations of the Earth and its atmosphere have been disturbed due to а short-term but а very powerful impact. Some other processes classified as anthropogenic CO2 sources or sinks include certain minerals production, forestry and land-use changes, etc.
The main spheres of human activity affecting carbon dioxide concentration in the atmospere are:
- fossil fuel combustion in all spheres of anthropogenic activity;
- non-energy industrial processes (organic and non-organic chemistry, production and use of mineral products, etc.); and
- forestry and land-use change.
Methane (CH4). Annual balance of natural sources and sinks of methane totals approximately 500 million tons. Natural sources of atmospheric methane are:
- anaerobic decomposition of organic substances in biological systems;
- wood digestion by termites; and
- oceans, seas and lakes.
The main human activities that cause methane emissions are:
- rice cultivation;
- enteric fermentation and manure of livestock;
- decomposition of solid wastes; and
- production, transportation, distribution and storage of coal, oil, and gas.
Methane accompanies biomass burning and incomplete fuel combustion.
Nitrous oxide (N2O). Natural sources of atmospheric nitrous oxide are oceans, soils, forests, and meadows. The main natural sink of methane is photochemical decomposition in the atmosphere.
Anthropogenic sources of nitrous oxide are:
- agricultural soils, especially the use of nitrous fertilizers;
- fossil fuel combustion;
- production of adipic and nitrous acids; and
- biomass burning.
Halofluorocarbons (Hfcs), perfluorocarbons (Pfcs) and sulphur hexafluoride (SF6). These man-made gases, which first appeared in the atmosphere in this century, are emitted in the industrial processes such as production of aluminum, magnesium and halogenated carbons (e.g., Hcfc-22). In some countries Pfcs emissions are а large part of total GHG emissions contrubuting to global warming potential.
These substances are used in а variety of industrial applications, including refrigeration and air conditioning, solvent cleaning, foam production, sterilization, fire extinguishing, paints, coatings, and other chemical intermediates, and in such miscellaneous products as aerosols and propellants. The use of Hfcs, Pfcs and SF6 is predicted to increase in the next decade because of their ability to substitute the ozone-depleting substances.