Greenhouse Gas Emissions
Greenhouse gases are present in the atmosphere naturally, released by natural sources, or formed from secondary reactions taking place in the atmosphere. They include carbon dioxide, methane, nitrous oxide and ozone. In the last 200 years, mankind has been releasing substantial quantities of greenhouse gases into the atmosphere. These extra emissions are increasing greenhouse gas concentrations in the atmosphere, enhancing the natural greenhouse effect, which is believed to be causing global warming. While man-made greenhouse gases include carbon dioxide, methane and nitrous oxide, some like the CFCs are completely new to the atmosphere.
Natural sources of carbon dioxide include the respiration (breathing) of animals and plants, and evaporation from the oceans. Together, these natural sources release about 150 billion tonnes of carbon dioxide each year, far outweighing the 7 billion tonnes of man-made emissions from fossil fuel burning, waste incineration, deforestation and cement manufacture. Nevertheless, natural removal processes, such as photosynthesis by land and ocean-dwelling plant species, cannot keep pace with this extra input of man-made carbon dioxide, and consequently the gas is building up in the atmosphere.
Methane is produced when organic matter decomposes in environments lacking sufficient oxygen. Natural sources include wetlands, termites, and oceans. Man-made sources include the mining and burning of fossil fuels, digestive processes in ruminant animals such as cattle, rice paddies and the burying of waste in landfills. Total annual emissions of methane are about 500 million tonnes, with man-made emissions accounting for the majority. As for carbon dioxide, the major removal process of atmospheric methane - chemical breakdown in the atmosphere - cannot keep pace with source emissions, and methane concentrations in the atmosphere are increasing.
Tropical soils are probably the single most important natural source of nitrous oxide to the atmosphere, although evaporation from the oceans is also a significant source. Nevertheless, man-made emissions, including the burning of organic waste, the use of agricultural fertilisers and industrial production of nylon, may account for up to 40% of total nitrous oxide sources (about 15 million tonnes per year). Nitrous oxide slowly breaks down in the atmosphere under the action of sunlight, but like carbon dioxide and methane, it is slowly accumulating in the atmosphere as a consequence of the extra man-made emissions.
Chlorofluorocarbons or CFCs are wholly man-made and do not exist naturally. Introduced in the 1930s they have been widely used in aerosols, foam manufacture, air conditioning and refrigeration. They are much stronger greenhouse gases than carbon dioxide, and they also destroy stratospheric ozone. The Montreal Protocol in 1987 has helped to phase out CFCs emissions in an attempt to protect the ozone layer, but because of their long atmospheric lifetimes, they will continue to influence the greenhouse effect for many years. Unfortunately, their replacements the hydrochlorofluorocarbons or HCFCs, whilst being relatively harmless to the ozone layer, are equally potent greenhouse gases, and at present their phase-out dates are not due for another 20 to 30 years.