Increased emissions of greenhouse gases in the atmosphere is thought to lead to global warming. The family of greenhouse gases most often referred to includes carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), sulphur hexaflouride (SF6), chloroflourocarbons (CFC), hydro-flourocarbons (HFC).
Water vapour is also considered a greenhouse gas and in fact is by far the largest overall contributor to the greenhouse effect. However, human emissions of water vapour are negligible in comparison with natural fluxes in the water cycle. Attention has therefore focused on the other greenhouse gases whose effects are subject to some degree of human influence.
Some greenhouse gases have both natural and human sources of emissions to the atmosphere: carbon dioxide (CO2), methane (CH4), nitrous oxide(N2O). The others do not exist naturally and their presence is due entirely to human activities.
Carbon dioxide is by far the dominant greenhouse gas emitted as a result of human activities.
In Canada (1999), carbon dioxide is the primary greenhouse gas, accounting for 78% of overall GHG emissions. Carbon dioxide is followed in importance by methane (13% of GHG emissions on a CO2 equivalent basis) and nitrous oxide which accounts for 8% of the national total.
The increased levels of carbon dioxide are believed to be due primarily to emissions resulting from fossil fuel use, and land use changes such as deforestation. The main argument in favor of this theory is that the increase in CO2 concentration parallels the increase in the use of fossil fuels with the industrial revolution, and deforestation in mid-latitudes and the tropics. Much of the incremental CO2 released as a result of human activities is absorbed in natural sinks. Some is taken up by vegetation, which grows more vigorously in an atmosphere enriched with carbon dioxide, and some is dissolved in the oceans.
Methane (CH4) concentrations have more than doubled, primarily due to raising cattle, the biological activity of bacteria in rice paddy fields, biomass burning, coal mining, venting of natural gas, and landfills.
Emissions of chlorofluorocarbons (CFCs), used as aerosol propellants, solvents, refrigerants and foam blowing agents are well quantified. Even if the manufacture of CFCs was to stop today, there is a vast inventory in existing products that has the potential of being released into the atmosphere in the future. CFCs are controlled under the 1987 Montreal Protocol on Substances That Deplete the Ozone Layer and so are considered separately from greenhouse gas emissions for reduction purposes.
