October 27, 2006

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Greenhouse Gases

Carbon Dioxide

On a worldwide basis, the anthropogenic emissions of CO₂ are known to be small. In comparison with the gross fluxes of carbon from natural systems they represent only a fraction (~2%) of total global emissions, but they are perceived to account for most of the observed accumulated CO₂ in the atmosphere. On the basis of available emissions information, the primary anthropogenic sources of CO₂ are fossil-fuel combustion (including both stationary and mobile sources), deforestation (resulting in permanent land use change), and industrial processes such as cement production. A global CO₂ emission rate of approximately 23.9 Gt has recently been estimated by the Carbon Dioxide Information and Analysis Centre (CDIAC). Deforestation, land use and subsequent soil oxidation have been estimated to account for about 23% of human-made CO₂ emissions. The primary natural sources include: respiration by plants and animals, decaying organic matter and fermentation, volcanos, forest/grass fires and oceans. On a net basis, natural carbon balancing processes such as photosynthesis and the oceanic reservoir remove most CO₂. Over the 45 years leading to 1996, global emissions of carbon dioxide grew from about 6.4 Gt to 23.9 Gt, almost a fourfold increase.

Methane

Excess global methane emissions resulting from human activities, are considered to have caused an increase of about 145% in atmospheric concentrations since the mid-1700s. The current annual rate of accumulation is estimated to range between 40 and 60 Mt CH₄/yr (~14 - 21 ppbv), or approximately 10% of total worldwide methane emissions. The anthropogenic CH₄ emissions, amounting to ~360 Mt per year, are primarily the result of activities such as livestock and rice cultivation, biomass burning, natural gas delivery systems, landfills and coal mining. Although several uncertainties exist in the actual contributions and relative importance of these sources, emission reductions of about 8% are thought to be required to stabilize methane concentrations at current levels.

Nitrous Oxide

At present, it has been estimated that approximately one third of global atmospheric nitrous oxide is of human origin, resulting primarily from the application of nitrogenous fertilizers and the combustion of fossil fuels and wood. The atmospheric concentration of nitrous oxide has grown by about 15% since the mid-1700s. Total annual emissions from all sources are estimated to be within the range of 10 to 17.5 Mt N₂O, expressed as N. Soil and water denitrification under anaerobic conditions is the primary natural source of N₂O. N₂O produced in this manner is readily taken up by plants. While it is generally recognized that nitrous oxide emission inventory data are more limited than carbon dioxide data and highly uncertain, efforts continue to improve the estimates.

HFCs, PFCs and SF₆

Currently some long-lived gases - particularly hydrofluorocarbons (HFCs, a CFC substitute), perfluorocarbons (PFCs) and sulfur hexafluoride (SF₆) - contribute little to radiative forcing, but their projected growth could contribute several per cent to radiative forcing during the 21st century (IPCC, 1996).

Canada's Contribution

While Canada contributes only about 2% of total global greenhouse gas (GHG) emissions, it is one of the highest per capita emitters, largely the result of our resource-based economy, our climate and our size. A Table of Provinicial Emissions, a Table of Canadian Emission Trends in CO₂ equivalent by Sector and a Table of Global CO₂ and other emissions are available. In 1990, Canadians released about 22 t CO₂ eq of GHGs per capita.