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The contribution of non-CO2 gases to total emissions can be estimated by expressing the emissions of all the gases in CO2-equivalent units. For a given gas, emissions expressed in mass are multiplied by its specific weighting factor, the Global Warming Potential (GWP), an estimate of the relative contribution of a kilogramme of that gas to global radiative forcing, as compared to the same amount of CO2, integrated over a fixed period of time (e.g. 100 years).

The UN Framework Convention on Climate Change (UNFCCC), following the Second Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), uses the 100-year GWPs of 21 for CH4, 310 for N2O and 23 900 for SF6. For the most common HFCs, GWPs vary between 140 and 3 000 (1 300 for HFC-134a). For the by-product HFC-23, the GWP is 11 700. The GWPs for PFCs vary between 6 500 (CF4) to 9 200 (C2F6). These two PFCs, the ones most commonly used, are also significant sources of byproduct emissions. This chapter expresses all emission data in CO2-equivalents using these GWP values.

In 2005, CO2 contributed 76% of global greenhouse-gas emissions, CH4 about 16%, N2O about 6% and the combined F-gases about 2% (Figure 1).

In 2005, the largest sources of greenhouse-gas emissions were the sectors of energy (66%, mainly CO2 fossil fuel use), and agriculture (11%, mainly CH4 and N2O, in comparable amounts). Other sources of greenhouse gases were CO2 from biomass burning (12%, mostly forest and peat fires and post-burn decay in non-Annex I countries), and CO2 from cement production (3%, of which 45% originated in China).

Figure 1. Global greenhouse-gas emissions by gas/source in 2005

N2O- N2O-OtherF-gas-All

Agriculture 2% 2%

Figure 1. Global greenhouse-gas emissions by gas/source in 2005

N2O- N2O-OtherF-gas-All

Agriculture 2% 2%

As seen in Figure 2, on an individual gas basis, the major global sources for methane (CH4) in 2005 were:

• agriculture (43%), mainly from enteric fermentation by animals and animal waste, from rice cultivation and from savannah burning;

• energy production and transmission (35%), mainly from coal production and gas production and transmission;

• waste (19%), from landfills and wastewater.

Figure 2. Global CH4 emissions in 2005

Other 3%

Figure 2. Global CH4 emissions in 2005

Other 3%

For nitrous oxide (N2O), agriculture contributed 64% of emissions in 2005, mainly from synthetic fertilisers and animal waste dropped on soils (either as animal manure or by animals during grazing) and agricultural waste burning (Figure 3). A much smaller source is fuel combustion (8%, mainly from coal, fuelwood and road transport). Another small source is N2O from industry (6%), mostly in Annex I countries.

Figure 3. Global N2O emissions in 2005

Energy 8%

Energy 8%

For the fluorinated gases (Figure 4), emissions are split between "use" and "by-products" because of the different ways they are produced. HFC use represented about half of the total in 2005, of which HFC 134a alone represented nearly half. Total by-product emissions of HFC contributed 20% and by-product emissions of PFCs another 10%. SF6 use represented 19%, while PFC use represented the remaining 3%. Most F-gas emissions are emitted by Annex I countries.

Figure 4. Global F-gas emissions in 2005

Figure 4. Global F-gas emissions in 2005

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