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close this bookThe Global Greenhouse Regime. Who Pays? (UNU, 1993, 382 p.)
close this folderPart I Measuring responsibility
close this folder3 Assessing emissions: five approaches compared
View the document(introduction...)
View the documentIntroduction
View the documentComprehensiveness compared
View the documentAccuracy by category
View the documentRegional and national emissions by source
View the documentConclusions
View the documentReferences
View the documentAppendix A: Estimates of greenhouse gas emissions
View the documentAppendix B: Calculating cumulative and current emissions

Accuracy by category

Table 3.2 summarizes the difficulties in estimating emissions from each of the source groupings; it includes the IPCC's ranges of uncertainty in estimating emissions by source and gas globally for each of the five emissions groups.

Cumulative CO2, energy only

CO2 emissions from energy use have been estimated at the country level between 1950 and 1988 (Marland et al. 1989) and between 1860 and 1950 (Subak and Clark 1990). Marland et al. estimate that the uncertainty of their inventory is 6-10 per cent at the country level (Marland et al. 1988). The accuracy of the pre-1950 data set is limited because of changes in geographical borders and sovereignty, lack of information on the type of coal used in the past, and because data on fossil fuels traded in certain regions are incomplete or unavailable. To relate historical emissions to current concentrations, a coefficient or 'discount rate' must be applied to adjust for the CO2 that has been removed from the atmosphere over time.

Table 3.2 Estimated accuracy of GHG emissions accounts

Estimated range
(IPCC 1990 unless noted)
1 Cumulative CO, Energy (1860-1986)
Fossil Fuel Consumption (CO2) Medium 175-215 GT C (10%)
2 Cumulative CO2 (1860-1986)
Fossil Fuel Consumption (CO2) Medium 175-215 GT C (10%)
Land Use Changes (CO2) Lowa 82-152 GT C (30%)
3 CO2 Energy (Current)
Fossil Fuel Consumption (CO2) High 5.4 GT C (Marland et al. 1990) (5%)
4 Partial CH4 CO2 (Current)
Above Plus:      
Landfills (CH4) Medium 20-70 MT CH4 (50%)
Land Use (CO2) Medium 1.1-3.6 GT C (Houghton 1991) (50%)
Energy Prod. and Distribution (CH4) Medium 44-100 MT CH4 (40%)
5 Comprehensiveb (Current)
Above Plus:      
Fossil Fuel Combustion (N2O, CO) Medium 0.5-1.4 MT N2O (IPCC 1992) (50%)
Cement Production (CO2) High    
Biomass Burning (CH4, CO, N2O) Low 20-180 MT CH4 0.3-1.6 MT N2O
(IPCC 1991)
Enteric Fermentation (CH4) Medium 65-100 MT CH4 (20%)
Animal and Human Wastes (CH4) Low    
Rice Cultivation (CH4) Low 25- 170 MT CH4 (80%)
Fertilizer Consumption (N2O) Low 0.01-2.20 MT N2O (100%)
Halocarbons (CFCs, Halons, HCFCs) High    
Nylon Production (N2O) Medium 0.6-0.9 MT N2O (IPCC 1991) (20%)

a In this cave, the uncertainty at the country level is far greater than the estimated global range.
b This inventory coos not include stratospheric water vapour, which is thought to contribute about 4 per cent of the climate forcing (IPCC 1990), and O3 precursors-NO and volatile organic compounds.

Despite these accounting difficulties, estimates of CO2 from energy use since 1860 are more accurate than those of current emissions from land use changes and agriculture. Keeling (1973) has estimated the uncertainty of historical global estimates for CO2 emissions from fossil fuels at 13 per cent. This range of uncertainty compares with 100 per cent or more for CO2 emissions from biota in the current period and equivalent or greater uncertainties in estimating CH4 from rice cultivation and biomass burning, and N2O, NOx, and CO from all sources (IPCC 1990; Logan et al. 1981).

Cumulative CO2. energy and biota

Estimating national historical emissions from energy and biota involves all of the technical difficulties of estimating cumulative emissions from fossil fuels outlined above, plus the challenge of estimating biotic contributions. While data are available to calculate emissions from fossil fuels on an annual basis, comprehensive international forest surveys are conducted less frequently, generally every decade since 1949. Much of the pre-1950 data are for changes in area devoted to agricultural uses only and therefore omit forest conversion to other uses such as settlements, etc. For all periods, many of the forest surveys are considered unreliable. It is unlikely that additional scientific research will significantly improve the accuracy of these estimates on the national level as investigations of historical trace gas concentrations such as ice core and tree ring analyses shed light on global historic concentrations rather than on nation-specific emissions.

Energy, CO2 (current)

This is the most practical, that is, measurable and verifiable approach of the five. Carbon dioxide emissions from current energy consumption are estimated to be accurate at the country level within an error range of only about 6-10 per cent (Marland et al. 1989). A comparison of the (ORNL) Marland and Rotty inventory (Marland et al. 1988), which is based on United Nations energy statistics with a new inventory of CO2 release (don Hippel et al. 1992) from energy consumption that was derived from OECD/IEA statistics (OECD/IEA 1990a) suggests that the error range may be higher for some countries. Regardless, the level of uncertainty in estimating emissions from this source is far lower than the uncertainty associated with inventories of the other gases and sources and should improve in the near future as a number of agencies are refining emission factor estimates and end-use data at the country level.

Partial CH4 and CO2

This approach is midway in practicality between the CO2/energy only approach and the comprehensive approach. The additional sources - land use changes, landfills, and fossil fuel extraction - cannot be estimated as accurately as energy consumption. The error range for estimating CO2 from land use changes and CH4 from landfills and fossil fuel extraction is + 40 to + 50 per cent at the global level (IPCC 1990), with developing countries generally at the higher end. Nevertheless, these sources of CO2 and CH4 should be easier to monitor than the agricultural sources and remaining gases. The landfill and coal mine sources of CH4 are also potentially important sources of natural gas (US/Japan Working Group on Methane 1992). Employing technology to recover and utilize natural gas from these sources should eventually enhance our capacity to control and monitor CH4 release.

For a number of countries in the tropics where CO2 emissions from deforestation far outweigh emissions from energy consumption, per capita estimates change a great deal depending on the assumptions used to estimate land clearing and biomass levels. As the FAO's once-a-decade study of tropical deforestation and tree plantation establishment and the Brazilian Space Institutes (INPE) detailed remote sensing survey of the Amazon Basin are due to be published in the next few years, estimates of emissions from land use changes should improve significantly. In addition, new international statistics on forest growth in temperate countries recently completed by the FAO/ECE, as well as new country studies for Northern and Central Europe, provide further information on the magnitude of CO2 uptake in northern forests.

Comprehensive emissions

The additional sources and gases not included in the above list are far more difficult to inventory. Generally, emissions from the minor greenhouse gas N2O, and CO - which oxidizes to become CO2 and affects the atmospheric residence time of CH4 - are highly uncertain. All of the agricultural sources are included in this approach. Of these, the factors that determine the release of CH4 from livestock enteric fermentation may be the best understood. But even in this case, the accuracy of national estimates for many countries is doubtful at present, because the controlling factors, which include livestock diet, breeding, and management practices, vary from country to country and accurate data are not available for many countries, particularly in the developing world. Measured CH4 release from rice cultivation varies widely according to soil type, fertilizer application, climate, and irrigation regime, but the net effect of all these conditions on emissions is not yet understood. Calculation of CH4 release from animal and human wastes has only started to be undertaken in the last two years, and estimates are rough, reflecting extrapolations based on only a few site-specific studies. Emissions of CH4, CO, and N2O from biomass burning vary with the extent of crop or forest burning, and the moisture and carbon and nitrogen content of the biota. Emissions of N2O and CO from the remaining sources are all highly uncertain.

Unlike the sources covered only in the partial CH4 and CO2 approach (energy, deforestation, and landfills), the additional sources covered here (livestock, rice cultivation, cement production, and fertilizer consumption) pose greater problems as abatement targets because their control would likely entail directly curtailing economic activities rather than reducing the residuals stemming from these activities. The agricultural and industrial activities they represent may be considered essential subsistence activities by many countries (Parikh et al. 1991), although in the case of livestock management for some animals, reducing CH4 emissions through changes in diet and breeding may be compatible with development goals (Leng 1991).

Unlike the three CO2 approaches, the partial and fully comprehensive approaches require an index to compare the heating effect of CH4 and CO2 emissions. The problems involved in evaluating the relative warming contribution of the gases include the difference in estimating the atmospheric lifetime of gases (particularly CO2), calculating indirect effects of the emitted gases, and specifying the most appropriate time period for which to calculate the warming effect (IPCC 1990). In practice, however, the choice of CO2 equivalent applied to these sources may have little effect on most countries' relative ranking by warming contribution.