David I. Stern* and Robert K. Kaufmann
Center for Energy and Environmental Studies, Boston University, 675 Commonwealth Avenue, Boston, Massachusetts 02215, U.S.A. (*current address: Centre for Resource and Environmental Studies, Australian National University, Canberra ACT 0200, Australia, http://cres.anu.edu.au/~dstern/index.html)
The authors provide the first estimates, by year, of global man-made emissions of methane, from 1860 through 1994. The methods, including the rationale for the various coefficients and assumptions used in deriving the estimates, are described fully in Stern and Kaufmann (1995, 1996), which provides the estimates for the period 1860-1993; the data presented here are revised and updated through 1994. Some formulae and coefficients were also revised in that process. Estimates are provided for total anthropogenic emissions, as well as emissions for the following component categories:
Changes in emissions over time were estimated by treating emissions as a function of variables (such as population or coal production) for which historical time series are available.
Flaring and Venting of Natural Gas
A ratio of 0.267 metric tons of CH4 per metric ton of CO2 released from flaring was assumed, for estimates from 1950 through 1994; that is, for each year t:
CH4t = 0.267Ft
where CH4 and F are metric tons of methane and carbon in carbon dioxide from flaring, respectively. The flaring data for this period are from Marland and Boden (1997). For estimates back to 1860, the following formulae was used for each year t:
CH4t = 0.267(0.478-0.0002193t)Ot
where CH4 and O are metric tons of methane from flaring and carbon in carbon dioxide from oil consumption, respectively. The oil consumption data for this period are from Keeling (1994).
Oil and Gas Supply Systems, Excluding Flaring
The emissions of methane, leaked from oil and gas supply systems, were calculated by assuming a coefficient of 0.0167 metric tons CH4 per metric ton of carbon emitted as carbon dioxide from natural gas consumption; that is, for each year t:
CH4t = 0.0167 Ct
where CH4 and C are metric tons of methane and carbon in carbon dioxide from natural gas consumption, respectively. The natural gas data are from Keeling (1994) and Marland and Boden (1997).
Methane emissions in metric tons were estimated differently for three time periods (1860-1948, 1949-1954, and 1955-1984, and 1985-1994), based on coal production data for the world, the U.S., the U.K., and the rest of the world (ROW), as follows for each year t (t = 1 for the year 1860):
CH4t = (0.238 + exp(-0.001216t))CWORLDt
where CWORLD is emissions in metric tons of carbon in carbon dioxide from global coal consumption.
CH4t = (0.238 + exp(-0.00871t))(CROWt + CUKt) + 0.00077SUSAt + 0.00989UUSAt
CH4t = 0.02445CROWt + 0.00077(SUSAt + SUKt) + 0.00989(UUSAt + UUKt)
CH4t = 0.02445CROWt + 0.00077SUKt + 0.00989UUKt + CH4USAt
where S and U are metric tons of coal mined from the surface and underground, respectively, and CH4USA is estimated U.S. methane emissions, obtained from EIA (1995), EPA (1995), and the U.S. Environmental Protection Agency's Greenhouse Gas Inventory Website.
Carbon emissions data are from Keeling (1994) and Marland and Boden (1997). Coal production data were obtained from EIA (1996) for U.S. coal production by mine type, and from the British Geological Survey (various years) and Central Statistical Office (various years). For 1993 and 1994, it was assumed that underground-mined coal accounted for 76% and 77% of total U.K. production, respectively, and data on coal production for the U.K. were obtained from EIA (1996).
Emissions of methane from biomass burning were extrapolated from the estimate in Subak et al.(1993) of 36 million metric tons of CH4 in 1985. The 1985 figure was extrapolated to 1860-1994 using estimates of anthropogenic carbon dioxide emissions from terrestrial biota published by Houghton et al. (1983) and a constant CH4/CO2 coefficient. The growth rate in each region for 1990-1994 was assumed to be the same as for 1989-1990. Yearly global estimates were summed from the regional estimates. Thus, for each year t:
CH4t = 0.021897Ct
where CH4t is emissions of methane, and C is emissions of carbon in carbon dioxide, from terrestrial biota, both in metric tons.
Methane emissions from enteric fermentation and animal wastes (in metric tons) were estimated for each year t (where t = 1 for the year 1500) as a function of human population, assuming that per-capita emissions declined linearly over time, as follows:
CH4t = (0.0213675 - 2.456E-06t)Pt
where P is world population. World population data for 1850, 1875, 1900, and 1925 are from McEvedy and Jones (1978), and for 1950-1994 are from the UN (1996); constant growth rates were assumed between data points.
Rice Farming and Related Activities
Emissions of methane from rice and other methane-emitting crops were estimated for each year t as a function of human population. A constant coefficient of per-capita emissions, 32 kg per year, was assumed for 1860-1900; the coefficient then declined, so that methane emissions in metric tons for each year t were calculated as:
CH4t = tPt
where P is world population and
t = t-1 + 0.03205[exp(-0.005829(1988-t)) - exp(- 0.005829(1988-t-1))]
Data on population are from sources listed above for livestock farming.
Methane emissions from landfills (or from anaerobic decomposition of garbage before modern landfills were in use) were extrapolated from the estimate in Subak et al. (1993) of 36 millions metric tons of CH4 in 1985. The 1985 figure was extrapolated to 1860-1985 assuming that emissions grew in proportion to economic growth (which was assumed to be 2.5% per year during this period). From 1985 to 1994 it was assumed that the growth rate was 1.25%, reflecting methane recovery in developed countries. Thus, methane emissions in metric tons were estimated for each year t as follows:
CH4t = CH4 1985exp[(t-1985)]
where is the growth rate in the two periods.
For flaring and venting of natural gas, estimated methane emissions rose from 0.0 in 1860 to a maximum of 29.3 million metric tons in 1973, then declined. For oil and gas supply systems, excluding flaring, estimated methane emissions rose from 0.0 in 1860 to a maximum of 18.0 million metric tons in 1994. For coal mining, estimated methane emissions rose from 2.2 million metric tons in 1860 to 49.5 million metric tons in 1989, then dropped slightly. For biomass burning, estimated methane emissions rose from 9.8 million metric tons in 1860 to 38.0 million metric tons in 1988 and subsequently declined slightly. For livestock farming, estimated methane emissions rose from 25.6 million metric tons in 1860 to 113.1 million metric tons in 1994; this appears to now be the largest individual anthropogenic source of methane emissions, having overtaken rice farming in the early 1980s. For rice farming and related activities, estimated methane emissions rose from 40.1 million metric tons in 1860 to 100.8 million metric tons in 1994. For landfills, estimated methane emissions rose from 1.6 million metric tons in 1860 to 40.3 million metric tons in 1994. Total estimated anthropogenic methane emissions rose from 79.3 million metric tons in 1860 to 371.0 million metric tons in 1994. During the period 1860-1994, the relative importance of the various component sources changed, with fossil fuels increasing and agriculture - although still dominant - declining in dominance. Within the agricultural sector, livestock replaced rice as the leading component.
The authors state these estimates to be a first approximation to actual emissions. As noted in the introduction to this section in Trends Online, the estimates for the 1980s of total anthropogenic methane emissions, and of emissions related to fossil fuels, are consistent with estimates from the Intergovernmental Panel on Climate Change. Uncertainty results from the use of proxy variables. To illustrate the range of confidence in them, the authors consider that of the proxy variables, the time series on extraction of fossil fuels are the most reliable, whereas estimates of pre-1950 gas flaring are "a guess." Also noted as sources of uncertainty are proxy data for world population (before recent years), world economic activity, and carbon dioxide emissions from terrestrial biota. Additional uncertainty results from estimates of emissions factors for years without published estimates.
31 August 1998