....-l.()
N 0 N
FIGURE 4. Anthropogenic CH4 emissions and net emissions of C02:
contribution of selected regions in percent (according to IPCC scenario IS92a). Source: Houghton et al., 1992.
Carbondioxide (C02) Total Emission: 326 Tg Total Emission: 6.0 GtC
10%
45% 47°/o
11% 0 OECD
0 USSR I Eastern Europe Total Emission: A65 Tg Total Emission: 10.7 GtC • China and CP Asia
l!lll Other
13%
57%
22%
While the precise geographic distribution of methane sources is still highly uncertain, one can at least assume a fairly homogenous distribution among countries with high and low population growth. In any case, how-ever, methane sources are geographically much more balanced than the sources of carbon dioxide emission. Therefore it is acceptable to use global estimates for calculating the contribution of population growth to future methane emission.
OPTIONS, CONSTRAINTS, AND RISKS FOR REDUCING METHANE EMISSION
In its recent report the Intergovernmental Panel on Climate Change (IPCC) concluded that global methane emissions would have to be reduced by only 15-20% to stop the rise of its atmospheric concentration (Houghton
129 GERHARD K. HEILIG
et al., 1992). This should be an easy target to reach if current mitigation estimates are correct. For instance, a United States/Japan IPCC working group recently found that most methane emissions could be reduced by between 30 and 90%. Several options are being discussed on how to re-duce methane emissions to the atmosphere:
(1) a modification of farming techniques in paddy rice agriculture;
(2) a reduction of livestock and the processing of manure (bio-gas pro-duction);
(3) the implementation of better technologies for reducing fuelwood burning and charcoal production;
(4) the prevention of forest and savannah fires;
(5) the reduction of methane losses in the petrochemical industry, as well as the reduction of methane ventilation from coal mines and methane degassing from coal during transport;
(6) the draining of natural wetlands; and
(7) the prevention of methane emission from landfills.
The above mentioned IPCC expert group found that methane emis-sions from animal waste and sewage systems could be reduced by up to 80% in the near future (before 1995). This would also partly be possible with relatively simple technologies. Primarily by stopping the waste of fuel-wood, one could decrease methane emission from biomass burning by between 20 and 80%. CH4 emission from ruminants could be reduced by up to 75~b and even emissions from flooded rice fields could decline
by
up to 30%. The range of methane emissions from rice paddies is enormous: It can be as low as 8 gm -2yr-1 (gram per square meter per year) as mea-sured in Thailand or as high as 170 gm-2yr-1 as documented for the TuZu Szechuan Province in China. A number of soil and water factors influence methane emission from paddy rice.fields, but we also know that emission varies significantly with cultivation methods. A change in the agricultural practices, such as not putting straw into the flooded field, can dramatically reduce methane emission.While the potential for reduction of anthropogenic methane emission is clearly documented we should not underestimate economic and social constrants that might prevent implementation of these measures. There are, for instance, limits to the reduction of methane emission from live-stock. In the arid rangelands of Africa and Asia-where a substantial pro-portion of the world livestock is located-the population has few alterna-tives for sustenance: Cattle ranging across vast areas (a practice which makes it nearly impossible to reduce methane emission from enteric fer-mentation and animal wastes) may be the only sustainable system of food production in these areas. One could also question the willingness of
peo-pie in Northern America and Europe to modify their animal protein-rich diets which could, in principle, reduce the livestock (and thus methane emission) in these regions. On the contrary-it is quite likely that we will observe a trend to meat based diets in Asia and Latin America ("Ham-burger Revolution") which would lift methane emission from livestock to soaringly high levels. In China, for instance, the meat supply grew tenfold between 1961 and 1989 (from 3.8 kg per person per year to 25.6 kg) and has reached a level that is now half as high as in Europe during the 1950s.
Worldwide, the meat supply per capita has increased from 22. 9 to 31 . 7 kg per capita between 1961and1990 (FAO, 1992).
There are also apparent environmental risks which might prevent mit-igation measures. For instance, a conversion of natural wetlands into agri-cultural land, which could reduce the methane emission from this natural source, would also destroy the innate biological richness of these ecosys-tems. Tropical wetlands, such as the Pantanal floodplain in southcentral Brazil, are refuges for several endangered species. The drainage of wet-lands and conversion into intensively farmed areas have already led to some of the most-decimated habitats, such as the Tule marshes in the Cen-tral Valley of the United States of America. The conversion of natural wet-lands could also affect watershed dynamics and increase the risk of flood-ing downstream. Agricultural modernization, as well, could trigger a broad range of environmental degradation-including soil erosion, ground water pollution, and salinization.
CONCLUSION
(1) Methane (CH4 ) is a powerful greenhouse gas. It affects the Earth's radiative balance by being oxidized to C02 . It is also important for the atmospheric chemistry since it controls the abundance of ozone (03) and the hydroxyl radical (OH) (which in turn affects the lifetime of other green-house gases, such as the CFCs). According to IPCC estimates, methane contributed some 15% to the changes in radiative forcing from 1980 to 1990.
(2) The concentration of methane in the atmosphere is still extremely minute (around 1.7 ppmv); but the gas has been accumulating in the atmo-sphere at the rate of around 1 % per year. Today the methane concentra-tion is about double that in the preindustrial era (of some 0.8 ppmv).
(3) During the 1980s the growth rate of atmospheric methane concen-tration has slowed down-from 1.3% per year in the late 1970s to some 0.75% in 1989. There is no generally accepted explanation for this de-cline.
131
GERHARD K. HEILIG
(4) About two-thirds of the global methane emrssron to the atmo-sphere is man-made. Fossil gas leakage from coal mines and oil wells, rice cultivation on flooded fields, livestock production systems, and biomass burning are the four most important anthropogenic sources of atmospheric methane.
(5) The increase of methane in the atmosphere is not inexorable. With appropriate technical and organizational measures the global methane emission could be stabilized. For example, burning methane from coal mines and oil wells in power generators-instead of just blowing it into the air-would not only reduce the direct emission, but also save other sources of energy. With clever agricultural practices methane emissions from flooded rice fields could be minimized; methane emission from ani-mal wastes could be reduced by treatment in biogas generation plants. We could also reduce leakage from natural gas pipelines and distribution net-works. We could promote landfill gas collection.
(6) While there is technical potential for reducing methane emission we also face constraints and risks. There is a worldwide trend to animal-based food which makes it quite unlikely that we could achieve a signifi-cant reduction of livestock-related methane emission. For environmental reasons, we must also refrain from draining natural wetlands (which are the largest natural source of atmospheric methane). This measure would not only destroy the biodiversity of these ecosystems, but also affect the balance of related watershed systems.
(7) Methane losses in the petrochemical industry, methane ventilation from coal mines and methane degassing from coal during transport, how-ever, can be reduced substantially. The emission from these sources is primarily due to technological inefficiency, which in turn depends on the lack of investment capital and know-how, lousy maintenance of equip-ment, poor monitoring of production processes, and-rather simply-a lack of "good-will" among those responsible. It is well documented that simple and cost-efficient measures could substantially reduce methane emission from these sources. Even if population growth (and spreading in-dustrialization) were to require an increase in fossil fuel exploration, the total methane emission from these sources could decline in absolute terms with better technology and intelligent process design.
8) Contrary to carbon dioxide, methane is not primarily emitted from the production and consumption sectors of affluent societies. Significant sources of atmospheric methane are in developing countries, where high population growth is projected for the next decades. According to the most recent IPCC projections, the anthropogenic emission of methane to the atmosphere would stagnate or even decline, if population had stabilized at its 1990 level. Through its link to food production, third world population
growth is a key factor of future methane emission. By the year 2025 (third world) population growth could increase methane emission by between 22 and 230%, as compared to a constant 1990 population.
(9) Methane is an attractive target for controlling the greenhouse ef-fect. First, there would be a fast response to emission cutbacks, since the gas has the relatively short lifetime in the atmosphere of only some 11 years. Second, it would require the reduction of only 15-20% of total methane emission to stop the atmospheric increase of this greenhouse gas Only recently it was estimated that reductions in methane emission "would be 20-60 times more effective in reducing the potential warming of the earth's atmosphere over the next century than reductions in C02 emis-sions" (Hogan et al., 1991; Yavitt, 1992). Even very modest measures, such as cutting natural gas leakage in half, would be sufficient to stabilize the current CH4 concentration in the atmosphere.
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