The contribution of other greenhouse gases

Most predictions of future changes in the intensity of the greenhouse effect are based solely on changes in the CO2 content of the atmosphere. Their accuracy is therefore questionable, since CO2 is not the only greenhouse gas, nor is it the most powerful. Methane (CH4), nitrous oxide (N2O) and the CFCs are the most important of the other greenhouse gases. Tropospheric ozone (O3) is also capable of enhancing the greenhouse effect, but its present concentrations are very variable in both time and place, and there is no clear indication of future trends (Bolle et al. 1986).

Methane is a natural component of the earth/ atmosphere system with its origin in the anaerobic decay of organic matter, mainly in the earth's natural wetlands. Significant amounts of CH4 are also produced by the global termite population (Crutzen et al. 1986). Levels of atmospheric CH4—at 1.72 ppmv—are very low compared to those of CO2. Molecule for molecule it is about twenty-one times more effective than CO2, however, and its concentration is increasing at about 0.8-1.0 per cent per year (Blake and Rowland 1988; Shine et al. 1990). The most important causes of this increase are to be found in agricultural development (see Table 7.1). Biomass burning, to clear land for cultivation, adds CH4 to the atmosphere, as does the world's growing population of domestic cattle, pigs and sheep, which release considerable amounts of CH4 through their digestive processes (Crutzen et al. 1986). By far the largest source of agriculturally-produced CH4, however, is rice cultivation. Rice paddies, being flooded and therefore providing an anaerobic environment for at least part of the year, act much like natural wetlands. Their total contribution to rising CH4 levels is difficult to measure since 60 per cent of the world's rice paddies are in India and China—both areas from which reliable data are generally unavailable. However, annual rice production has doubled over the past 50 years, and it is likely that CH4 emissions have increased in proportion (Watson et al

1990), although perhaps not by as much as was once thought (Houghton 1992).

The energy industry is another important source of anthropogenic CH4. As a by-product of the conversion of vegetable matter into coal, it is trapped in coal-bearing strata, to be released into the atmosphere when coal is mined. It is also one of the main components of natural gas, and escapes during drilling operations or through leaks in pipelines and at pumping stations (Cicerone and Oremland 1988). Together these sources may account for 15 per cent of global CH4 emissions (Hengeveld

1991). The disposal of organic waste in landfill sites, where it undergoes anaerobic decay, is also considered to be a potentially significant source of CH4. Attempts to provide accurate estimates of emissions, however, are hampered by the absence of appropriate data on the nature and amounts of organic waste involved (Bingemer and Crutzen 1987).

The lifespan of CH4 in the atmosphere averages 10 years. It is removed by reaction with hydroxyl radicals (OH) which oxidize it to water vapour and CO2, both of which are greenhouse gases, but less potent than CH4 (Watson et al. 1990). Atmospheric OH levels are currently declining as a result of reactions with other anthropogenically produced gases such as carbon monoxide (CO), causing a reduction in the rate of removal of CH4 (Hengeveld 1991). The total impact of decreased concentrations is difficult to assess, but CH4 emissions into the atmosphere continue to grow, and it has been estimated that an immediate reduction in emissions of 15-20 per cent would be required to stabilize concentrations at their current levels (Watson et al. 1990). The IPCC Supplementary Report noted some evidence that the rate of growth in CH4 concentration in the atmosphere may be already beginning to slow down (Houghton 1992). Even with this, however, potential feedbacks working through such elements as soil moisture levels and rising high latitude temperatures, could result in significant increases in future CH4 emissions. All of these trends suggest that CH4 will continue to contribute to the enhancement of the greenhouse effect well into the future.

The current atmospheric concentration of N2O—at 310 parts per billion by volume (ppbv)—is about a thousand times less than that of CO2, and it is increasing less rapidly than either CO2 or CH4. N2O is released naturally into the atmosphere through the denitrification of soils, and is removed mainly through photochemical decompostion in the stratosphere, in a series of reactions which contribute to the destruction of the ozone layer (see Chapter 6). It is thought to owe its present growth to the increased use of fossil fuels and the denitrification of agricultural fertilizers. The IPCC assessment has concluded, however, that past estimates of the contribution of fossil fuel combustion to the increase are too large—by perhaps as much as ten times—and N2O production rates during agricultural activity are difficult to quantify. Thus, although the total increase of N2O can be calculated, the amounts attributable to specific sources cannot be predicted with any accuracy. It is even possible

Figure 7.10 Greenhouse gas contributions to global warming (a) 1880-1980 (b) 1980s

Figure 7.10 Greenhouse gas contributions to global warming (a) 1880-1980 (b) 1980s

Source: After Mintzer (1992)

that there are sources yet to be identified (Watson et al. 1990). As a result the global N2O budget remains poorly understood, and its future concentration is therefore difficult to predict.

CFCs and other halocarbons released from refrigeration units, insulating foams, aerosol spray cans and industrial plants are recognized for their ability to destroy the stratospheric ozone layer, but they are also among the most potent greenhouse gases. For example, CFC-11 is about 12,000 times more effective than CO2 (Houghton et al. 1990). The CFCs are entirely anthropogenic in origin, and should therefore be much easier to monitor and control than some of the other gases. Their concentrations in the atmosphere range from CFC-115 at 5 parts per trillion by volume (pptv) to CFC-12 at 484 pptv, and have been growing at rates between 4-10 per cent per annum. Other halocarbons, such as Halon-1211 and Halon-1301, used mainly in fire extinguishers, with current concentrations of less than 2 pptv are growing at rates as high as 15 per cent per year (Watson et al. 1990). Recent international agreements to reduce the use of

CFCs (see Chapter 6) are aimed at preventing further damage to the ozone layer, but they will also have some impact on the greenhouse effect. However, CFCs have a long residence time in the atmosphere—up to 400 years in the case of CFC-13 and CFC-115—and even as emission rates fall, they will continue to contribute to global warming for some time to come.

The presence of these other greenhouse gases introduces a number of uncertainties into the predictions of future greenhouse levels. None of them is individually as important as CO2. It has been suggested, however, that their combined influence on the greenhouse effect is already equivalent to half that of CO2 alone (Bolle et al. 1986), and by early next century their contribution to global warming could be equal to that of CO2 (Ramanathan et al. 1985). Their impact would become increasingly important in the low CO2 emission scenarios envisaged by some investigators. The involvement of the CFCs and N2O in the depletion of the ozone layer adds a further complication. Attempts to mitigate the effects of these gases on the ozone layer would also impact on the greenhouse effect. Thus, although such gases as CH4, N2O and the CFCs have received much less attention than CO2 in the past, it is clear that plans developed to deal with global warming must include consideration of all the greenhouse gases, not just CO2 (see Figure 7.10).

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