Reduction in sources of greenhouse gases other than carbon dioxide

Methane, nitrous oxide and the halocarbons are greenhouse gases, less important than carbon dioxide, all of which show increases at the present time. In Figures 6.1 and 6.2 and Table 6.1 are shown the emissions, atmospheric concentrations and radiative forcing of these gases estimated for the twenty-first century under the various SRES scenarios, assuming no special action to reduce them. Is it possible that these further increases can be slowed or eliminated? We consider them in turn.

Methane contributes about 15% to the present level of global warming (Figure 10.1). The stabilisation of its atmospheric concentration would contribute a small but significant amount to the overall problem. Because of its much shorter lifetime in the atmosphere (about 12 years compared with 100-200 years for carbon dioxide), only a relatively small reduction in the anthropogenic emissions of this gas, about 8%, would be required to stabilise its concentration at the current level. Of the various sources of methane listed in Table 3.2, there are four sources arising from human activities that could rather easily be reduced at small cost.18

Firstly, methane emission from biomass burning would be cut by, say, one-third if deforestation were drastically curtailed. Secondly, methane production from landfill sites could be cut by at least a third if more waste were recycled or used for energy generation by incineration or if arrangements were made on landfill sites for the collection of methane gas (it could then be used for energy production or if the quantity were insufficient it could be flared, turning the methane into carbon dioxide which molecule-for-molecule is less effective than methane as a greenhouse gas). Waste management policies in many countries already include the encouragement of such measures.

Thirdly, the leakage from natural gas pipelines from mining and other parts of the petrochemical industry could at little cost (probably even at a saving in cost) also be reduced by, say, one-third. An illustration of the scale of the

As wastes break down, methane is produced that can be captured and burnt for power generation. The carbon dioxide then released has much less impact as a greenhouse gas than the methane that would otherwise be released.

leakage is provided by the suggestion that the closing down of some Siberian pipelines, because of the major recession in Russia, has been the cause of the fall in the growth of methane concentration in the atmosphere from about 1992. Improved management of such installations could markedly reduce leakage to the atmosphere.

Fourthly, with better management, options exist for reducing methane emissions from sources associated with agriculture, for instance, by adjustments to the diet of cattle or to the details of rice cultivation.19

Reductions from these four sources could reduce anthropogenic methane emissions by more than 60 million tones per year - enough to stabilise methane concentration in the atmosphere at or below the current level. Put another way, the reduction in methane emissions from these sources would be equivalent to a reduction in annual carbon dioxide emissions of about 1.4 Gt20 or about 3% of total greenhouse gas emissions - a useful contribution towards the solution of the global warming problem.21

It was noted in Chapter 3 (page 50) that the growth in global atmospheric methane concentrations had largely halted since the late 1990s. However, since early 2007 there is evidence of renewed growth22 which may be more pronounced in the northern hemisphere. Some increase from methane emissions from natural sources may be expected because of the influence of global warming on various natural methane reservoirs (see Chapter 3). Prominent among these are the very large reservoirs under the tundra at high latitudes (see box on page 48-9 and Table 7.5). As the cover of Arctic summer sea ice has reduced and as northern Siberia has warmed, more evidence of local methane emissions has emerged. Further studies are needed to ascertain whether these are connected with the recent growth in global methane concentration. As the Arctic warms further, the possibility exists of much larger releases in the longer term, especially if global average temperature rise is not halted.

Turning now to nitrous oxide which contributes about 7% to the present level of global warming and which is growing at about 0.25% per year. Much of its growth appears to rise from emissions associated with the use of nitrogen fertilisers. Careful management of the use of such fertilisers and other changes in agricultural practice could largely stem the continuing increase.

For halocarbons for many of which the manufacture is being phased out, the most important concern is that disposal of products containing these gases, for instance of foams or refrigeration equipment, is carefully controlled so as to minimise leakage to the atmosphere and to ensure that their atmospheric concentration gradually reduces during the twenty-first century.

In this section we have seen that options are available for stemming the growth and possibly reducing the concentrations of greenhouse gases other than carbon dioxide. In the following section, stabilisation of carbon dioxide concentrations will be considered together with stabilisation of the combined effect of greenhouse gases considered together. Some reductions in the concentrations of methane, nitrous oxide and halocarbons will be seen as important contributions to achieving this overall stabilisation.

Because the lifetime of methane in the atmosphere is relatively short, a small reduction in methane emissions will quickly lead to its stabilisation as required by the Climate Convention objective. The same is not true of the stabilisation of carbon dioxide concentration with its much longer and rather complicated lifetime. It is to that we shall now turn.

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