Reduction in thesources of methane 253reduction that could be achieved in the twenty-first century through the
enhancement of carbon uptake by land-use change has been estimated
at 40–70 ppm^11 (equivalent to a storage of 85–150 Gt). Compare this
with the range in carbon dioxide concentration in 2100 of about 400 ppm
that results from different SRES emission scenarios (Figure 6.1) or with
the increase of up to 300 ppm by 2100 because of the possible effect of
climate-carbon cycle feedbacks (see Figure 3.5).
Reduction in the sources of methane
Methane is a less important greenhouse gas than carbon dioxide, con-
tributing perhaps fifteen per cent to the present level of global warming.
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 (abouttwelve years compared with
100–200 years for carbon dioxide), only a relatively small reduction in
the anthropogenic emissions of this gas, about eight per cent, would be
required to stabilise its concentration at thecurrent level.
In Figures 6.1 and 6.2 are shown the emissions and the atmospheric
concentrations of methane estimated for the various SRES scenarios,
assuming no special action to reduce them. Of the various sources of
methane listed in Table 3.3, there are three sources arising from human
activities that could rather easily be reduced at small cost.^12 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 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 1992 to 1993. Improved management of such
installations could markedly reduce leakage to the atmosphere, perhaps
by as much as one-quarter overall.