254 A strategyfor action to slowand stabiliseclimate change
Fourthly, with better management, options exist for reducing
methane emissions from sources associated with agriculture.^13
Reductions from these four sources could reduce anthropogenic
methane emissions by over 60 000 000 tonnes per annum which would
be more than adequate to stabilise the concentration of methane in the
atmosphere at about 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 producing about one-
third of a gigatonne of carbon^14 or a little less than five per cent of total
greenhouse gas emissions – a useful contribution towards the solution
of the global warming problem.
Because the lifetime of methane in the atmosphere is relatively short,
a small reduction in methane emissions will quickly lead to its stabilisa-
tion as requiredby the Climate Convention objective. The same, however,
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.Stabilisation of carbon dioxide concentrations
Carbon dioxide, as we have seen, is the most important of the greenhouse
gases that result from human activities. Under all the SRES scenarios,
the concentration of carbon dioxide rises continuously throughout the
twenty-first century and apart from scenario B1 none come anywhere
near to stabilisation of concentration by 2100.
What sort of emissions scenario would stabilise the carbon dioxide
concentration? Suppose for instance that it were possible to keep global
emissions for the whole of the twenty-first century at the same level as
in the year 2000, would that be enough? Stabilising concentrations is,
however, very different from stabilising emissions. With constant emis-
sions after the year 2000, the concentration in the atmosphere would
continue to rise and would approach 500 ppm by the year 2100. After
that carbon cycle models predict that, because of the long time con-
stants involved, the carbon dioxide concentration would still continue to
increase, although more slowly, for many centuries.
Examples of scenarios that would lead to stabilisation of atmospheric
carbon dioxide concentration at different levels are shown in Figure 10.1.
Note that stabilisation at any level shown in the figure, even at an ex-
tremely high level, requires that anthropogenic carbon dioxide emissions
eventually fall to a small fraction of current emissions. This highlights
the fact that to maintain a constant future carbon dioxide concentra-
tion, emissions must be no greater than the level of persistent natural