252 A strategyfor action to slowand stabiliseclimate change
much carbon could be sequestered between the years 1995 and 2050 by
a programme of afforestation on this land. It is estimated to be between
50 and 70 Gt of carbon, to which a further 10–20 Gt can be added if the
rate of tropical deforestation were to be slowed. Estimates of the cost of
carrying out the programme have also emerged from the studies; they
are considerably lower than those estimated earlier in the 1990s. When
expressed per tonne of carbon sequestered they typically fall between
$US 1 and 10 (the lower values in developing countries) not includ-
ing land and transaction costs, but also not including the value of local
benefits (for instance, watershed protection, maintenance of biodiver-
sity, education, tourism and recreation) which might be derived from
the programme and which, in some circumstances, might offset most of
the programme’s cost. Compare this figure with the estimate given in
Chapter 9 of between $US 50 and 100 for the cost per tonne of carbon
of the likely damage dueto global warming. The programme therefore
appears as a potentially attractive one for alleviating the rate of change
of climate due to increasing greenhouse gases in the relatively short
term.
Let me insert here a note of caution. As with many environmental
projects the situation, however, may not be as simple as it seems at first.
One complicating factor is that introducing forest can change the albedo^9
of the Earth’s surface. Dark green forests absorb more of the incoming
solar radiation than arable cropland or grassland and so tend to warm the
surface. This is particularly noticeable in winter months when unforested
areas may possess highly reflecting snow cover. Calculations show that,
particularly at high latitudes, the warming due to this ‘albedo effect’ can
offset a significant fraction of the cooling that arises from the additional
carbon sink provided by the forest.^10
A possible afforestation programme has been presented in order to
illustrate the potential for carbon sequestration.Once the trees are fully
grown, of course, the sequestration ceases. What happens then depends
on the use that may be made of them. They may be ‘protection’ forests, for
instance for the control of erosion or for the maintenance of biodiversity;
or they may be production forests, used for biofuels or for industrial tim-
ber. If they are used as fuel for energy generation (see Chapter 11), they
add to the atmospheric carbon dioxide but, unlike fossil fuels, they are a
renewable resource. As with the rest of the biosphere where natural recyc-
ling takes place on a wide variety of timescales, carbon from wood fuel
can be continuously recycled through the biosphere and the atmosphere.
However, although there is a useful potential contribution from af-
forestation to the mitigation of climate change, it can only provide a
small part of what is required. An approximate upper bound for the