Forests 251150 000 km^2 per annum over the decades of the 1980s and 1990s (see
box above) about 1.2 Gt of carbon would enter the atmosphere as carbon
dioxide. Although there are substantial uncertainties in the numbers,
they approximately tally with the IPCC estimate, quoted in Chapter 3
(see Table 3.1), of the carbon as carbon dioxide entering the atmosphere
each year from land-use change (mostly deforestation) of 1.7±0.8 Gt
per year – a significant fraction of the current total emissions of carbon
dioxide into the atmosphere from human activities.
Reducing deforestation can therefore make a substantial contribution
to slowing the increase of greenhouse gases in the atmosphere, as well as
the provision of other benefits such as guarding biodiversity and avoiding
soil degradation. These benefits are being increasingly recognised and
developing countries in tropical regions where there are large areas of
natural forest are beginning to concentrate seriously on the management
of their forests, on limitingthe extent of deforestation or planning for
substantial afforestation. Other large areas of forest lie at higher latitudes
where developed countries are also taking action to increase forest area
so as to contributeto the mitigation of global warming.
Let us look at the possibilities for afforestation. For every square
kilometre, a growing forest fixes between about 100 and 600 tonnes of
carbon per year fora tropical forest and between about 100 and 250
tonnes for a boreal forest.^7 To illustrate the effect of afforestation on
atmospheric carbon dioxide, suppose that an area of 100 000 km^2 , a little
more than the area of the island of Ireland, were planted each year for
forty years – starting now. By the year 2045, 4 000 000 km^2 would have
been planted; that is roughly half the area of Australia. During that forty
years, the forests would continue to grow and uptake carbon for twenty
to fifty years or more after planting (the actual period depending on the
type of forest and site conditions) – and, assuming a mixture of tropical,
temperate and boreal forest, between about 20 and 50 Gt of carbon
from the atmospherewould have been sequestered. This accumulation of
carbon in the forests is equivalent to between about five and ten per cent
of the likely emissions due to fossil fuel burning up to 2045.
But is such a tree planting programme feasible and is land on the scale
required available? The answer is almost certainly, yes. Studies have been
carried out that have identified land which is not presently being used for
croplands or settlements, much of which has supported forests in the past,
of an area totalling about 3 500 000 km^2.^8 About 2 200 000 km^2 of this
total is land that is technically suitable at mid and high latitudes – all of
this is deemed to be available. In tropical regions, of the 22 000 000 km^2
actually deemed suitable, only six per cent or 1 300 000 km^2 is consid-
ered to be actually available because of additional cultural, social and
economic constraints. These studies have also considered in detail how