250 A strategyfor action to slowand stabiliseclimate change
countries the development of forest areas has been the only hope of
subsistence for many people. Unfortunately, because the soils and other
conditions were often inappropriate, some of this forest clearance has not
led to sustainable agriculture but to serious land and soil degradation.^3
Measurements on the ground and observations from orbiting satel-
lites have been combined to provide estimates of the area of tropical
forest lost. Over the decades of the 1980s^4 and 1990s the average loss
was about one per cent per year (see box below) although in some
areas it was considerably higher. Such rates of loss cannot be sustained
if much forest is to be left in fifty or a hundred years’ time. The loss of
forests is damaging, not only because of the ensuing land degradation
but also because of the contribution that loss makes to carbon emissions
and therefore to global warming. There is also the dramatic loss in bio-
diversity (it is estimated that over half the world’s species live in tropical
forests) and the potential damage to regional climates (loss of forests can
lead to a significant regional reduction in rainfall – see box on page 173).
For every square kilometre of a typical tropical forest there is about
25 000 tonnes of biomass (total living material) above ground, containing
about 12 000 tonnes of carbon.^6 It is estimated that burning or other
destruction from deforestation turns about two-thirds of this carbon into
carbon dioxide. Approximately the same amount of carbon is also stored
below the surface in the soil. On this basis, from the destruction of aboutThe world’s forests and deforestation^5
The total area covered by forest is almost one-third of the world’s land
area, of which ninety-five per cent is natural forest and five per cent
planted forest. About forty-seven per cent of forests worldwide are trop-
ical, nine per cent subtropical, eleven per cent temperate and thirty-three
per cent boreal.
At the global level, the net loss in forest area during the 1990s was an
estimated 940 000 km^2 (2.4% of total forest area). This was the combined
effect of a deforestation rate of about 150 000 km^2 per year and a rate of
forest increase of about 50 000 km^2 per year. Deforestation of tropical
forests averaged about one per cent per year.
The area under forest plantations grew by an average of about
3000 km^2 per year during the 1990s. Half of this increase was the result
of afforestation on land previously under non-forest land use, whereas
the other half resulted from conversion of natural forest.
In the 1990s, almost seventy per cent of deforested areas changed
to agricultural land, predominantly under permanent rather than shifting
systems.