Global Warming

The impact onecosystems 173

Forests, deforestation and climate change

Extensive changes in the area of forests due to de-

forestation can seriously affect the climate in the

region of change. Also, changes in temperature or

rainfall that occur because of long-term changes in

climate can also have a major impact on forests. We

look at these effects in turn.

Changes in land use such as those brought about

by deforestation can affect the amount of rainfall,

for three main reasons. Over a forest there is a lot

more evaporation of water (through theleaves of

the trees) than there is over grassland or bare soil,

hence the air will contain more water vapour. Also,

a forest reflects twelve to fifteen per cent of the sun-

light that falls on it, whereas grassland will reflect

about twenty per cent and desert sand up to forty

per cent. A third reason arises from the roughness

of the surface where vegetation is present.

An American meteorologist, Professor Jules

Charney, suggested in 1975 that, in the context of

the drought in the Sahel, there could be an impor-

tant link between changes of vegetation (and hence

changes of reflectivity) and rainfall. The increased

energy absorbed at the surface when vegetation is

present and the increased surface roughness both

tend to stimulate convection and other dynamic ac-

tivity in the atmosphere so leading to the production

of rainfall.

Early experiments with numerical models that

included these physical processes demonstrated the

effect and indicated, for instance, a reduction of

about fifteen per cent in rainfall if the forest north of

30 ◦S in South Americawere removed and replaced

by grassland.^44 Similar model experiments for Zaire

over a smaller region showed an average reduction

in rainfall of over thirty per cent.^45 A much more

drastic experiment in which the Amazonian forest

was removed and replaced with a desert surface

showed a reduction in rainfall by seventy per cent

to levels similar to those of the semi-arid regions

of the Sahel part of Africa.^46 Such a model exper-

iment does not represent a realistic situation, but

it illustrates the significant impact that widespread

deforestation could have on the local climate.

More recent work has been with interactive

models that include not just the effect of changes

in land use or forestation on the climate but also,

in a dynamic way, the effect on forests and other

vegetation of changes in climate. In an experiment

with such a modelthat assumes carbon dioxide

emissions following the IS 92a scenario,^47 substan-

tial reductions in precipitationare projected for

areas of Amazonia, that lead to die-back of the

Amazonian forest and significant release of carbon

to the atmosphere (one of the positive climate feed-

backs mentioned in the box on page 40). As the

forest dies back, the rainfall is further reduced be-

cause of the change in properties of the land sur-

face, leading, by the end of the twenty-first century,

to the replacement of much of the forest cover by

semi-arid conditions. Such results are still subject

to considerable uncertainties (for instance, those

associated with the model simulations of El Ni ̃no

events under climate change conditions and the con-

nections between these events and the climate over

Amazonia), but they illustrate the type of impacts

that might occur and emphasisethe importance of

understanding the interactions between climate and

vegetation.

from heavy industry, power stations and motor cars. Not all damage to
trees, however, is thought to have this origin. Studies in several regions
of Canada, for instance, indicate that the die-back of trees there is related
to changes in climatic conditions, especially to successions of warmer
winters and drier summers.^48 In some cases it may be the double effect of
pollution and climate stress causing the problem; trees already weakened