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WORLDWIDE FOREST/BIODIVERSITY CAMPAIGN NEWS

Lost Forests Leave West Africa Dry

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Forest Networking a Project of Ecological Enterprises

http://forests.org/

1/18/97

The New Scientist Magazine makes the case that the droughts in West Africa

may be related to rainforest destruction. Rainforests are largely

responsible for tropical rainfall such as West Africa's monsoons. This is

a photocopy for your personal use only.

g.b.

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RELAYED TEXT STARTS HERE:

P L A N E T S C I E N C E

Lost forests leave West Africa dry

Fred Pearce

From New Scientist,

c Copyright IPC Magazines 1997

18 Jan 97

DROUGHTS in West Africa over the past 20 years may have been caused by the

destruction of rainforests in countries such as Nigeria, Ghana and Cote

d'Ivoire, according to a new study.

Further deforestation in the region "could cause the complete collapse of

the West African monsoon", says Xinyu Zheng of the Centre for Global Change

Science at the Massachusetts Institute of Technology.

Rainforests need high rainfall to grow. But they also help to generate

rainfall elsewhere. Half or more of the rain falling on the forest quickly

evaporates from the forest canopy, providing moisture in the air to form

clouds that produce rainfall further downwind. In this way, West African

coastal rainforests, which receive copious amounts of rain from winds

coming off the Atlantic Ocean, have helped to maintain rainfall in the

drier lands of the interior.

At the beginning of this century, the West African coastal rainforests

covered around 500 000 square kilometres. Since then, up to 90 per cent

have disappeared to make way for farms and other kinds of human activity

such as mining. Overgrazing, expansion of arable land and the substantial

growth of the timber industry are the main culprits. As the forests are cut

down, more of the rain falling on coastal regions percolates into soils or

flows directly to the sea. Evaporation is reduced, which affects rainfall

in drought-prone countries of the interior such as Mali and Niger.

Several studies have predicted that deforestation of the Amazon basin will

have a similar impact in Brazil, but Zheng and coauthor Elfatih Eltahir,

also of MIT, say that the effect may already be happening in West Africa.

They point out that the proportion of total forest cover that has been

cleared is much greater in West Africa than in the Amazon.

In Geophysical Research Letters this week (p 155), the researchers report

on a statistical model of the hydrological cycle of the West African

monsoon that takes into account such features as energy flows, rainfall and

evaporation in the coastal region, and condensation as new clouds form

inland. It also predicts the position of the Inter-Tropical Convergence

Zone (ITCZ), the permanent weather front which is the source of most of the

rain on the coast of West Africa.

The model confirms an old theory, first developed 20 years ago by MIT's

Jule Charney, that the loss of vegetation on the edge of the Sahara Desert

in the West African interior could reduce rainfall. But the authors say

this effect is much smaller than that of coastal deforestation, which until

now has been virtually unresearched.

The model predicts that as forests are lost, the coastal rainfall will no

longer be recycled to create rain inland. And worse, the ITCZ, which

normally moves across the land during the summer monsoon, "stays over the

ocean".

The "worst possible scenario for tropical deforestation in West Africa",

the authors say, would see "all the forests replaced by savanna". This,

according to Zheng's model, "could cause the collapse of the monsoon

system".

So far that has not happened, and the authors admit that their model is

fairly crude. But they point out that since 1970, rainfall over the whole

of West Africa has been lower than before, apparently confirming their

predictions.

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