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 C么te
d鈥橧voire, according to a new study.
Further deforestation in the region 鈥渃ould cause the complete collapse of the
West African monsoon鈥�, 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.
Advertisement
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鈥檚 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, 鈥渟tays over the ocean鈥�.
The 鈥渨orst possible scenario for tropical deforestation in West Africa鈥�, the
authors say, would see 鈥渁ll the forests replaced by savanna鈥�. This, according to
Zheng鈥檚 model, 鈥渃ould cause the collapse of the monsoon system鈥�.
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.