The Remote Effects Of Deforestation

Studies using GCMs (general circulation models, see Chapter 1) suggest that the influence of deforestation can extend very far afield, and the distant effects are often stronger than the local ones. Decreases in the amount of tropical forest seem likely to lead to cooler temperatures in the mid and high latitudes, especially the northern hemisphere, in winter. This is mainly because there is less evaporation of water from the tropical land surface when the forest is gone. Evaporated water is a great store of heat, and when it reaches cooler regions it can condense back out as water droplets in clouds, which releases the heat and tends to keep the air from cooling further. So, the tropical forests provide a "heat subsidy" to the higher latitudes, and when the tropical forest is reduced this long-distance source of heat also diminishes. Not only temperature is affected by the change in heat and water flux to the atmosphere.

Another remote effect predicted from deforestation close to the equator is a decrease in monsoon rainfall farther to the north. For instance, deforestation in equatorial Indonesia and Malaysia could weaken the monsoon over southern Asia (India and Bangladesh, through to Thailand and Vietnam). However, not all models predict the same effects. One model suggests that, if there was more forest in tropical Asia, Australia and east Africa, the monsoon over India would actually be weaker. The workings of the climate system on the broad scale are so complex that the slight differences in assumptions between different models can lead to entirely different results!

Such remote effects do not necessarily take the complete destruction of the rainforests to become noticeable. Even the amount of tropical deforestation that has already occurred over the last few thousand years, compared with the present natural state, is predicted by one model to have produced significant changes in climate in other parts of the world. The model—by T.N. Chase and colleagues at the University of Colorado—suggests that there would have been several degrees' centigrade cooling in winter temperature in the mid-latitudes, in such areas as Europe and North America. Much of this remote effect occurs through changes in the amount of latent heat leaving the tropics in water vapor. With less forest, there is less evaporation, and less long-distance export of latent heat to the mid-latitudes. If tropical deforestation continues, it may significantly change the climate for people living in parts of the world that seem entirely remote from these "Third World'' problems. Interestingly, these cooling effects work at the same time as CO2 released from the cleared forests (Chapter 7) is tending to warm the world, setting off a sort of "tug of war'' between the two effects.

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