Deforesting the Amazon

E. Linacre

3/’02

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Climate model simulations have been performed to test the effect of removing entirely the Amazon rainforest and substituting it by grassland (1,2). It turns out that the two different land surface conditions produce some differences in regional climate, however the magnitude of the change is uncertain. Grassland has a higher albedo than rainforest, and the Bowen ratio (sensible heat flux divided by latent heat flux) is larger. As a result, Amazon deforestation would increase the screen-level temperature by 2.3 K, on average, according to the General Circulation Model (GCM) of the National Center for Atmospheric Research (1). The average evaporation rate over the Amazon Basin would be reduced by 0.76 mm/d (which is 18% of the present rate), and rainfall lessened by 0.27 mm/d, which is only 4% of the rate now. The Hadley Centre GCM predicts larger rainfall reductions over the lower Amazon Basin (Fig 1).

These changes imply an only slightly larger mean run-off. However, more detailed estimates show that the figures are very sensitive to the soil infiltration rate. Compaction of the ground by cattle lowers infiltration, and that would further reduce the evaporation rate and enhance the run-off. 

The climatological, biological, hydrological and geochemical changes resulting from the current and anticipated deforestation of the Amazon rainforest are the subject of a large multinational research project, the Large Scale Biosphere-Atmosphere Experiment in Amazonia (LBA).

Fig 1. The annual mean rainfall (mm per day) change if the entire Amazon forest was replaced by pasture. The estimate is based on simulations of the Hadley Centre GCM (2). A rainfall deficit of more than 1 mm/d is predicted in the brown region near the Amazon mouth. Areas in blue should receive more rainfall.

References

(1) Lean, J. and P.R. Rowntree 1997. Understanding the sensitivity of a GCM simulation of Amazonian deforestation to the specification of vegetation and soil characteristics. J. Climate 10, 1216-35.

(2) Gash, J.H.C. and C.A. Nobre 1997. Climatic effects of Amazonian deforestation: some results from ABRACOS. Bull. Amer. Meteor. Soc., 78, 823-830.