MODELING THE EFFECTS OF ALBEDO CHANGE ASSOCIATED WITH TROPICAL DEFORESTATION

The effects in climate models of changes in albedo and soil moisture which are expected to follow from tropical deforestation are reviewed. The major results from experiments with tropical forest regions (a) forested (`Experiment F') and (b) grass-covered (`Experiment G') are compared over South America and Central Africa. Then, Experiment F is compared with `Experiment 2' which has constant snowfree albedo of 0.2, close to that of grassland; in both South America and Zaire regions, the albedo changes for (G-F) and (2-F) are similar and, as Experiment 2 is an eight year run, assessments of the statistical significance can be made. The results for December to February and March to May show a decrease of rainfall over the Amazon region and also to the southeast outside the area which has changed albedos in (G-F). These decreases are common to both (G-F) and (2-F) and are statistically significant in the comparison with the eight year record for Experiment 2. Deforestation in the Zaire regions is associated with similar changes, which, though less extensive, are generally larger in relation to the albedo changes. In June to August, changes are small south of the equator because the rainbelt has moved north; in South America larger, significant decreases are obtained north of the equator. The drying with increased albedo is due to decreases in both evaporation and moisture convergence. The decreases in evaporation result both from a reduction in absorbed energy at the surface and an increase in surface resistance to evaporation due to greater soil moisture deficits arising from earlier decreases in rainfall.