Why does air passage over forest yield more rain? Alternative interpretations of Spracklen et al. 2012 Nature 489: 282
Spracklen et al. recently presented a pan-tropical study of rainfall and land-cover that showed that satellite-derived rainfall measures were positively correlated with the degree to which model-derived air trajectories had been exposed to forest cover. This result confirms the influence of vegetation on regional rainfall patterns suggested in previous studies. However, we find that the conclusion of Spracklen et al. -- that differences in rainfall reflect air moisture content resulting from evapotranspiration -- appears undermined by methodological inconsistencies. We discuss some alternative explanations that require investigation. These include the distinct role of forest evapotranspiration in creating low pressure systems that draw moisture from the oceans to the continental hinterland. This alternative physical process is consistent with the empirical findings of Spracklen et al. but underlines a greater potential danger of forest loss than is suggested by their analyses of moisture recycling.