There is now a wide appreciation of the damage being done to the environment through human activity. This damage is sometimes global, as in the case of the depletion of the ozone layer and of the increase in greenhouse gas emissions. Often the damage is regional, as with pest degradation of native forests, or the erosion of river catchments. Often too, the damage is local. Pollution from motor vehicles, contamination of soil by chemicals, sewage discharges; these and hundreds of other human activities stress the environment