The SCOPE/UNEP International program of Studies on the Biogeochemical Cycles has four principal subjects: Nitrogen, Phosphorus, Carbon and Sulphur. The study is being undertaken to find the major complex interactions between the cycles of these substances. The purpose is to demonstrate the resulting overall limits, which govern our lives and the future of mankind. The emerging climate changes caused by the greenhouse effect make it increasingly clear that our Earth is an integrated system. But environmental systems can be stretched by mankind's actions to limits at which changes in them can occur rapidly and perhaps irreversibly. It is possible that some of nature's thresholds are already being crossed.