Despite the improved ability of climate models to simulate observed trends, there are still considerable uncertainties in key factors, including the magnitude and patterns of natural variability, the effects of human influence, and the rates of carbon sequestration. There are also new questions to be resolved. For example, is the observed increasing magnitude of El Niño events during recent decades related to human-induced climate change? To what extent do reductions in sulphur emissions, required to reduce the acid rain problem, offset warming by greenhouse gases by reducing sulphate aerosols in the atmosphere?This strategy features in the framework of Agenda 21 as formulated at UNCED (Rio de Janeiro, 1992), now coordinated by the United Nations Commission on Sustainable Development and implemented through national and local authorities.