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The possibility that human activities are affecting climate on a regional or global scale has been studied with increasing diligence during the past 20 years. Monitoring programs have been started and expanded, and mathematical models of climatic change, ofever increasing complexity, have been developed as computer capability has increased. Examination of all published information shows that the atmospheric carbon-dioxide content has increased by about 3 percent between 1958 and 1975 as a result of fossil-fuel combustion, that the solid-particle loading of the atmosphere has risen noticeably downwind of large urbanindustrial complexes in developed countries, although volcanoes and wind still provide nearly all the dust loading on a global scale, and that waste heat from man's activities now amounts to about 0.016 percent, or 1 part in 6000, of the average input of solar energy to the planet. Human influences are felt most strongly in and downwind of cities; in such areas the mean temperatures are higher, the mean diurnal temperature range is smaller, and the annual precipitation is higher than they would be were the cities absent. The urban influence is felt for only tens or a few hundred kilometers from these source regions.