"Human activities are transforming the global environment, and these global changes have many faces: ozone depletion, tropical deforestation, acid deposition, and increased atmospheric concentrations of gases that trap heat and may warm the global climate. For many of these troubling transformations, data and analyses are fragmentary, scientific understanding is incomplete, and long-term implications are unknown. Yet, even against a continuing background of uncertainty, it is abundantly clear that human activities - burning fossil fuels, emitting pollutants from industry, and clearing forests that are the habitats for plant and animal species, for example - now match or even surpass natural processes as agents of change in the planetary environment."
Frank Press, President, National Academy of Sciences, in the Preface to "One Earth, One Future: Our Changing Global Environment", National Academy of Sciences, 1990.