Discovery of carbon dioxide accumulation in the atmosphere, 1957 – Roger Revelle and Charles Keeling of Scripps Institution of Oceanography began documenting rise of CO2 from 315 parts per million (ppm) base that year. (See Spencer Weart’s Discovery of Global Warming, Harvard University Press, 2008.)
Publication of Silent Spring by Rachel Carson, Sept. 27, 1962 — The book that catalyzed the worldwide environmental movement, Silent Spring called for an end to indiscriminate pesticide use and, on a broader level, a change in the way we view nature. (See Silent Spring Wikipedia article)
Discovery of atmospheric ozone depleting chemicals, 1974 — F. Sherwood Rowland and Mario J. Molina describe the way refrigerants (CFCs or chlorofluorocarbons) break up ozone in a catalytic cycle in the June issue of Nature. (See the Nobel Prize page; also EPA pages about the Montreal Protocol ).