A growing number of environmental problems arose after World War II, when many American industries--both old and new--underwent unprecedented growth. The publication of Aldo Leopold's (1886-1948) A Sand Country Almanac (1949), which describes the world as a collection of interlinked communities, marked a turning point in the American environmental movement. Thereafter, the relatively new science of ecology, the study of the interrelationship of organisms and their environments, came to play an integral role. During the 1950s and 1960s an expanding group of scientists voiced their concerns about pesticide poisoning, pollution, overpopulation, and nuclear fallout. The book Silent Spring (1962), by the biologist Rachel Carson (1907-1964), drew attention to the fact that harmful chemicals such as the insecticide DDT travel up the food chain to significantly affect human and animal health. During the widespread social unrest of the 1960s, environmental abuse was a prominent issue that prompted widespread protest. The environmental movement was rejuvenated as a result of the heightened activism of the period.
Though the Clean Air Act was passed in 1955 and the Wilderness Preservation Act was passed in 1964, the government did not act on behalf of the environment in a significant way until the 1970s, following the first Earth Day, on April 22, 1970. This event, initiated by Wisconsin senator Gaylord Nelson, was observed in thousands of rallies, demonstrations, and cleanup initiatives around the country. Earth Day motivated a larger public to get involved in environmental affairs and established environmentalism as a worldwide movement. Following the event, membership in environmental organizations swelled to unprecedented levels, and politicians suddenly scurried to respond to voters' concerns. A number of federal laws were passed, beginning with the National Environmental Policy Act, the Occupational Safety and Health Act, and the Solid Waste Disposal Act in 1970 and the Endangered Species Act in 1973. President Richard Nixon (1913-1994) also created the Environmental Protection Agency (EPA) in 1970 to enforce the new laws.