evidence: While all eyes were turned on carbon dioxide, almost by chance a few researchers discovered that other gases emitted by human activity could have serious global impacts. In the 1970s attention centered on damage to the atmosphere's protective ozone layer, and nations combined to diminish the risk. In the mid 1980s scientists realized that other gases, especially methane and nitrates, added together could be as important as carbon dioxide in bringing global warming. The sources and interactions of the gases were multiple and complex, and research brought many surprises; for example, the world's livestock turned out to be a major problem. As methane levels soared ever higher, researchers warned that feedbacks such as increased emissions from warming soils could dangerously accelerate global warming. The warnings had little impact on policy. (This essay is supplementary to the core essay on The Carbon Dioxide Greenhouse Effect For the most important greenhouse gas, water vapor, see the essay on Simple Models of Climate.) Keywords: climate change, global warming, CO2, CH4, methane, ozone, nitrates, CFCs