The two major global-scale environmental issues of international concern since the 1970s have been the threats of global stratospheric ozone reduction and global warming. As discussed in Chapter 11, the threat to the global ozone layer has been controlled to some degree because national and international regulations have required the chemical industry to use alternatives to chlorocarbons and bromocarbons, the chemicals primarily responsible for much of the ozone-layer reductions. Regulations are similarly responsible for improvements in air quality and acid deposition problems in the United States and many western European countries since the early 1970s (e.g., the U. S. Clean Air Act Amendments of 1970 motivated U.S. automobile manufacturers to develop the catalytic converter in 1975, which led to improvements in urban air quality; regulations through the Clean Air Act Amendments and the 1979 Geneva Convention on Long-Range Transboundary Air Pollution led to reductions in sulfur emissions and an amelioration of some acid deposition problem in the 1980s and 1990s). The second major issue of international concern, the threat of global warming, has been addressed at international meetings, but progress in regulating emissions responsible for the problem has been slow. In this chapter, global warming is discussed and distinguished from the natural greenhouse effect. Climate responses to enhanced gas and aerosol particle concentrations are examined. Historical and recent temperature trends, both in the lower and upper atmosphere, are described. National and international efforts to curtail global warming are also discussed, as are potential effects of global warming.