Global change: state of the science

Only recently, within a few decades, have we realized that humanity signifi cantly influences the global environment. In the early 1980s, atmospheric m easurements confirmed basic concepts developed a decade earlier. These basi c concepts showed that human activities were affecting the ozone layer. Lat er measurements and theoretical analyses have clearly connected observed ch anges in ozone to human-related increases of chlorine and bromine in the st ratosphere. As a result of prompt international policy agreements, the comb ined abundances of ozone-depleting compounds peaked in 1994 and ozone is al ready beginning a slow path to recovery. A much more difficult problem conf ronting humanity is the impact of increasing levels of carbon dioxide and o ther greenhouse gases on global climate. The processes that connect greenho use gas emissions to climate are very complex. This complexity has limited our ability to make a definitive projection of future climate change. Never theless, the range of projected climate change shows that global warming ha s the potential to severely impact human welfare and our planet as a whole. This paper evaluates the state of the scientific understanding of the glob al change issues, their potential impacts, and the relationships of scienti fic understanding to policy considerations. (C) 1999 Elsevier Science Ltd. All rights reserved.