Global warming is almost certainly the most important problem addressed by the atmospheric community today and has recently become one of the hottest on the political agenda. Because it is such a vast and complex problem, the majority of atmospheric researchers are carrying out investigations on some aspect of it. The results of these studies are assimilated in computer-intensive global circulation models to simulate the earth's climate. Despite such intensive efforts, there is still much contention over the nature and degree of global warming, over long-term climate prediction, and over the implementation of mitigating measures. In this paper, I review the scientific basis for global warming, present outstanding issues in climate modeling, and examine the reasons for the uncertainty in future predictions. Global impacts—those on California in particular—are discussed together with the measures that have been taken worldwide and at the state level to combat it.