This is an introduction to the scientific consensus on the human role in global warming.The vast majority of scientists agree that human activity has significantly increased greenhouse gases in the atmosphere - most dramatically since the 1970s. In February 2007 the Intergovernmental Panel on Climate Change found that global warming is "unequivocal" and that human-produced carbon dioxide and other greenhouse gases are chiefly to blame, to a certainty of more than 90 percent. Yet global warming skeptics and ill-informed elected officials continue to dismiss this broad scientific consensus. In "What We Know About Climate Change", MIT atmospheric scientist Kerry Emanuel outlines the basic science of global warming and how the current consensus has emerged.Although it is impossible to predict exactly when the most dramatic effects of global warming will be felt, he argues, we can be confident that we face real dangers. Emanuel, whose work was widely cited in media coverage of Hurricane Katrina, warns that global warming will contribute to an increase in the intensity and power of hurricanes and flooding and more rapidly advancing deserts.But just as our actions have created the looming crisis, so too might they avert it. Emanuel calls for urgent action to reduce greenhouse gases and criticizes the media for playing down the dangers of global warming (and, in search of "balance," quoting extremists who deny its existence).An afterword by environmental policy experts Judith Layzer and William Moomaw discusses how the United States could lead the way in the policy changes required to deal with global warming.