How will the growth of atmospheric concentrations of greenhouse gases affect the climate? When we try to foresee the future global climate, we are not simply extrapolating past behavior. We are using our scientific understanding of how the climate system works. Keeping that distinction in mind, then, the typical benchmark figure by which climate scientists now predict the climate will warm in response to a doubling of CO2 is a range rather than a single number. A range often quoted is 1.5 to 4.5 degrees Celsius (which is 2.7 to 8.1 degrees Fahrenheit). Not long ago, a widely quoted consensus number was the midpoint of this range, 3 degrees Celsius (which is 5.4 degrees Fahrenheit). A similar range, also often quoted, is 2 to 5 degrees Celsius (or 3.6 to 9.0 degrees Fahrenheit). There is more than one way to arrive at such a range, both from observations and from models, and the details are important to scientists doing this research. For other people, it is more important to be familiar with the approximate range, because estimates of climate sensitivity will continue to vary as more research is done.