Carbon dioxide, water vapor, methane, nitrous oxide, and chlorofluorocarbons in the earth's lower atmosphere prevent much of the warmth from each day's sunlight from radiating back into space, thus stabilizing the earth's temperature. Without them there would be a 60-degree Fahrenheit temperature difference between day and night. But by pumping more of these greenhouse gases into the atmosphere, we're preventing too much heat from escaping, throwing the earth's energy balance out of whack.
Until the 1950s, nobody knew where the CO2 generated by human activity was stored after it left our smokestacks and tailpipes: atmosphere, oceans, plants? But then Charles Keeling of the California Institute of Technology began measuring atmospheric CO2 in locations that were remote from sources of industrial emissions, such as Antarctica and the Mauna Loa volcano in Hawaii. Within two years Keeling's data showed that even in these pristine environments, CO2 concentrations were rising each year to levels that could not be explained by natural variability, revealing for the first time that our industrial emissions were being stored, in part, in the atmosphere.
Readings from Mauna Loa indicate that the annual rise in CO2 levels has accelerated over the past 50 years. Today CO2 concentrations are higher than at any time in the past 400,000 years. The U.N. Intergovernmental Panel on Climate Change projects that without new controls on emissions, CO2 levels may more than double by 2100, raising global temperatures by as much as 10.8 degrees Fahrenheit from their 1990 levels.