Of all the greenhouse gases in Earth’s atmosphere, including water vapor, carbon dioxide exerts the most control on climate, according to a study by Andrew A. Lacis and colleagues at NASA’s Goddard Institute for Space Studies (Science 2010, 330, 356). The finding confirms previous modeling of Earth’s climate and counters some studies that suggest anthropogenic CO2 stemming from burning fossil fuels is not relevant to climate change. Water vapor contributes more to Earth’s greenhouse effect than CO2—50% for water vapor and 25% for clouds, relative to 20% for CO2, Lacis and coworkers explain. But because water condenses and precipitates from the atmosphere, it plays a different role from those of CO2 and other noncondensing greenhouse gases, such as ozone, methane, nitrous oxide, and chlorofluorocarbons. The NASA researchers conducted a set of idealized climate model experiments in which they added and subtracted different greenhouse gases and aerosols from the atmosphere to determine their roles in controlling air temperature. They found that, without CO2, Earth’s greenhouse effect would collapse and the planet would plunge into a frozen state, and that rising levels of CO2 would drive up temperatures. In effect, CO2 serves as the primary “control knob” for Earth’s temperature, making control of rising atmospheric CO2 levels “a serious and pressing issue,” they write.