The hydrocarbons we combust in the service of human needs have always come from the atmosphere—albeit on extended time scales. In fossil fuels, the carbon was converted into biomass millions of years ago; in biofuels, the carbon was fixed by photosynthesis during a recent growing season. Society relies on the incredible energy density of these everyday fuels. But their hydrocarbons accumulated over millennia, and the planet now suffers from the glut of carbon returned to the atmosphere at a breakneck—and catastrophic—rate.