Fortunately for us, carbon monoxide (CO)—a toxic gas—is a very minor constituent of the atmosphere. It is produced by incomplete burning of fossil fuels and biomass (such as dry leaves and wood) and by the oxidation of methane and other volatile hydrocarbons in the atmosphere. On page 1663 of this issue, Wang et al. (1) present high-precision measurements, taken from air trapped in Antarctic ice, of how atmospheric concentrations of CO have changed over the past 650 years. Their findings offer a striking and surprising look at the history of fire in the Southern Hemisphere, and some hints at future global fire trends.