The concentrations of methane (CH4) and carbon dioxide (CO2) gases in the atmosphere have both risen dramatically since the start of the Industrial Revolution. However, unlike its more familiar greenhouse-gas cousin, atmospheric methane has recently stopped increasing in abundance. This development wasn’t entirely unanticipated, given that the rate of increase has been slowing for at least a quarter-century. The recent stabilization of methane levels is something that some scientists are trying very hard to explain. Methane has many sources. Some are natural, such as wetlands and plants, and some are the consequences of modern society, such as landfills and wastewater treatment. Methane is destroyed principally by its reaction with the hydroxyl radical (OH) in the lower atmosphere. One theory about the stabilization of methane levels is that deforestation has reduced the number of plants contributing to atmospheric methane. Another idea is that an increase in the prevalence of tropical thunderstorms may have raised the amounts of the various nitrogen oxides high in the atmosphere. There, these gases have the side effect of boosting the production of OH, which in turn acts to destroy methane.