Credit: © ISTOCKPHOTO / GRAFIKERAY

Thanks to a complicated set of chemical reactions, increases in the atmospheric concentration of nitrous oxide, a powerful and long-lived greenhouse gas, trigger a decline in methane, another climate-warming culprit, an analysis suggests.

Using a computer model that links the chemistry of the stratosphere with that of the underlying troposphere, atmospheric scientists Michael Prather and Juno Hsu of the University of California, Irvine, have assessed in detail how anthropogenic emissions of nitrous oxide affect methane1. The intricate chemical cascade is as follows, the researchers explain: increased nitrous oxide in the stratosphere depletes ozone at altitudes between 25 and 40 kilometres, which allows larger amounts of ultraviolet radiation to reach lower layers of the atmosphere. That, in turn, boosts the tropospheric concentration of hydroxyl radicals, highly reactive compounds that help cleanse the air of methane and many pollutants.

The net result of a 50-parts-per-billion bump in nitrous oxide concentration since pre-industrial times is that methane concentrations sit about 18 ppb lower than they would be otherwise, the researchers say.