The atmosphere, and more precisely the troposphere, is the largest sink for methane. Methane in the troposphere reacts with hydroxyl (OH) radicals, forming mainly water and carbon dioxide.
In total this reaction accounts for about 500 million tonnes of methane each year. An indirect effect of atmospheric methane oxidation is that it can magnify the effects of other pollutants. Increased methane in the atmosphere means fewer OH radicals and so less oxidizing power in the atmosphere as a whole. Some methane, about 40 million tonnes a year, is oxidized in the same way, but in our stratosphere.
Human Impact
Overall the direct human impact on the atmospheric destruction of methane is relatively minor. However, our emissions of other atmopsheric pollutants, such as nitrogen oxide (NOx) gases (see NOx page) may reduce the levels of OH radicals in our atmopshere, so prolonging the lifetime of methane in our atmosphere. Additionally, our past use of ozone depleting aerosols, and the destruction of ozone in the stratosphere, may lead to increases in tropospheric ozone and so to an overall lowering of methane concentrations.
Potential for control
Our potential for control of the atmospheric methane sink is minimal and essentially lies in reduction of the amounts of methane emitted to the atmosphere in the first place. Recent trends indicate that the rate of methane increase in our atmosphere has accelerated and it has been suggested that this is due to increased methane emissions from fracking, more methane from wetlands, more from ruminants, or a decrease in the atmospheric methane sink. It may well be that a combination of these factors is actually causing the increase
