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.
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
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