The amount of methane in the atmosphere is the result of a balance between the production of methane on the Earth's surfaceits sourceand the destruction or removal of methane, mainly in the atmosphereits sink in an atmospheric chemical process.
Another major natural sink is through oxidation by methanotrophic or methane-consuming bacteria in Earth's soils.
These 2005 NASA computer model simulationscalculated based on data available at that timeillustrate how methane is destroyed as it rises. 
As air rises in the tropics, methane is carried upwards through the tropospherethe lowest portion of Earths atmosphere which is  to  from the Earths surface, into the lower stratospherethe ozone layerand then the upper portion of the stratosphere. 
This atmospheric chemical process is the most effective methane sink, as it removes 90% of atmospheric methane. This global destruction of atmospheric methane mainly occurs in the troposphere. 
Methane molecules react with hydroxyl radicals (OH)the "major chemical scavenger in the troposphere" that "controls the atmospheric lifetime of most gases in the troposphere". Through this CH oxidation process, atmospheric methane is destroyed and water vapor and carbon dioxide are produced. 
While this decreases the concentration of methane in the atmosphere, it also increases radiative forcing because both water vapor and carbon dioxide are more powerful GHGs factors in terms of affecting the warming of Earth. 
This additional water vapor in the stratosphere caused by CH oxidation, adds approximately 15% to methane's radiative forcing effect. 
By the 1980s, the global warming problem had been transformed by the inclusion of methane and other non-CO trace-gasesCFCs, NO, and O on global warming, instead of focusing primarily on carbon dioxide. Both water and ice clouds, when formed at cold lower stratospheric temperatures, have a significant impact by increasing the atmospheric greenhouse effect. Large increases in future methane could lead to a surface warming that increases nonlinearly with the methane concentration.
Methane also affects the degradation of the ozone layerthe lowest layer of the stratosphere from about  above Earth, just above the troposphere. NASA researchers in 2001, had said that this process was enhanced by global warming, because warmer air holds more water vapor than colder air, so the amount of water vapor in the atmosphere increases as it is warmed by the greenhouse effect. Their climate models based on data available at that time, had indicated that carbon dioxide and methane enhanced the transport of water into the stratosphere.
Atmospheric methane could last about 120 years in the stratosphere until it is eventually destroyed through the hydroxyl radicals oxidation process.