Global methane concentrations increased by 12 ppb per year in the 1980s. This rise slowed in the 1990s, and then stabilised entirely from 1999 to 2006, before concentrations began to increase again at a rate of 6 ppb per year.
As would be expected, methane growth rate varies across different regions of the globe. For instance, growth has been above global levels in the southern tropics since 2007 and the authors suggest that this may have been due to wet summers stimulating the expansion of wetland area. Additionally, they point out the large increase in Arctic methane that occurred solely during 2007, but also suggest that catastrophic emission scenarios of methane from hydrates are unlikely. Modelling of methane concentrations thus shows that tropical wetlands were responsible for driving atmospheric concentration growth in 2007, and that since then the tropics and northern mid-latitudes have been important contributors. There are also anthropogenic sources to consider. Coal mining activities have expanded across some areas of the globe, particularly China, and fracking has increased in popularity in the US.
Additional data are needed to reconcile top-down and bottom-up estimates of atmospheric methane, and more isotope studies that would allow emission sources to be identified. The authors conclude by warning that funding for the monitoring of greenhouse gases is shrinking at a time when they are sorely needed.
Nisbet, E.G., Dlugokencky, E.J., Bousquet, P. 2014. Science, 343, 493-495.
Author: Ed Dlugokencky, NOAA CMDL. http://www.esrl.noaa.gov/gmd/Photo_Gallery/GMD_Figures/ccgg_figures/tn/ch4_surface_color.png.html (PDF version) --- This plot shows methane observations from 1993 to 2007 showing the growth of methane, the seasonal variations and the difference between northern and southern hemispheres