In 2007 after a seven year plateau atmospheric methane concentration began rising. Research has shown that the proportion of carbon-13 in methane has been decreasing since 2007. Since carbon-12 is taken up preferentially by photosynthetic processes, the decreasing carbon-13 fraction suggests biogenic sources such as wetlands or agriculture including livestock as opposed to fossil fuel burning or biomass burning. Alternatively, a shift from biomass burning to fossil fuels may contribute, for example, there is a major program in India to replace solid fuels with gas for indoor use. There is evidence that the source of the increased methane could be tropical wetlands where record high temperatures increased methane production. Another possibility is reduced chemical reactions in the atmosphere, primarily with hydroxyl ions, that destroys methane.