NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION (NOAA) “Trends in atmospheric methane”: http://www.esrl.noaa.gov/gmd/ccgg/trends_ch4/index.html .
December 2014: 1833.2 ppb CH4
December 2015: 1843.2 ppb CH4
Atmospheric CH4 increased in 1983-1998 by up to 13 ppb (parts per billion) per year, increased much more slowly in the period 1999-2006 (up to 3 ppb per year, the 2001-2005 average being 0.5 ppb/year), and has increased more rapidly from 2007 onwards, reaching 12.5 ppb per year in 2014. Atmospheric CH4 increased to 1,843 ppb CH4 in December 2015 as compared to a pre-Industrial Revolution level of 700 ppb CH4 ( “2011 climate change course”: https://sites.google.com/site/300orgsite/2011-climate-change-course ).