Graph showing the increases in the concentration of methane in the atmosphere - as measured in ppbv (parts per billion by volume) - roughly parallel world population growth. As indicated above, over the last two centuries (i.e., since the industrial revolution), methane's concentration in the atmosphere has more than doubled. Over the past decade (1990-2000), although methane concentrations have continued to increase, the overall rate of methane growth has slowed. In the late 1970ís, the growth rate was approximately 20 ppbv/year. In the 1980ís, growth slowed to 9-13 ppbv/year. The 1990ís have seen growth of between 2 and 8 ppbv/year.