The concentration of atmospheric methane has been steadily increasing since the dawn of the industrial age - except for a mysterious plateau between 1999 and 2006.

A new study suggests that this plateau was a result of lower industrial output, and that it ended due to an increase in biological sources, for example agricultural activity.

Methane is a potent greenhouse gas, and increasing levels can affect atmospheric chemistry, ozone generation, and the water vapor cycle.

Atmospheric methane levels have almost tripled since preindustrial times, yet researchers remain puzzled as to why this trend plateaued between 1999 and 2006.

Hinrich Schaefer et al. reconstructed the global history of methane sources over the past 35 years by using carbon isotope data from ice cores, archived air, and a global network of monitoring stations.

Using the data to run multiple different simulations, the authors found that the best explanation for the beginning of the plateau is a reduction in thermogenic emissions, which include emissions from fossil fuel use, as well as a potential increase in the capacity of natural methane sinks.

As for the end of the plateau and resumed increases in methane levels, this can likely be attributed to biogenic sources.

Although the exact biological source cannot be determined, the authors propose that agriculture may be a key contributor, based on the distribution of methane sources.

Research paper: A 21st century shift from fossil-fuel to biogenic methane emissions indicated by 13CH4