Recent methane rise mostly due to biogenic sources

There is an interesting new paper by Nisbet et al. in the AGU publication Global Biochemical Cycles titled “Rising atmospheric methane: 2004-2014 growth and isotopic shift” . The fossil fuel industry (oil extraction, fracking….) is often blamed for rising methane emissions, and this argument went somehow into limbo as the atmospheric mixing ratio, after a period of clear rising,  was stable for many years:

This picture documents the rise from 1984 to about 1999, the following plateau and finally a new lower rise from 2005 to 2015 (the lower plot shows the derivative = the change in mixing ratio per year).

One fingerprint in detecting the origin of the methane (from fossil fuels or from biogenic sources) is the isotopic composition: biogenic methane has a higher component of the 13C (carbon-13) isotope than the methane from fossil sources which are more depleted in 13C. Usually the isotopic fingerprint is given as delta_13C/12C in per mil (°/°°): the next figure (from Wikipedia) shows the exact defintion.

 

More negative values point to dominant biogenic sources, less negative values to fossil methane. For instance this paper gives a delta_13C/12C of about -60 for methane from ruminants (cattle) and marsh gas (wetlands). The next table (right column) has an overview from different sources:

Clearly methane from landfills or natural gas leaks and vents have a less negative delta_13C/12C.

The following picture from the Nisbet paper shows how this delta_13C/12C has evolved during the last 18 years:

 

CDIAC gives the series of 4 measurement stations (from North (Barrow) to South) which is consistent with the previous plots:

 

Clearly (in the 3 given regions) there was a general plateau until 2005, followed by a marked decrease. Nisbet et al. conclude that the dominant cause of this decrease was biogenic: greater rainfalls in the tropics increased wetlands, and helped increasing agricultural surfaces and livestock. But the contribution of the latter is estimated more gradual and lower, so that the main cause seems to be a meteorological driven increase in the tropical wetlands.