Global methane emissions from terrestrial plants

Recent measurements suggest that the terrestrial plant community may be an important source of methane with global contributions between 62 and 236 Tg CH4 y(-1). If true, terrestrial plants could rival wetlands as being the largest global source of methane forcing us to rethink the methane budget. While further measurements are needed to confirm the methane release rates from this source and their dependencies, in this work we use the preliminary measurements to assess the potential impact of the methane release from this source globally. Using novel techniques we extrapolate the initially reported chamber measurements to the global scale and calculate the global methane emissions from the terrestrial plant community to be in the range 20 to 69 Tg CH4 y(-1). The spread in emissions is largely due to the sensitivity of the global flux to the prescribed temperature dependence of the plant emission rate, which is largely unknown. The spread of calculated emissions is in good agreement with the upper limit imposed on the source during the late pre-industrial period, which we estimate to range from 25 to 54 Tg CH4 y(-1) during the years 0 to 1700 A.D. using the published atmospheric delta(CH4)-C-13 record. In addition, if we assume that plant emissions have been constant at the mean value of 45 Tg CH4 y(-1), we find that the methane release from wildfires and biomass burning during the pre-industrial span 0-1000 A.D. must be near 12 Tg CH4 y(-1), which would be in better agreement with previous estimates of the pyrogenic source during this time than a methane budget missing the plant source. We conclude that methane release from the terrestrial plant community as presently understood does not require major innovations to the global methane budget.