Natural methane emissions from wetlands and fire, and soil uptake of methane,
simulated using the Canadian Land Surface Scheme and Canadian Terrestrial
Ecosystem (CLASS-CTEM) modelling framework, over the historical 1850–2008
period, are assessed by using a one-box model of atmospheric methane burden.
This one-box model also requires anthropogenic emissions and the methane sink
in the atmosphere to simulate the historical evolution of global methane
burden. For this purpose, global anthropogenic methane emissions for the
period 1850–2008 were reconstructed based on the harmonized representative
concentration pathway (RCP) and Emission Database for Global Atmospheric
Research (EDGAR) data sets. The methane sink in the atmosphere is represented
using bias-corrected methane lifetimes from the Canadian Middle Atmosphere
Model (CMAM). The resulting evolution of atmospheric methane concentration
over the historical period compares reasonably well with observation-based
estimates (correlation  =  0.99, root mean square error  =  35 ppb). The
modelled natural emissions are also assessed using an inverse procedure where
the methane lifetimes required to reproduce the observed year-to-year
increase in atmospheric methane burden are calculated based upon the
specified global anthropogenic and modelled natural emissions that we have
used here. These calculated methane lifetimes over the historical period fall
within the uncertainty range of observation-based estimates. The present-day
(2000–2008) values of modelled methane emissions from wetlands (169 Tg CH4 yr−1) and fire (27 Tg CH4 yr−1),
methane uptake by soil (29 Tg CH4 yr−1), and the budget terms associated with overall anthropogenic and
natural emissions are consistent with estimates reported in a recent global
methane budget that is based on top-down approaches constrained by observed
atmospheric methane burden. The modelled wetland emissions increase over the
historical period in response to both increases in precipitation and
in atmospheric CO2 concentration. This increase in wetland emissions
over the historical period yields evolution of the atmospheric methane
concentration that compares better with observation-based values than the
case when wetland emissions are held constant over the historical period.</p