Abstract
Abstract. 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.
Highlights
Earth system models (ESMs) represent physical climate system processes and their interactions with biogeochemical processes focusing primarily on the carbon cycle in the context of carbon dioxide (CO2)
Extent which increases by 8 % from 7.5 to 8.1 million km2 from 1850s to the present day, (2) the simulated fire methane emissions decrease from their 1850s value by 20 % from about 34 to 27 Tg CH4 yr−1 for the present day, and (3) the soil methane uptake more than doubles from its 1850s value of about 14 to 29 Tg CH4 yr−1 for the present day
The offline evaluation of natural methane fluxes simulated by the Canadian Land Surface Scheme (CLASS)-Canadian Terrestrial Ecosystem Model (CTEM) modelling framework presented here is the first step in making atmospheric methane concentration a prognostic variable in the family of Canadian earth system models
Summary
Earth system models (ESMs) represent physical climate system processes and their interactions with biogeochemical processes focusing primarily on the carbon cycle in the context of carbon dioxide (CO2). The model years per wall clock day simulated by the atmospheric component of the second generation Canadian Earth System Model (CanESM2; Arora et al, 2011) are reduced by a factor of around 6 when atmospheric chemistry is turned on Despite these two challenges there are ways forward to model [CH4] as a fully prognostic variable and be able to use comprehensive ESMs to ask questions that the climate modelling community has asked so far in the context of CO2. We evaluate the simulated time evolution of the global sums of these fluxes for the 1850–2008 period by using a one-box model of atmospheric CH4 burden This one-box model requires anthropogenic CH4 emissions, emissions from other natural sources that are not modelled in the CLASS-CTEM framework, and a representation of atmospheric sinks.
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