Climate-CH ₄ feedback from wetlands and its interaction with the climate-CO ₂ feedback

The existence of a feedback between climate and methane (CH ₄) emissions from wetlands has previously been hypothesized, but both its sign and amplitude remain unknown. Moreover, this feedback could interact with the climate-CO ₂ cycle feedback, which has not yet been accounted for at the global scale. These interactions relate to (i) the effect of atmospheric CO ₂ on methanogenic substrates by virtue of its fertilizing effect on plant productivity and (ii) the fact that a climate perturbation due to CO ₂ (respectively CH ₄) radiative forcing has an effect on wetland CH ₄ emissions (respectively CO ₂ fluxes at the surface/atmosphere interface). We present a theoretical analysis of these interactions, which makes it possible to express the magnitude of the feedback for CO ₂ and CH ₄ alone, the additional gain due to interactions between these two feedbacks and the effects of these feedbacks on the difference in atmospheric CH ₄ and CO ₂ between 2100 and pre-industrial time (respectively ΔCH ₄ and ΔCO ₂). These gains are expressed as functions of different sensitivity terms, which we estimate based on prior studies and from experiments performed with the global terrestrial vegetation model ORCHIDEE. Despite high uncertainties on the sensitivity of wetland CH ₄ emissions to climate, we found that the absolute value of the gain of the climate-CH ₄ feedback from wetlands is relatively low (<30 % of climate-CO ₂ feedback gain), with either negative or positive sign within the range of estimates. Whereas the interactions between the two feedbacks have low influence on ΔCO ₂, the ΔCH ₄ could increase by 475 to 1400 ppb based on the sign of the C-CH ₄ feedback gain. Our study suggests that it is necessary to better constrain the evolution of wetland area under future climate change as well as the local coupling through methanogenesis substrate of the carbon and CH ₄ cycles - in particular the magnitude of the CO ₂ fertilization effect on the wetland CH ₄ emissions - as these are the dominant sources of uncertainty in our model.