Response of methane emissions from wetlands to the Last Glacial Maximum and an idealized Dansgaard–Oeschger climate event: insights from two models of different complexity

Abstract

Abstract.The role of different sources and sinks of CH 4 in changes in atmospheric methane ([CH 4 ]) concentration during the last 100 000 yr is still not fully understood. In particular, the magnitude of the change in wetland CH 4 emissions at the Last Glacial Maximum (LGM) relative to the pre-industrial period (PI), as well as during abrupt climatic warming or Dansgaard-Oeschger (D-O) events of the last glacial period, is largely unconstrained. In the present study, we aim to understand the uncertainties related to the parameterization of the wetland CH 4 emission models relevant to these time periods by using two wetland models of different complexity (SDGVM and ORCHIDEE). These models have been forced by identical climate fields from low-resolution coupled atmosphere-ocean general circulation model (FA-MOUS) simulations of these time periods. Both emission models simulate a large decrease in emissions during LGM in comparison to PI consistent with ice core observations and previous modelling studies. The global reduction is much larger in ORCHIDEE than in SDGVM (respectively −67 and −46 %), and whilst the differences can be partially explained by different model sensitivities to temperature, the major reason for spatial differences between the models is the inclusion of freezing of soil water in ORCHIDEE and the resultant impact on methanogenesis substrate availability in boreal regions. Besides, a sensitivity test performed with ORCHIDEE in which the methanogenesis substrate sensitivity to the precipitations is modified to be more realistic gives a LGM reduction of −36 %. The range of the global LGM decrease is still prone to uncertainty, and here we underline its sensitivity to different process parameterizations. Over the course of an idealized D-O warming, the magnitude of the change in wetland CH 4 emissions simulated by the two models at global scale is very similar at around 15 Tg yr −1 , but this is only around 25 % of the icecore measured changes in [CH 4 ]. The two models do show regional differences in emission sensitivity to climate with much larger magnitudes of northern and southern tropical anomalies in ORCHIDEE. However, the simulated northern and southern tropical anomalies partially compensate each other in both models limiting the net flux change. Future work may need to consider the inclusion of more detailed wetland processes (e.g. linked to permafrost or tropical floodplains), other non-wetland CH 4 sources or different patterns of D-O climate change in order to be able to reconcile emission estimates with the ice-core data for rapid CH 4 events.

Limited response of peatland CH<sub>4</sub> emissions to abrupt Atlantic Ocean circulation changes in glacial climates

“…However, it appears that the dependence of CH 4 emissions on the dynamic position of the active layer is crucial for fully resolving the magnitude of the transient changes in emissions in these simulations. A weak CH 4 response to abrupt AMOC variations has also been found in prior work using SDGVM and ORCHIDEE models forced with FAMOUS climate output (Hopcroft et al, 2011;Ringeval et al, 2013), and in a newer version of LPJ-WHyMe forced with a freshwater scenario under modern climatic conditions using a different model, CSM1.4 . A recent model intercomparison quantified the sensitivities of 10 CH 4 emissions models including LPJ-WHyMe, SDGVM and ORCHIDEE.…”

Methane emissions from floodplains in the Amazon Basin: challenges in developing a process-based model for global applications

“…This pattern could be explained by the common use of the remote-sensing-derived inundation data set of Papa et al (2010) as prescribed or prognostic wetland map in most of the WETCHIMP models. SDGVM is characterized by an extreme imbalance between the contribution of each component (CH 4 flux densities and wetland extent) as compared to other WETCHIMP models Ringeval et al, 2013). LPX wetland extents (Fig.…”

“…Thus, inundation has no effect on vegetation, carbon pools and heterotrophic respiration. Instead, the mean value of the heterotrophic respiration over the entire grid cell is used to compute the CH 4 flux density (see Melton et al, 2013;Ringeval et al, 2013).…”

Present state of global wetland extent and wetland methane modelling: methodology of a model inter-comparison project (WETCHIMP)

“…A fraction, α, of the natural labile carbon pool computed by ORCHIDEE is used to estimate the methanogenesis substrate. The α parameter also encompasses the methanogenesis base rate at the reference temperature (see Ringeval et al, 2013). α has been optimized against three sites then extrapolated on all grid cells sharing the same vegetation type (boreal, temperate or tropical).…”

“…In the WETCHIMP simulations, a Q 10 equal to 3 (close to the mean value in Ringeval et al, 2010) has been chosen for all grid cells. As in Ringeval et al (2013), the reference temperature for methanogenesis is defined as the mean surface temperature computed by ORCHIDEE when forced by the 1960-1991 CRUNCEP climatology.…”