Modeled Microbial Dynamics Explain the Apparent Temperature Sensitivity of Wetland Methane Emissions

Abstract

Methane emissions from natural wetlands tend to increase with temperature and therefore may lead to a positive feedback under future climate change. However, their temperature response includes confounding factors and appears to differ on different time scales. Observed methane emissions depend strongly on temperature on a seasonal basis, but if the annual mean emissions are compared between sites, there is only a small temperature effect. We hypothesize that microbial dynamics are a major driver of the seasonal cycle and that they can explain this apparent discrepancy. We introduce a relatively simple model of methanogenic growth and dormancy into a wetland methane scheme that is used in an Earth system model. We show that this addition is sufficient to reproduce the observed seasonal dynamics of methane emissions in fully saturated wetland sites, at the same time as reproducing the annual mean emissions. We find that a more complex scheme used in recent Earth system models does not add predictive power. The sites used span a range of climatic conditions, with the majority in high latitudes. The difference in apparent temperature sensitivity seasonally versus spatially cannot be recreated by the non-microbial schemes tested. We therefore conclude that microbial dynamics are a strong candidate to be driving the seasonal cycle of wetland methane emissions. We quantify longer-term temperature sensitivity using this scheme and show that it gives approximately a 12% increase in emissions per degree of warming globally. This is in addition to any hydrological changes, which could also impact future methane emissions. Plain Language Summary Wet soils such as bogs, fens, and other wetlands emit methane gas. Methane is a powerful greenhouse gas that adds to climate warming. It is important to understand its net production and also how this might change as the Earth warms. Generally, scientists have found that warmer soils emit more methane. However, there is a discrepancy between comparing warm versus cold sites-where the effect of the temperature difference is relatively small-and comparing warmer and colder seasons of the year, where the effect of temperature is much stronger. Since methane emissions are caused by microbes, we investigated whether their behavior might provide an explanation for this

A new approach to simulate peat accumulation, degradation and stability in a global land surface scheme (JULES vn5.8_accumulate_soil)

“…The initial JULES simulation (JULES, Table 2) is based on the configuration in Chadburn et al (2020), but now has 20 soil layers extending to around 7.9 m, with thicknesses given in Supplementary Table S3. This was originally derived from the JULES-ES configuration (see https://jules.jchmr.org/content/core-configurations), with extra processes added to enhance the simulation, particularly for high latitudes.…”

“…This work has taken a step forwards by enabling the peat soils to react appropriately to long term changes in hydrology and to include some of the key feedbacks that the soils then have on the water flows. We have also recently developed an improved methane emissions scheme (Chadburn et al, 2020).…”

“…which are the volumetric soil moisture at the wilting point and critical point respectively (defined in terms of hydraulic suction) are functions of the other parameters and are recalculated when the other parameters are updated (see e.g. Chadburn et al (2015a)).3 Simulations and evaluation dataWe used a large suite of simulations at 24 sites that have been use for JULES development and evaluation inChadburn et al (2017Chadburn et al ( , 2020;Nakhavali et al (2018); Gao et al, in prep. ; Smith et al, in prep., along with Scotty Creek (Helbig et al, 2016, Pleistocene Park…”

Regional variation in the effectiveness of methane-based and land-based climate mitigation options

“…These model developments not only improve the seasonality of the emissions, but more importantly for this study capture the release of carbon as CH4 from deep soil layers, including thawed permafrost. Further evaluation of the multi-layer scheme can be found in Chadburn et al (2020).…”

Comment on gmd-2021-263

“…. (2021);Miller et al (2008);Miller and Fujii (2010);Chadburn et al (2020) ZackenbergGreenland TundraElberling et al (2008);Chadburn et al (2017) Table2. JULES simulations conducted.…”