A new process-based methanogenesis routine in the JSBACH land surface model – a pan-Arctic view of the CO2:CH4 ratio

The Arctic is warming at an increased rate. This will lead to large-scale permafrost thaw, thereby potentially releasing significant amounts of greenhouse gases to the atmosphere through the decomposition of previously frozen organic material in the soils. To accurately gauge the impact of these emissions, it is crucial to know the emission ratio between the two most important greenhouse gases CO2 and CH4. While CO2 is projected to be the dominant gas, it is important to consider the amplified climatic forcing of CH4, which is a stronger greenhouse gas than CO2. Laboratory incubations and in situ studies have shown that the CO2:CH4 ratio is highly variable. Despite this, most land surface models use a pre-set ratio factor to simulate methanogenesis, thus making it impossible to capture the dynamics of the CO2:CH4 ratio from the start, and, in consequence, making prediction of the Arctic carbon budget more uncertain. Methanogenesis is a complicated framework of different microbial processes, most importantly the two main production pathways – acetoclastic and hydrogenotrophic methanogenesis – and fermentation, which produces the substrate for the two former processes. The inclusion of these processes into models has been studied on the small, i.e. lab to site-level, scale but this has rarely been explored on a pan-Arctic scale. Here, we augmented the JSBACH land surface model’s methane routine, which normally uses a predefined CH4 production ratio factor, by including the three aforementioned microbial processes to study the CO2:CH4 ratio on a pan-Arctic scale. We present the new model routine, show how it performs against the base model, and how the results compare to other estimates from the literature. We discuss the uncertainty of the new results and highlight the difficulties in upscaling many of the factors that influence the CO2:CH4 production ratio.