Modelling Methane Emissions from Arctic Lakes
Climate change in the Arctic is moving at a greater rate than the rest of the world, and the urgency of characterizing greenhouse gas emissions from water bodies located in high latitudes has become a subject of intensive research during the last two decades. Methane (CH4) emissions from freshwater systems are believed to be the most important source of uncertainty in the global greenhouse gas budget, and their positive feedback has been excluded from earth systems models. This powerful gaseous fuel can be emitted to the atmosphere via diffusion or ebullition, with fieldwork suggesting the latter is the primary mode of release. Previous process-based models have been developed in the past to determine these emissions; however, they often require several inputs and lakes characteristics that are not readily available. Bearing these limitations in mind, this study quantifies ebullition during the ice-free season from four kettle oligotrophic lakes located in Arctic Alaska. By using a one-dimensional bulk mixed-layer thermodynamic lake model, temperature profiles were obtained, and a CH4 subroutine was also included to determine ebullition and diffusion fluxes as well as CH4 concentrations in the water column. Following model validation, meteorological data from a general circulation model was then implemented to predict future CH4 sediment emissions and their resulting atmospheric fluxes. Results were validated against published observed data from the literature; the model calculated the temperature profile of the lakes with root mean square errors of <4°C. Accordingly, CH4 water concentrations were modelled with root mean square errors of <<1µM, in both deep and shallow systems. Ebullition fluxes showed a high interannual variability which further validated previous evidence of the high heterogeneity of this process. Results of increases in atmospheric fluxes were significant; under three Representative Concentration Pathways, emissions from all systems are expected to increase at least 21% over the next 80 years. This study presents a simple formulation with limited constraints to estimate emissions via ebullition; it is expected that this model will be embedded into land surface schemes of global and regional climate models to predict emissions from the Arctic and potentially from the rest of the globe.