Numerical Modelling of Methane Emissions From Peatlands

Abstract

Peatlands could play a significant role in global warming for two reasons: 1) peatlands release methane (CH4), which is a very potent greenhouse gas estimated to be responsible for a quarter of the temperature rise the world has experienced to date, and 2) peatlands are a natural source of atmospheric greenhouse gases, which means that unlike anthropogenic sources, they cannot be controlled through regulations or sustainable policies. Because emissions from peatlands are inevitable, it is paramount that they are quantified, and accounted for in the climate models used to determine emission reduction targets. Quantifying CH4 releases from peatlands is challenging because peatlands are highly variable ecosystems, and as a result they generate highly variable emissions. CH4 gets released from peatlands by diffusion in the aqueous phase, or by ebullition in the gas phase. Since diffusion is a constant process, it is more easily measured in the field, and therefore most previous emission estimates have been based solely on diffusion. However, it is now believed that ebullitive emissions make up the majority of methane emissions from peatlands. Ebullitive releases are poorly understood and are therefore challenging to predict. In order to better understand peatland CH4 emissions, a numerical model of CH4 flux from the water table in peatlands was developed. The model is based on physical processes and simulates diffusion, ebullition and gas-water mass transfer, and is populated by parameters measured in peat. The model was used to conduct a sensitivity analysis on the effect that different conceptual models of the peat profile have on emissions. It was found that changing the physical parameters of peat, such as its degree of decomposition or heterogeneity, did not affect the total flux from the water table, but did affect the magnitude and distribution of ebullition events. This is an important finding because large release events may bypass consumption above the water table and result in considerably more CH4 reaching the atmosphere than previously anticipated. It was also found that if only CH4 was modelled, and the carbon dioxide that is also present in peat was ignored, the outputs were comparable to reducing the CH4 generation rate by an order of magnitude.