Increasing Methane Emissions From Natural Land Ecosystems due to Sea-Level Rise

Abstract

Atmospheric methane (CH4) is one of the most important greenhouse gases. However, there is still a large uncertainty in simulating CH4 emissions from terrestrial ecosystems. Different from modeling studies focusing on response of CH4 emissions to various environmental changes in land ecosystems, this study analyzed the response of CH4 emissions to sea-level rise (SLR). To do so, a large-scale surface water routing module was incorporated into an existing CH4 model. This allowed the model to simulate the effect of SLR on river flows and inland water levels. This study focused on these freshwater systems and did not address saltwater intrusion or coastal wetland impacts. Both the annual maximum inundation extent and CH4 emissions at the global level showed a steadily growing trend, with an increase of 1.21 × 105 km2 in extent and an increase of 3.13 Tg CH4/year in CH4 emissions, in a 22-year SLR experiment from 1993 to 2014. Most of new inundation and methane source areas were located near rivers' deltas and along downstream reaches of rivers. The increase in the inundation extent is primarily influenced by precipitation, channel geomorphic characteristics, and topography of riverside area. The increase of CH4 emissions due to the SLR is largely determined by the inundation extent, but other factors such as air temperature and carbon storage also play roles. Although the current SLR-induced increases in the inundation extent and CH4 emissions only accounted for 1.0% and 1.3% of their global totals, these increases contributed 7.0% and 17.3% of the mean annual variability in both, respectively, during the study period. Considering that SLR has a long-term increasing trend, future SLR under a changing climate could play a more important role in global CH4 emissions.