Natural methane emissions feedbacks in MAGICC v. 7.6

Literature estimates of natural methane emissions, particularly from wetlands, have a wide range of uncertainty. Meanwhile, few Earth System Models (ESMs) explicitly model wetlands as a potential source of methane. As a result, Simple Climate Models that aim to emulate the behaviour of ESMs have little to constrain their present and future contributions. MAGICC, as of version 7.5.3, fixed natural methane concentrations as constant after the historical period. Two studies that model wetland methane emissions over the 21st century both find a relationship between those emissions and global temperature, though disagree on the extent of this temperature sensitivity. An updated version of MAGICC has been created that uses this evidence to include a linearised representation of the relationship between wetland methane emissions and global temperature. The temperature-sensitivity parameter in this relationship has been parametrised in a way such that its distribution encompasses the uncertainty in both modelling literature and carbon budget studies, reflecting the currently high degree of uncertainty in wetland emissions. Our results show how incorporating a temperature feedback in methane emissions leads to both higher temperature projections for all scenarios used here, and a widening of the uncertainty in global temperature response.