| Summary: | Methane (CH4) is a major greenhouse gas (GHG), second only to carbon dioxide (CO2) in the contribution to historical climate forcing. Yet, the level of understanding of how CH4 will influence the future climate remains low because CH4 processes are generally not represented in Earth system models used for future climate projections. The objective of this thesis is to investigate the importance of CH4 for future climate change with a focus on CH4 mitigation as well as wetland CH4 emissions from thawing permafrost soils, and their respective impact on global warming. The thesis includes a description of a new model for wetland CH4 emissions implemented in an Earth system model of intermediate complexity (EMIC) and applications of the EMIC (including a simplified representation of the CH4 cycle) to: (i) investigate the importance of CH4 mitigation to comply with stringent global warming limits, and (ii) project the additional warming due to wetland CH4 emissions from previously frozen carbon following gradual permafrost thaw over the next three centuries. Salient results of this thesis are: (i) immediate cuts in anthropogenic CH4 emissions, alongside CO2 mitigation, are needed to increase the likelihood of limiting global warming to 2°C above pre-industrial levels; (ii) the warming due to wetland CH4 emissions from thawing permafrost soils is projected to be small (<0.05°C) throughout the 21st century independent of the future anthropogenic emission scenario, (iii) the warming due to such permafrost CH4 emissions has the potential to increase substantially beyond the 21st century, reaching 0.09 (0.01-0.24) °C in the year 2300 under a scenario of high anthropogenic emissions. Overall, by incorporating a simplified representation of the CH4 cycle in Earth system model simulations, this thesis suggests that (i) delaying CH4 mitigation to after the year 2040 will constitute a challenge for limiting global warming to 2°C even if anthropogenic CO2 emissions were reduced aggressively, (ii) reducing anthropogenic GHG emissions will allow to limit the warming due to wetland CH4 emissions from thawing permafrost soils to well below 0.1°C over the next three centuries.
|
|---|