Methane emissions in a chemistry-climate model: feedbacks and response

Slides Methane emissions in a chemistry-climate model: feedbacks and response Paul Griffiths, Ines Heimann, Nicola Warwick, Luke Abraham, Alex Archibald, John Pyle

We examine methane emissions in a climate model under present day and future climate conditions. The origin of methane feedbacks are briefly discussed and we explore these feedbacks in a chemistry-climate model. We explore the impact of the treatment of model boundary conditions for methane, and show that, depending on other factors, such as CO emissions, satisfactory agreement may be obtained with two separate CH4 emission datasets. This highlights the difficulty in unambiguous choice of model emissions in a coupled chemistry model with strong feedbacks. The feedbacks in the CH4CO-OH system, and their uncertainties, also play a critical role in the projection of possible futures. Using a series of single-forcing experiments we show that in a future driven by large increases in greenhouse gas forcing, increases in tropospheric temperature drive an increase in water vapour and, hence, [OH]. In the absence of methane emission changes this leads to a significant decrease in methane compared to the year 2000. However, adding a projected increase in anthropogenic methane emissions from the RCP8.5 scenario leads to a large increase in methane abundance. This is further modified by changes to CO and NOx emissions. Clearly future levels of methane are uncertain and depend critically on climate change and on the future emission pathways of methane and ozone precursors.