Apostolos Voulgarakis

The Atmospheric Chemistry and Climate Modeling Intercomparison Project 
(ACCMIP): An overview and what it can tell us about oxidants and methane

The simulations performed for the Climate Model Intercomparison Project 3 
(CMIP3) in support of the IPCC AR4 provided a tremendously useful resource for 
exploring past, present and future climate. However, the radiative forcings imposed 
in the simulations varied from model to model due to differences in emissions of 
short-lived species, physical processes affecting them, and differences in which 
processes and constituents were included at all. The CMIP5 project similarly will 
have little information on aerosols or on gases other than carbon dioxide. Hence 
there is a need for characterization of the short-lived forcings imposed in the CMIP5 
historical and future simulations, and for diagnostics to allow us to understand the 
causes of the differences in constituent concentrations and forcings from model to 
model, and of their changes in historical and future times. ACCMIP is a project that 
attempts to contribute towards these directions. Different ACCMIP analysis groups 
focus on topics such as tropospheric ozone and its forcing, aerosol processing and 
forcing, atmospheric oxidation processes, nitrogen/sulfur deposition, and air 
quality. In this talk, the basic aspects of the ACCMIP project will be outlined, and 
some initial representative results from different areas of analysis will be presented. 
Furthermore, there will be a focus on the analysis of the changes of the hydroxyl 
radical (OH), and its role in the methane budget. OH is the main oxidant in the 
atmosphere, removing a wide range of pollutants, affecting aerosol and ozone 
production, and controlling the atmospheric lifetime of methane, the most 
important greenhouse gas after carbon dioxide. Because of its crucial role, it is 
important to understand how OH has changed from preindustrial times to present 
day, and how it may change in the future in response to different scenarios.