Ace in the hole or a house of cards: Will a DeCK experiment help atmospheric chemistry?

 Over the past few decades the global atmospheric chemistry modelling community has collectively simulated 100000s of model years, producing petabytes of output, using increasingly complex  chemistry and aerosol schemes and higher resolution models. Yet, our understanding of key aspects of global atmospheric composition change has not evolved at the same pace as the tools we use to study it. Answers to key questions remain as uncertain now as they were two decades ago, including the strength of the methane self-feedback and the past and possible future evolution of tropospheric ozone in response to changing emissions and climate. Here, we will review the progress in understanding that has been generated in model intercomparison experiments (MIPs) from the last three IPCC assessment cycles: ACCENT (AR4), ACCMIP (AR5), and CCMI and AerChemMIP (AR6). We conclude that the aims and experimental design in these MIPs can be improved to reduce  the uncertainty in some of the outstanding questions in atmospheric chemistry. To this end  we propose a new set of experiments, specifically targeted at the atmospheric chemistry modelling community, that will go towards resolving outstanding challenges and integrate the wealth and expertise of chemistry transport and chemistry climate models. These experiments emulate the CMIP DeCK experiments and are designed to provide a continuing legacy for the community in understanding model evolution and process understanding. We aim to elicit  feedback and input into the experimental design from the community  with this presentation.