A case for an emissions driven approach for projecting future climate

Humans emit carbon dioxide into the atmosphere by burning fossil fuels, leading to increases in CO2 concentrations and impacts on climate. Climate models used to project future climate as part of the coupled model intercomparison project (CMIP6, CMIP5, etc.) have for the most part been simulating climate in response to specified concentrations of CO2 in the atmosphere, with the concentrations of CO2 typically selected through simulations from simple climate models. However, many climate models are capable of simulating emergent concentrations in response to specified emissions, by including representations of land and ocean carbon cycling. These emergent concentrations frequently differ from those that were specified from simpler models. In this talk I will discuss the importance of moving towards emissions-forced climate simulations as our standard approach for future iterations of CMIP. Policy starts with emissions, not concentrations, and specifying concentrations of CO2 hides real uncertainty in how emissions are translated into concentrations by the Earth system. I will also introduce two key metrics for determining the amount and timing of warming, the transient climate response to emissions and the zero emissions commitment and discuss how they depend on carbon cycling.