The TIPMIP Earth system model experiment protocol: phase 1

We describe a new Earth system model (ESM) experiment protocol, as part of the international Tipping Points Modelling Intercomparison Project (TIPMIP) project. We propose this as a protocol for the Coupled Model Intercomparison Project 7 (CMIP7). The protocol requires ESMs to run in CO2-emission mode, with atmospheric CO2 a predicted variable. Forcing for the protocol consists solely of a constant emission of CO2, based on each model’s transient climate response to cumulative emissions of carbon dioxide (TCRE) value, to give a common global mean warming rate of 2 °C per century. This positive emission experiment is started from the pre-industrial state of a given model. When the ramp-up run first exceeds a specified level of global warming (2 °C and 4 °C) relative to the model’s pre-industrial global mean surface air temperature (GMSAT), CO2 emissions are set to zero and the positive emission run is branched into a zero-emission run. The zero-emission runs continue for 300 years. At 50 years into each zero-emission run, CO2 emissions are set to the negative of the positive emission rate and the model run until GMSAT cools below the original pre-industrial value. Additionally, when the negative emission run started from global warming level (GWL) = 4 °C first drops below GWL = 2 °C, a zero-emission run is branched off this, completing the set of experiments. Using this protocol, we are able to control the rate of global warming and cooling across participating models. TIPMIP experiments will support a range of analyses, including; an assessment of abrupt/rapid Earth system change, the long-term response to zero CO2 emissions, the response to negative CO2 emissions, the efficacy of negative emissions in driving cooling, and the reversibility of Earth system change under a pathway of positive (warming), zero (stabilization), and negative (cooling) CO2 emissions.