Context for Interpreting Equilibrium Climate Sensitivity and Transient Climate Response from the CMIP6 Earth System Models
For the current generation of earth system models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6), the range of equilibrium climate sensitivity (ECS, a hypothetical value of global warming at equilibrium for a doubling of CO2) is 1.8°C-5.6°C, the largest of any generation of models dating to the 1990s. Meanwhile, the range of transient climate response (TCR, the surface temperature warming around the time of CO2 doubling in a 1% per year CO2 increase simulation) for the CMIP6 models of 1.7°C (1.3°C-3.0°C) is only slightly larger than for the CMIP3 and CMIP5 models. We provide a historical context for these numbers and identify causes for their changes over time.
Even though the range of ECS has increased, the average values of ECS have remained about the same (near 3.5°C +/- 0.2°C) for all generations of models. For TCR, starting with the CMIP2 models in the AR3, both the range and average values have remained relatively stable, with the average varying around 2.0°C +/-0.2°C, and the range staying near 1.7°C +/- 0.2°C. The high end of the assessed range of ECS, 4.5°C, has been exceeded by the high end of the multi-model range in every IPCC assessment except for the AR4.
New sophisticated cloud schemes in the CMIP6 models have improved the overall simulation of clouds, though newly included interacting processes, including cloud-aerosol feedbacks, have increased the uncertainty (increased range) in ECS representing the long-term earth system response to increasing greenhouse gases. Meanwhile, TCR, which is more typically related to the multi-decadal response, has not appreciably changed. Additional knowledge from new observations and improved modeling would be desired to quantify the details of how clouds interact with and are affected by aerosols, both natural and anthropogenic.