CMIP6 climate models imply high committed warming

摘要

A question often asked by many, and across different strands of society, is what would happen to global warming levels if atmospheric greenhouse gases (GHGs) are held at their current concentrations. Existing research, e.g. as summarised by Meehl et al. (2007), estimates potentially considerable additional warming resulting from stable present-day atmospheric GHG concentrations. We use the Equilibrium Climate Sensitivity (ECS) values of the latest set of Earth System Models (ESMs) in the CMIP6 ensemble, to estimate equilibrium global warming. Radiative forcing characterises the combined effect of human-induced changes to all GHGs and aerosols in a single metric. We find that based on the ECS values and contemporary radiative forcing, additional warming would be substantial. Approximately half of CMIP6 ESMs project an equilibrium global warming greater than two degrees (compared to pre-industrial times) for the year 2019 radiative forcing. This statistic increases to around 80% for the risk of exceeding the 1.5 degrees C threshold. Our analysis is specific to what is sometimes referred to as the "constant composition commitment". For comparison, "net-zero" emissions imply no overall net emissions by humans, which would involve even larger reductions in fossil fuel burning. Achieving net-zero implies that natural land- and ocean-atmosphere exchanges would modulate GHG concentrations and likely including the removal of some atmospheric CO2.