AR6 updates to RF by GHGs and aerosols lowers the probability of accomplishing the Paris Agreement compared to AR5 formulations

Abstract. We provide a reduced complexity climate model (RCM) evaluation of how the IPCC WG1 Sixth Assessment Report (AR6) updates to the time series of the future atmospheric concentrations of greenhouse gases (GHGs), the effective radiative forcing (ERF) of GHGs, and the ERF of tropospheric aerosols (ERFAER) affect attributable anthropogenic warming rate, climate sensitivity, and the likelihood of achieving either the target (1.5 °C) or upper limit (2 °C) global warming thresholds of the Paris Agreement (PA). This evaluation is conducted for four selected Shared Socioeconomic Pathway (SSP) scenarios: SSP1–1.9, SSP1–2.6, SSP4–3.4, and SSP2–4.5. Throughout, we compare and contrast these AR6 updates to the state of knowledge that existed prior to the publication of AR6, and provide data-driven probabilistic model simulations based on an evaluation of the impact in the uncertainty of ERFAER and climate feedback. Our most important findings are that the modeled rate of human-induced warming between 1975 and 2014 is 0.18 (0.13 to 0.21) °C per decade within the AR6 framework (range reflects the 5th and 95th percentiles), which is considerably lower than values found by many Earth system models (ESMs) that participated in Phase 6 of the Coupled Model Intercomparison Project (CMIP6). Effective climate sensitivity (EffCS) inferred from the historical global mean surface temperature (GMST) record was found to be 2.29 (1.54 to 3.11) °C using the ERF datasets from AR6 as model inputs. Upon adoption of the AR6 best estimate for the pattern effect (that is, 0.5 Wm-2°C-1), we find values for equilibrium climate sensitivity (ECS) of 3.24 (1.92 to 5.15) °C, which is quite similar to the AR6 assessment of 3.0 (2.0 to 5.0) °C for ECS. The hallmark of our RCM is the ability to conduct large (here, 160 000 member) ensemble forecasts of global warming. These calculations show that AR6 updates to the ERF of GHGs and aerosols result in a considerable decline in the likelihood of limiting warming to either 1.5 or 2 °C of the PA, compared to prior knowledge, for the same future emissions scenarios of GHGs. The likelihood of limiting global warming to 2.0 °C by the end of the century is found to be 100 %, 85 %, 40 %, and 8 % for the SSP1–1.9, SSP1–2.6, SSP4–3.4, and SSP2–4.5 scenarios, respectively, based on the AR6 ERF datasets. Similarly, the ensembles run using the AR6 updates yield likelihoods of 70 %, 32 %, 3 %, and 0 % of limiting warming to 1.5 °C by the end of the century for the same four SSPs. For society to have high confidence in achieving at least the upper limit of 2 °C warming of the PA, the radiative forcing of climate due to GHGs must be placed close to the SSP1–2.6 pathway over the coming decades.