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

Abstract. Many Reduced complexity climate models (RCMs) and Earth System Models (ESMs) use prescribed concentrations or Effective Radiative Forcing (ERF) of Greenhouse Gases (GHGs) and tropospheric aerosols as inputs for projections. Revisions to these datasets, made in Chapter 7 and Annex III of the Sixth IPCC Assessment Report: The Physical Science Basis (AR6, 2021) are vital to ensure the accuracy of climate model forecasts. AR6 provided updates to the formulation of ERF for most GHGs and tropospheric aerosols, relative to values in AR5 (2013). In this work, we provide a comprehensive assessment of how the changes to the ERF datasets impact projections of future warming, using our multiple linear regression energy balance RCM, the Empirical Model of Global Climate (EM-GC). We provide an analysis of the rate of human−induced warming (AAWR) between 1974 and 2014, and Effective Climate Sensitivity (EffCS) from the regression to the observation-based historical climate record with ERF datasets predating the AR6 report (which we term Baseline Framework) and AR6 ERF data (AR6 Framework). Probabilistic projections on future warming that consider the uncertainty in the magnitude of climate feedback and ERF from tropospheric aerosols are provided for four policy-relevant Shared Socioeconomic Pathway (SSP) scenarios. We find AAWR within the AR6 Framework to be 0.18 [0.13 to 0.21 °C decade⁻¹, 5−95 % range], a slight increase to the values of 0.16 [0.12 to 0.20 °C decade⁻¹] within the Baseline Framework. The central estimate of EffCS is found to be nearly identical between the two Frameworks, but a narrower range is found for the AR6 Framework at 2.29 [1.54 to 3.11 °C, 5−95 % range] relative to 2.26 [1.45 to 4.37 °C] within the Baseline Framework. We find Equilibrium Climate Sensitivity (ECS) to be 3.24 [1.92 to 5.15 °C] for the AR6 best estimate of the pattern effect. Our estimates of AAWR, EffCS and ECS are highly consistent with recent studies and observationally constrained CMIP6 model output. Projections of future warming for the AR6 Framework compared to the Baseline Framework show an increase of 0.2 to 0.4 °C in the end-of century median warming for the SSP scenarios studied. This increase corresponds to a significantly lowered possibility of accomplishing the goals of the Paris Agreement (PA). In particular, the SSP2−4.5 scenario, that is widely considered to be consistent with current climate policies, only offers an 8 % chance of accomplishing the PA upper limit of 2.0 °C warming by the end of the century within the AR6 Framework.