Earth’s transient climate sensitivity S_tr is the rapid change, plateauing at ~5 yr, in global mean surface temperature GMST per change in forcing (e.g., Held et al., JGR, 2010). S_tr is readily evaluated from time series of total forcing F and temperature anomaly ΔT as the slope of a regression of ΔT vs F, with ΔT from model or observations and F generally modeled based on change in atmospheric composition. Prior estimates of S_tr have varied quite widely, mainly because of uncertainty in aerosol forcing. F is evaluated as total non-aerosol forcing, dominated by positive GHG forcing, plus negative aerosol forcing; large magnitude aerosol forcing results in small F and in turn high S_tr, and vice versa. Forcing time series derived from the Fifth IPCC Assessment Report (AR5, 2013) resulted in best estimate S_tr 0.35 K (W m^-2)^-1; 5% to 95% uncertainty range 0. 27 to 0.55 (Schwartz, JGR, 2018). New time series of total forcing from the (2021) Sixth IPCC Assessment report (AR6) permit similar evaluation of S_tr, Figure 1, as 0.46 (0.36 to 0.50) K (W m^-2)^-1. The increase in best-estimate S_tr is due to increased magnitude of best-estimate aerosol forcing in AR6 vs. AR5. Poor long-term correlation of forcing time series and observed ΔT for the 5% forcing estimate (large negative aerosol forcing added to GHG forcing, yielding low total forcing) suggests that that the corresponding bound on aerosol forcing magnitude may be an over-estimate, with the correlation substantially improved for best estimate and even more so for lowest estimate of aerosol forcing magnitude, thus more consistent with lower values of S_tr. A somewhat higher range of S_tr, 0.42 to 0.75 K (W m^-2)^-1, is obtained using time series of forcings obtained with individual models (Smith et al., ACP, 2020). Figure 1. Time series of total forcing F and as convolved with 5-year decaying exponential F_c (left); correlations of observed temperature anomaly ΔT (GISS) vs F_c (center); slope denotes transient sensitivity S_tr; and time series of ΔT (right; left axis) and F_c (right axis, scaled to ΔT by S_tr). Top row, lower 5% bound on forcing time series; middle row, best estimate forcing; bottom row, 95% bound. Forcing data from draft AR6 report, expected release August 9, 2021, potentially subject to change.