Earth's Transient Climate Sensitivity Evaluated From AR6 Estimates of
Total Forcing and Observed Time Series of Global Temperature Change

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.