Earth’s transient climate sensitivity Str 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).
Str 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
Str 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 Str, and vice versa. Forcing time series derived
from the Fifth IPCC Assessment Report (AR5, 2013) resulted in best
estimate Str 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 Str, Figure 1, as 0.46 (0.36 to
0.50) K (W m-2)-1. The increase in
best-estimate Str 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
Str. A somewhat higher range of Str,
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 Fc (left);
correlations of observed temperature anomaly ΔT (GISS) vs
Fc (center); slope denotes transient sensitivity
Str; and time series of ΔT (right; left axis) and
Fc (right axis, scaled to ΔT by Str).
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.