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\begin{document}

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



\author[1]{Stephen Schwartz}%
\affil[1]{Brookhaven National Laboratory}%


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  \date{June 16, 2024}


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\begin{abstract}
Earth's transient climate sensitivity S\textsubscript{tr} is the rapid
change, plateauing at \textasciitilde{}5 yr, in global mean surface
temperature GMST per change in forcing (e.g., Held et al., JGR, 2010).
S\textsubscript{tr} is readily evaluated from time series of total
forcing F and temperature anomaly \selectlanguage{greek}Δ\selectlanguage{english}T as the slope of a regression of \selectlanguage{greek}Δ\selectlanguage{english}T
vs F, with \selectlanguage{greek}Δ\selectlanguage{english}T from model or observations and F generally modeled based
on change in atmospheric composition. Prior estimates of
S\textsubscript{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\textsubscript{tr}, and vice versa. Forcing time series derived
from the Fifth IPCC Assessment Report (AR5, 2013) resulted in best
estimate S\textsubscript{tr} 0.35 K (W
m\textsuperscript{-2})\textsuperscript{-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\textsubscript{tr}, Figure 1, as 0.46 (0.36 to
0.50) K (W m\textsuperscript{-2})\textsuperscript{-1}. The increase in
best-estimate S\textsubscript{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 \selectlanguage{greek}Δ\selectlanguage{english}T 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\textsubscript{tr}. A somewhat higher range of S\textsubscript{tr},
0.42 to 0.75 K (W m\textsuperscript{-2})\textsuperscript{-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\textsubscript{c} (left);
correlations of observed temperature anomaly \selectlanguage{greek}Δ\selectlanguage{english}T (GISS) vs
F\textsubscript{c} (center); slope denotes transient sensitivity
S\textsubscript{tr}; and time series of \selectlanguage{greek}Δ\selectlanguage{english}T (right; left axis) and
F\textsubscript{c} (right axis, scaled to \selectlanguage{greek}Δ\selectlanguage{english}T by S\textsubscript{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.%
\end{abstract}%



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