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\begin{document}

\title{CO2 increase experiments using the Community Earth System Model (CESM):
Relationship to climate sensitivity and comparison of CESM1 to CESM2}



\author[1]{Julio T. Bacmeister}%
\author[1]{Cecile Hannay}%
\author[2]{Brian Medeiros}%
\author[1]{Andrew Gettelman}%
\author[1]{Richard Neale}%
\author[3]{Hege-Beate Fredriksen}%
\author[2]{William H. Lipscomb}%
\author[1]{Isla Ruth Simpson}%
\author[1]{David Anthony Bailey}%
\author[1]{Marika M Holland}%
\author[1]{Keith Lindsay}%
\author[1]{Bette L Otto-Bliesner}%
\affil[1]{National Center for Atmospheric Research (UCAR)}%
\affil[2]{National Center for Atmospheric Research}%
\affil[3]{Department of Physics and Technology, UiT The Arctic University of Norway}%


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  \date{May 15, 2024}


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\begin{abstract}
We examine the response of the Community Earth System Model versions 1
and 2 (CESM1 and CESM2) to abrupt quadrupling of atmospheric CO\$\_2\$
concentrations (4xCO2) and to 1\% annually increasing CO2 concentrations
(1\%CO2). Different estimates of equilibrium climate sensitivity (ECS)
for CESM1 and CESM2 are presented. All estimates show that the
sensitivity of CESM2 has increased by 1.5K or more over that of CESM1.
At the same time the transient climate response (TCR) of CESM1 and CESM2
derived from 1\%CO2 experiments has not changed significantly - 2.1K in
CESM1 and 2.0K in CESM2. Increased initial forcing as well as stronger
shortwave radiation feedbacks are responsible for the increase in ECS
seen in CESM2. A decomposition of regional radiation feedbacks and their
contribution to global feedbacks shows that the Southern Ocean plays a
key role in the overall behavior of 4xCO2 experiments, accounting for
about 50\% of the total shortwave feedback in both CESM1 and CESM2. The
Southern Ocean is also responsible for around half of the increase in
shortwave feedback between CESM1 and CESM2, with a comparable
contribution arising over tropical ocean. Experiments using a
thermodynamic slab-ocean model (SOM) yield estimates of ECS that are in
remarkable agreement with those from fully-coupled earth system model
(ESM) experiments for the same level of CO2 increase. Finally, we show
that the similarity of TCR in CESM1 and CESM2 masks significant regional
differences in warming that occur in the 1\%CO2 experiments for each
model.%
\end{abstract}%



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