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\begin{document}

\title{Proxy-Model Comparison for the Eocene-Oligocene Transition in Southern
High Latitudes}



\author[1]{Emily Tibbett}%
\author[2]{Natalie J Burls}%
\author[3]{David Hutchinson}%
\author[1]{Sarah J Feakins}%
\affil[1]{University of Southern California}%
\affil[2]{George Mason University}%
\affil[3]{University of New South Wales}%


\vspace{-1em}



  
  \date{May 27, 2024}


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\selectlanguage{english}
\begin{abstract}
The Eocene-Oligocene Transition (EOT) marks the shift from greenhouse to
icehouse conditions at 34 Ma, when a permanent ice sheet developed on
Antarctica. Climate modeling studies have recently assessed the drivers
of the transition globally. Here we revisit those experiments for a
detailed study of the southern high latitudes in comparison to the
growing number of mean annual sea surface temperature (SST) and mean air
temperature (MAT) proxy reconstructions, allowing us to assess
proxy-model temperature agreement and refine estimates for the magnitude
of the~\emph{p}CO\textsubscript{2} forcing of the EOT. We compile and
update published proxy temperature records on and around Antarctica for
the late Eocene (38-34 Ma) and early Oligocene (34-30 Ma). Compiled SST
proxies cool by up to 3\selectlanguage{ngerman}°C and MAT by up to 4°C between the timeslices.
Proxy data were compared to previous climate model simulations
representing pre- and post-EOT, typically forced with a halving
of~\emph{p}CO\textsubscript{2}. We scaled the model outputs to identify
the magnitude of~\emph{p}CO\textsubscript{2} change needed to drive a
commensurate change in temperature to best fit the temperature proxies.
The multi-model ensemble needs a 30 or 33\% decrease
in~\emph{p}CO\textsubscript{2}, to best fit MAT or SST proxies
respectively, a difference of just 3\%. These proxy-model
intercomparisons identify~\emph{p}CO\textsubscript{2} as the primary
forcing of EOT cooling, with a magnitude (-200 or -243 ppmv) approaching
that of the~\emph{p}CO\textsubscript{2} proxies (-150 ppmv). However
individual model estimates span -66 to -375 ppmv, thus proxy-model
uncertainties are dominated by model divergence.%
\end{abstract}\selectlanguage{ngerman}%



\sloppy


\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure1-10-28/Figure1-10-28}
\caption{{Proxy data compiled for Antarctica, the Southern Ocean and the southern
high latitudes spanning the Eocene and Oligocene. a) Map depicting
location of records used in this study. b) Land surface mean air
temperature (MAT) data including BayMBT (Tibbett et al., 2021a, 2022b),
pollen climate reconstructions including NLR (Amoo et al., 2022; Francis
et al., 2008; Macphail \& Truswell, 2004; Passchier et al., 2013; Poole
et al., 2005; Thompson et al., 2022; Truswell \& Macphail, 2009),
S-index (Passchier et al., 2013, 2017), and MBT/CBT (Douglas et al.,
2014). c) Sea surface temperature (SST) data from BAYSPAR calibrated
TEX86 data (Douglas et al., 2014; Hartman et al., 2018; Houben et al.,
2019; Lauretano et al., 2021; Pagani et al., 2011), SSTs from BAYSPLINE
calibrated Uk'37 (Houben et al., 2019; Liu et al., 2009; Pagani et al.,
2011; Plancq et al., 2014), and \selectlanguage{greek}Δ\selectlanguage{english}47 from bivalves (Douglas et al., 2014;
Petersen \& Schrag, 2015). d) Air Temperature anomaly and e) sea surface
temperature anomaly for each Southern Ocean sector with data normalized
by the respective datasets Eocene mean in standard deviation units (Z
scores), combined for each Southern Ocean sector, and then interpolated
to a common interval with a spline fit to the data. Only datasets
covering the late Eocene and early Oligocene were used for the
temperature anomaly plot. f) \selectlanguage{greek}δ\selectlanguage{english}18Obenthic spline (Westerhold et al.,
2020) and pCO2 compiled from \selectlanguage{greek}δ\selectlanguage{english}11B (blue) and alkenone (red) proxies (Rae
et al., 2021).~ ~
{\label{155836}}%
}}
\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/FIgure2-10-28/FIgure2-10-28}
\caption{{Southern hemisphere model MAT for the a) Eocene (4x pCO2), b) Oligocene
(2x pCO2 model runs), and c) the difference across the transition
(2x-4x) showing results for the unscaled multi model ensemble mean. The
circles correspond to proxy mean annual air temperature records while
the dotted areas show the source area used to compare the model
temperature to the proxy record.~ d) The RMSE for pCO2 model runs for
MAT, for individual model mapped output see Figure S1. Red lines are the
RMSE for each model after the pCO2 scaling for RMSE values see Table
S1.~ ~
{\label{518774}}%
}}
\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/FIgure3-10-27/FIgure3-10-27}
\caption{{Southern Ocean sea surface temperatures (SST) for the a) Eocene (4x
pCO2), b) Oligocene (2x pCO2 model runs), and c) the difference across
the transition (2x-4x) showing results for the unscaled multi model
ensemble mean. The circles correspond to proxy mean annual air
temperature records while the dotted areas show the source area used to
compare the model temperature to the proxy record.~ d) Summarizing the
RMSE for pCO2 model runs for SST, for individual model mapped output see
Figure S3. Red lines are the RMSE for each model after the pCO2 scaling
for RMSE values see Table S1.~
{\label{717923}}%
}}
\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure4-10-28/Figure4-10-28}
\caption{{Summary of average RMSE across the model experiments a) Eocene runs for
CO2 using 4x pCO2 (Table 3), ice runs contain no ice with pCO2 of 560
ppmv, Eocene paleogeography runs (Table 3) b) Oligocene runs for pCO2
using 2x pCO2, ice runs containing the model prescribed ice sheet with
pCO2 of 560 ppmv, and Oligocene paleogeography runs c) is the difference
model outputs between the Oligocene and Eocene runs. (Table 3). The x
axis headers correspond as follows: CO2 distinguishes the proxy-model
comparison based on pCO2 changes, ice for the with and without an ice
sheet, and geo for the model runs with paleogeographic changes. MAT and
SST correspond to either air or sea surface temperature comparison Mean
is the orange line with outliers as dots. Dots are outliers with both
from the GFDL CM2.1 model. Blue line is the ensemble mean from the pCO2
scaling.~
{\label{631699}}%
}}
\end{center}
\end{figure}

~\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/figure5-10-28/figure5-10-28}
\caption{{Proxy-model [?]MAT comparison after scaling~ he pCO2 forcing to achieve
best fit to the magnitude of cooling in the proxy MATs, b) Proxy-model
[?]SST comparison after scaling the pCO2 forcing to achieve best fit to
the magnitude of cooling in the proxy SSTs. In the Oligocene run the
catchment is restricted for Prydz Bay and the Ross Sea on the basis of
ice expansion reducing the effective catchment area. The grey area in
the Eocene timeslice identifies the separate source regions used in
Prydz Bay based on the proxy type.~ In the Oligocene run the catchment
is restricted on the basis of ice expansion reducing the effective
catchment area. In the Oligocene-Eocene panel the grey panel represents
the Eocene area used for Prydz Bay while the line for source region
corresponds to proxy sourcing areas for the Oligocene.~ ~
{\label{695436}}%
}}
\end{center}
\end{figure}

\selectlanguage{english}
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