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Abstract Early Late Cretaceous (∼90–100 Ma) Sea surface temperatures (SST) records suggest extremely warm Southern Hemisphere high latitudes and a meridional gradient as low as 5°C, attributed to elevated atmospheric CO₂. Climate models have been unable to reproduce such extreme warmth, questioning model performance and/or the validity of SSTs reconstructions. Indeed, the latter partly rely on the measurement of oxygen isotopic composition of marine organisms (δ¹⁸O_c), a proxy that requires knowledge of the δ¹⁸O of past seawater (δ¹⁸O_sw). Here we use the water isotope‐enabled Community Earth System Model (iCESM1.2) to investigate how paleogeography and pCO₂affect δ¹⁸O_swdistribution and our understanding of Cenomanian‐Turonian SSTs. Our simulations suggest that the semi‐isolation of southern South Atlantic‐Indian Ocean resulted in locally very negative δ¹⁸O_swexplaining low δ¹⁸O_cmeasured on planktonic foraminifera. Accounting for this δ¹⁸O_swspatio‐temporal variability increases the estimated meridional temperature gradient by 5°C and narrows the gap between model and proxy‐based reconstructions. 

Simulating the warmth and equability of past hothouse climates has been a challenge since the inception of paleoclimate modeling. The newest generation of Earth system models (ESMs) has shown substantial improvements in the ability to simulate the early Eocene global mean surface temperature (GMST) and equator-to-pole gradient. Results using the Community Earth System Model suggest that parameterizations of atmospheric radiation, convection, and clouds largely determine the Eocene GMST and are responsible for improvements in the new ESMs, but they have less direct influence on the equator-to-pole temperature gradient. ESMs still have difficulty simulating some regional and seasonal temperatures, although improved data reconstructions of chronology, spatial coverage, and seasonal resolution are needed for more robust model assessment. Looking forward, key processes including radiation and clouds need to be benchmarked and improved using more accurate models of limited domain/physics. Earth system processes need to be better explored, leveraging the increasing ESM resolution and complexity.