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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. 

Abstract The Miocene (23.03–5.33 Ma) is recognized as a period with close to modern‐day paleogeography, yet a much warmer climate. With large uncertainties in future hydroclimate projections, Miocene conditions illustrate a potential future analog for the Earth system. A recent opportunistic Miocene Model Intercomparison Project 1 (MioMIP1) focused on synthesizing published Miocene climate simulations and comparing them with available temperature reconstructions. Here, we build on this effort by analyzing the hydrological cycle response to Miocene forcings across early‐to‐middle (E2MMIO; 20.03–11.6 Ma) and middle‐to‐late Miocene (M2LMIO; 11.5–5.33 Ma) simulations with CO₂concentrations ranging from 200 to 850 ppm and providing a model‐data comparison against available precipitation reconstructions. We find global precipitation increases by ∼2.1 and 2.3% per degree of warming for E2MMIO and M2LMIO simulations, respectively. Models generally agree on a wetter than modern‐day tropics; mid and high‐latitude, however, do not agree on the sign of subtropical precipitation changes with warming. Global monsoon analysis suggests most monsoon regions, except the North American Monsoon, experience higher precipitation rates under warmer conditions. Model‐data comparison shows that mean annual precipitation is underestimated by the models regardless of CO₂concentration, particularly in the mid‐ to high‐latitudes. This suggests that the models may not be (a) resolving key processes driving the hydrological cycle response to Miocene boundary conditions and/or (b) other boundary conditions or processes not considered here are critical to reproducing Miocene hydroclimate. This study highlights the challenges in modeling and reconstructing the Miocene hydrological cycle and serves as a baseline for future coordinated MioMIP efforts. 

Abstract The Miocene epoch, spanning 23.03–5.33 Ma, was a dynamic climate of sustained, polar amplified warmth. Miocene atmospheric CO₂concentrations are typically reconstructed between 300 and 600 ppm and were potentially higher during the Miocene Climatic Optimum (16.75–14.5 Ma). With surface temperature reconstructions pointing to substantial midlatitude and polar warmth, it is unclear what processes maintained the much weaker‐than‐modern equator‐to‐pole temperature difference. Here, we synthesize several Miocene climate modeling efforts together with available terrestrial and ocean surface temperature reconstructions. We evaluate the range of model‐data agreement, highlight robust mechanisms operating across Miocene modeling efforts and regions where differences across experiments result in a large spread in warming responses. Prescribed CO₂is the primary factor controlling global warming across the ensemble. On average, elements other than CO₂, such as Miocene paleogeography and ice sheets, raise global mean temperature by ∼2°C, with the spread in warming under a given CO₂concentration (due to a combination of the spread in imposed boundary conditions and climate feedback strengths) equivalent to ∼1.2 times a CO₂doubling. This study uses an ensemble of opportunity: models, boundary conditions, and reference data sets represent the state‐of‐art for the Miocene, but are inhomogeneous and not ideal for a formal intermodel comparison effort. Acknowledging this caveat, this study is nevertheless the first Miocene multi‐model, multi‐proxy comparison attempted so far. This study serves to take stock of the current progress toward simulating Miocene warmth while isolating remaining challenges that may be well served by community‐led efforts to coordinate modeling and data activities within a common analytical framework.