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The Miocene epoch, spanning 23.03–5.33 Ma, was a dynamic climate of sustained, polar amplified warmth. Miocene atmospheric CO2 concentrations 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 CO2 is 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 CO2 concentration (due to a combination of the spread in imposed boundary conditions and climate feedback strengths) equivalent to ∼1.2 times a CO2 doubling. 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. Plain Language Summary: As human activity continues to increase atmospheric carbon dioxide concentrations, scientists turn to warm intervals in Earth's history to develop insight into the behavior of the climate system under elevated carbon dioxide and temperature. One such interval is the Miocene epoch which has become increasingly relevant as reconstructions of Miocene atmospheric CO2 concentrations point to values ranging between current concentrations of ∼400 ppm and those projected for the end of this century under Shared Socioeconomic Pathways 3 and 4. In this study, we evaluate the surface warming patterns simulated by a range of different climate models configured with Miocene paleogeography and CO2 concentrations spanning 200–850 ppm. We also synthesize available Miocene surface temperature reconstructions. The primary factor controlling the amount of global warming seen across the Miocene simulations analyzed is the CO2 concentration that was prescribed within a given simulation. On average, Miocene elements other than CO2, such as Miocene paleogeography and ice sheets, raise global mean temperature by ∼2°C. While some Miocene simulations with high CO2 forcing overlap with the reconstructed global mean surface temperature estimates for their target Miocene interval, they still generally fail to capture the reconstructed pattern of warming. Key Points: A synthesis of Miocene modeling efforts, and surface temperature reconstructions, is presented within a single analysis frameworkMiocene global mean surface temperature estimates span ∼5.3°C–11.5°C higher than preindustrial, only ∼2°C is explained by non–CO2 boundary conditions in climate modelsSome simulations overlap with reconstructed global mean surface temperature estimates but fail to capture the weak temperature gradient [ABSTRACT FROM AUTHOR]

The Miocene epoch (23.03–5.33 Ma) was a time interval of global warmth, relative to today. Continental configurations and mountain topography transitioned toward modern conditions, and many flora and fauna evolved into the same taxa that exist today. Miocene climate was dynamic: long periods of early and late glaciation bracketed a ∼2 Myr greenhouse interval—the Miocene Climatic Optimum (MCO). Floras, faunas, ice sheets, precipitation, pCO2, and ocean and atmospheric circulation mostly (but not ubiquitously) covaried with these large changes in climate. With higher temperatures and moderately higher pCO2 (∼400–600 ppm), the MCO has been suggested as a particularly appropriate analog for future climate scenarios, and for assessing the predictive accuracy of numerical climate models—the same models that are used to simulate future climate. Yet, Miocene conditions have proved difficult to reconcile with models. This implies either missing positive feedbacks in the models, a lack of knowledge of past climate forcings, or the need for re‐interpretation of proxies, which might mitigate the model‐data discrepancy. Our understanding of Miocene climatic, biogeochemical, and oceanic changes on broad spatial and temporal scales is still developing. New records documenting the physical, chemical, and biotic aspects of the Earth system are emerging, and together provide a more comprehensive understanding of this important time interval. Here, we review the state‐of‐the‐art in Miocene climate, ocean circulation, biogeochemical cycling, ice sheet dynamics, and biotic adaptation research as inferred through proxy observations and modeling studies. Plain Language Summary: During the Miocene time period (∼23–5 million years ago), Planet Earth looked similar to today, with some important differences: the climate was generally warmer and highly variable, while atmospheric CO2 was not much higher. Continental‐sized ice sheets were only present on Antarctica, but not in the northern hemisphere. The continents drifted to near their modern‐day positions, and plants and animals evolved into the many (near) modern species. Scientists study the Miocene because present‐day and projected future CO2 levels are in the same range as those reconstructed for the Miocene. Therefore, if we can understand climate changes and their biotic responses from the Miocene past, we are able to better predict current and future global changes. By comparing Miocene climate reconstructions from fossil and chemical data to climate simulations produced by computer models, scientists are able to test their understanding of the Earth system under higher CO2 and warmer conditions than those of today. This helps in constraining future warming scenarios for the coming decades. In this study, we summarize the current understanding of the Miocene world from data and models. We also identify gaps in our understanding that need further research attention in the future. Key Points: Miocene floras, faunas, and paleogeography were similar to today and provide plausible analogs for future climatic warmingThe Miocene saw great dynamism in biotic and climate systems, but the reasons for these shifts are still not well understoodThe pCO2‐temperature‐ice relationships during major Miocene climate oscillations and transitions warrant further research [ABSTRACT FROM AUTHOR]

Across the middle Miocene, Earth's climate underwent a major cooling and expansion of the Antarctic ice sheet. However, the associated response and development of the tropical climate system is not fully understood, in part because this is influenced by both global climate and also low‐latitude tectonic gateways and paleoceanography. Here we use combined δ18O and Mg/Ca of planktic foraminifera to reconstruct the thermal history and changes in hydrology from the Indo‐Pacific region from 16.5 to 11.5 Ma. During the warmth of the early middle Miocene, our records indicate a dynamic ocean‐atmosphere system in the Indo‐Pacific region, with episodes of saltier and warmer tropical surface waters associated with high pCO2 and retreat of the Antarctic ice sheet. We show that across the Middle Miocene Climate Transition (MMCT) surface ocean temperatures in the Indo‐Pacific cooled by ~2°C, synchronous with the advance of the Antarctic ice sheet. The associated cooling in the Southern Ocean appears to have started earlier and was stronger. Further, we show that western Pacific Ocean warmed and eastern tropical Indian Ocean freshened following the MMCT, likely caused by the constriction of the Indonesian Seaway and reduced connectivity between the Pacific and Indian Oceans following Antarctic glaciation. The MMCT therefore represented a key phase in the evolution of the West Pacific Warm Pool and associated tropical climate dynamics. Key Points: Low‐latitude Indo‐Pacific sea surface temperatures cooled synchronous with the advance of the Antarctic ice sheetEastern tropical Indian Ocean freshened following the Middle Miocene Climate TransitionSea level fall and changing paleogeographic conditions constricted the Indonesian Seaway modifying the Tropical Indo‐Pacific Ocean climate [ABSTRACT FROM AUTHOR]

The middle Miocene is an important analogue for potential future warm climates. However, few independent deep ocean temperature records exist, though these are important for climate model validation and estimates of changes in ice volume. Existing records, all based on the foraminiferal Mg/Ca proxy, suggest that bottom water temperatures were 5–8°C warmer than present. In order to improve confidence in these bottom water temperature reconstructions, we generated a new record using carbonate clumped isotopes (Δ47) and compared our results with Mg/Ca‐based estimates for the Indian Ocean at ODP Site 761. Our results indicate temperatures of 11.0 ± 1.7°C during the middle Miocene Climatic Optimum (MCO, 14.7–17 Ma) and 8.1 ± 1.9°C after the middle Miocene Climate Transition (MCT, 13.0–14.7 Ma), values 6 to 9°C warmer than present. Our record also indicates cooling across the MCT of 2.9 ± 2.5°C (uncertainties 95% confidence level). The Mg/Ca record derived from the same samples indicates temperatures well within uncertainty of Δ47. As the two proxies are affected by different non‐thermal biases, the good agreement provides confidence in these reconstructed temperatures. Our Δ47 temperature record implies a ~0.6‰ seawater δ18O change over the MCT, in good agreement with previously published values from other sites. Our data furthermore confirm overall high seawater δ18O values across the middle Miocene, at face value suggesting ice volumes exceeding present‐day despite the warm bottom water temperatures. This finding suggests previously underappreciated additional influences on seawater δ18O and/or a decoupling of ice volume and ocean temperature. Plain Language Summary: In the context of understanding global warming, the middle Miocene (approximately 12 to 18 million years ago) is an important time in Earth's geological history because atmospheric carbon dioxide levels in parts of this time period were comparable to those predicted for the near future. An accurate understanding of the temperature of deep ocean water in the past is an important constraint for climate studies because that information is needed in order to understand important processes such as ocean heat transport and sea level changes. In this study, we compared the only two temperature proxies available for ancient bottom water, the Mg/Ca and carbonate clumped isotope paleothermometers, to help understand their accuracy in estimating temperatures from the middle Miocene period. We found that the two proxies agree well at our study site (Ocean Drilling Program Site 761 in the Eastern Indian Ocean) despite unresolved issues with both proxies, suggesting that the warm temperatures reconstructed are realistic (approximately 11°C between 15 and 17 Ma, and about 8°C between 11.5 and 13 Ma, compared to approximately 2°C today). Key Points: Benthic foraminiferal clumped isotope and Mg/Ca temperature proxies are compared in the context of middle Miocene climate changeResults of the two proxies agree well at ODP Site 761, exhibiting relatively warm bottom water temperatures up to 9°C warmer than todayThe warm temperatures lead to reconstruction of relatively heavy seawater δ18O compared to modern, especially after ~13 Ma [ABSTRACT FROM AUTHOR]

Sea ice and associated feedback mechanisms play an important role for both long- and shortterm climate change. Our ability to predict future sea ice extent, however, hinges on a greater understanding of past sea ice dynamics. Here we investigate sea ice changes in the eastern Bering Sea prior to, across, and after the Mid-Pleistocene transition (MPT). The sea ice record, based on the Arctic sea ice biomarker IP25 and related open water proxies from the International Ocean Discovery Program Site U1343, shows a substantial increase in sea ice extent across the MPT. The occurrence of late-glacial/deglacial sea ice maxima are consistent with sea ice/land ice hysteresis and land−glacier retreat via the temperature−precipitation feedback. We also identify interactions of sea ice with phytoplankton growth and ocean circulation patterns, which have important implications for glacial North Pacific Intermediate Water formation and potentially North Pacific abyssal carbon storage. [ABSTRACT FROM AUTHOR]