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1. Glacial‐Interglacial Controls on Ocean Circulation and Temperature During the Permo‐Carboniferous

   https://doi.org/10.1029/2022PA004417  

   Macarewich, Sophia I. ; Poulsen, Christopher J. ( October 2022 , Paleoceanography and Paleoclimatology)

   The apex of Earth's penultimate icehouse during the Permo‐Carboniferous coincided with dramatic glacial‐interglacial fluctuations in atmospheric CO₂, sea level, and high‐latitude ice. Global transformations in marine fauna also occurred during this interval, including a rise to peak foraminiferal diversity, suggesting that glacial‐interglacial climate change impacted marine ecosystems. Nevertheless, changes in ocean circulation and temperature over the Permo‐Carboniferous and their influence on marine ecosystem change are largely unknown. Here, we present simulations of glacial and interglacial phases of the latest Carboniferous‐early Permian (∼305‐295 Ma) using the Community Earth System Model version 1.2 to provide estimates of global ocean circulation and temperature during this interval. We characterize general patterns of glacial and interglacial surface ocean currents, temperature, and salinity, and compare them to the documented abundance and distribution of Permo‐Carboniferous marine fauna as well as a preindustrial climate simulation. We then explore how glacial‐interglacial changes in atmospheric CO₂, sea level, and high‐latitude ice extent impact thermohaline circulation. We find that glacial‐interglacial changes in equatorial surface temperatures are consistently ∼3–6°C. Ocean circulation is stronger overall in the glacial simulation, particularly as lower atmospheric CO₂enables deep convection in the Northern Hemisphere. Wind‐driven circulation, heat transport, and upwelling intensity are stronger overall in the Permo‐Carboniferous superocean relative to the preindustrial oceans at the same level of atmospheric CO₂. We also find that CO₂‐induced glacial conditions of the early Permian may have promoted foraminiferal diversity through increased thermal gradients and suppressed riverine input in marine shelf environments.

    

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2. Warm High‐Elevation Mid‐Latitudes During the Miocene Climatic Optimum: Paleosol Clumped Isotope Temperatures From the Northern Rocky Mountains, USA

   https://doi.org/10.1029/2020PA003991  

   Methner, K. ; Mulch, A. ; Fiebig, J. ; Krsnik, E. ; Löffler, N. ; Bajnai, D. ; Chamberlain, C. P. ( June 2021 , Paleoceanography and Paleoclimatology)

   Interrupting a long‐term Cenozoic cooling trend, the Miocene Climatic Optimum (MCO; ca. 17–15 Ma) represents a time interval characterized globally by warmer than present temperatures, lower ice volume, and elevated pCO₂levels. Establishing quantitative Neogene temperature estimates is an important element in the effort to explore the long‐term changes in the carbon cycle and associated climate feedbacks, yet terrestrial temperature records are still sparse. Here, we present a clumped isotope (Δ₄₇) temperature record of the MCO from intermontane basins in the Northern Rocky Mountain (NRM) region. Arikareean (22.7–21.5 Ma) to Barstovian (16.9–14.7 Ma) paleosol carbonates from the Hepburn's Mesa Formation (Montana), supplemented with data from fossil localities in western Idaho. These records yield Δ₄₇‐temperatures ranging from 17°C to 24°C, which are rather warm given the high elevation sites and are further relatively stable (mean of 21 ± 2°C) leading into and during the MCO until ca. 14.7 Ma. At ca. 14.7 Ma, we observe low Δ₄₇‐temperatures (8°C–10°C) concomitantly with elevated Δ₄₇‐temperatures (ca. 22°C). In line with recently suggested climate stability in the NRM region leading into the MCO, our Δ₄₇‐temperature record, combined with carbon isotope (δ¹³C) and reconstructed soil water oxygen isotope (δ¹⁸O_sw) values, indicates rather stable climate and environmental conditions throughout the MCO. Combining available records from inland sites in the western United States (NRM, Mojave region) points to prevailing stable continental climates even during the MCO.

    

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3. Simulating Miocene Warmth: Insights From an Opportunistic Multi‐Model Ensemble (MioMIP1)

   https://doi.org/10.1029/2020PA004054  

   Burls, N. J. ; Bradshaw, C. D. ; De Boer, A. M. ; Herold, N. ; Huber, M. ; Pound, M. ; Donnadieu, Y. ; Farnsworth, A. ; Frigola, A. ; Gasson, E. ; et al ( May 2021 , Paleoceanography and Paleoclimatology)

   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.

    

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4. Miocene Evolution of North Atlantic Sea Surface Temperature

   https://doi.org/10.1029/2019PA003748  

   Super, James_R ; Thomas, Ellen ; Pagani, Mark ; Huber, Matthew ; O'Brien, Charlotte_L ; Hull, Pincelli_M ( May 2020 , Paleoceanography and Paleoclimatology)

   We reconstruct sea surface temperatures (SSTs) at Deep Sea Drilling Project Site 608 (42.836°N, 23.087°), north of the Azores Front, and Ocean Drilling Program Site 982 (57.516°N, 15.866°), under the North Atlantic Current, in order to track Miocene (23.1–5.3 Ma) development of North Atlantic surface waters. Mean annual SSTs from TEX₈₆and U^K′₃₇proxy estimates at both sites were 10–15 °C higher than modern through the Miocene Climatic Optimum (17–14.5 Ma). During the global cooling of the Middle Miocene Climate Transition (~14.5–12.5 Ma), SSTs at midlatitude Site 608 cooled by ~6 °C, whereas high‐latitude Site 982 cooled by only ~2 °C, resulting in an ~4 Myr collapse of the SST gradient between the two sites. This regional pattern is inconsistent with an increased latitudinal surface temperature gradient, as generally associated with global cooling episodes linked to decreasingpCO₂levels. Instead, the pattern is best explained by enhanced ocean heat transport into the high‐latitude North Atlantic superimposed on the global cooling trend, probably due to enhanced Atlantic meridional overturning circulation and/or a stronger North Atlantic Current. During global late Miocene cooling (~8–7 Ma), surface waters cooled by ~6 °C at Site 982 while minimal change occurred at Site 608, reestablishing the North Atlantic SST gradient. The collapse and reemergence of the SST gradient between the middle‐ and high‐latitude North Atlantic suggests that interaction between changes in regional ocean circulation and the global response to changes in greenhouse gas concentration was important in Miocene climate evolution.

    

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5. Southern Ocean drives multidecadal atmospheric CO 2 rise during Heinrich Stadials

   https://doi.org/10.1073/pnas.2319652121  

   Wendt, Kathleen A ; Nehrbass-Ahles, Christoph ; Niezgoda, Kyle ; Noone, David ; Kalk, Michael ; Menviel, Laurie ; Gottschalk, Julia ; Rae, James_W B ; Schmitt, Jochen ; Fischer, Hubertus ; et al ( May 2024 , Proceedings of the National Academy of Sciences)

   The last glacial period was punctuated by cold intervals in the North Atlantic region that culminated in extensive iceberg discharge events. These cold intervals, known as Heinrich Stadials, are associated with abrupt climate shifts worldwide. Here, we present CO₂measurements from the West Antarctic Ice Sheet Divide ice core across Heinrich Stadials 2 to 5 at decadal-scale resolution. Our results reveal multi-decadal-scale jumps in atmospheric CO₂concentrations within each Heinrich Stadial. The largest magnitude of change (14.0 ± 0.8 ppm within 55 ± 10 y) occurred during Heinrich Stadial 4. Abrupt rises in atmospheric CO₂are concurrent with jumps in atmospheric CH₄and abrupt changes in the water isotopologs in multiple Antarctic ice cores, the latter of which suggest rapid warming of both Antarctica and Southern Ocean vapor source regions. The synchroneity of these rapid shifts points to wind-driven upwelling of relatively warm, carbon-rich waters in the Southern Ocean, likely linked to a poleward intensification of the Southern Hemisphere westerly winds. Using an isotope-enabled atmospheric circulation model, we show that observed changes in Antarctic water isotopologs can be explained by abrupt and widespread Southern Ocean warming. Our work presents evidence for a multi-decadal- to century-scale response of the Southern Ocean to changes in atmospheric circulation, demonstrating the potential for dynamic changes in Southern Ocean biogeochemistry and circulation on human timescales. Furthermore, it suggests that anthropogenic CO₂uptake in the Southern Ocean may weaken with poleward strengthening westerlies today and into the future.

    

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