RII Track-4:NSF: Evaluating the Role of Deep Ocean Equilibration in Warmer Climates

这项研究基础设施改善轨道4 EPSCoR研究人员项目将为布朗大学的一名高级研究助理提供奖学金，并为一名研究生提供培训。这项工作将与美国国家大气研究中心（NCAR）的研究人员合作进行。政府间气候变化专门委员会（IPCC 2021）预测，到本世纪末，大气中的二氧化碳分压可能达到940-1490 ppm。因此，全球气温预计将上升1.5-4.5摄氏度。这种二氧化碳分压水平与气候模式估计的变暖幅度之间的关系被称为气候敏感性。然而，模型是建立在对地球气候系统不完全了解的基础上的，而且，在某种程度上，由于我们有限的计算能力无法完全掌握系统的巨大复杂性，模型是建立在对驱动气候反馈的生物地球化学和物理过程的简化基础上的。另一方面，利用深时间档案重建过去的气候变化，尽管在时间和空间上都很稀疏，但却提供了平衡气候对二氧化碳分压升高的反应的基本事实，使气候学家能够对气候模型的性能进行基准测试。在这项拟议的研究中，PI和NCAR的研究人员将合作整合1500万年前的古气候重建。中新世中期)和气候模式模拟。目前的研究表明，中新世中期的温度比今天高~6-10°C，而估计的二氧化碳分差仅高出200-300 ppm。这表明，二氧化碳分压每增加一倍，气候敏感性为升温6-10°C，远高于IPCC 2021年提出的1.5-4.5°C。这项新研究旨在探索这种模型数据差异的可能解释，希望我们从研究过去温暖条件下的气候过程中获得的见解可以提高我们对未来气候变化的认识。对未来全球变暖的预测在很大程度上取决于我们对气候敏感性的认识，而对过去温暖气候的重建不断完善了我们对气候敏感性的认识。中新世中期气候最适期（MMCO, ~17.5-13.9 Ma）就是这样一个时期，它最近激发了古气候模拟的发展，旨在提高我们对估计的二氧化碳分压高于今天的气候动力学的理解。然而，古气候记录对气候模式提出了一些挑战，包括高气候敏感性和平坦的经向温度梯度，这两者都难以在模式中重现。这一建议提出了一个假设，即一个可能被忽视的导致MMCO变暖和模式数据不一致的因素是深海在二氧化碳强迫升高下的平衡响应。与海温不同，中新世深海温度和环流没有受到太多关注。然而，越来越多的证据表明，深海的平衡响应不同于瞬态响应，并显著影响地表变暖的时空格局。随着海温模式和平衡程度的变化而变化的反馈反过来影响平衡气候敏感性。本研究旨在通过(1)与NCAR（主办机构）的Jiang Zhu合作进行长时间的MMCO模拟（6000年）来解决这一差距，以产生深海的平衡响应；(2)将模拟的MMCO海洋与底栖生物δ13C和钕同位素（Nd）等古代用物进行比较，探讨中新世海洋环流模式；(3)研究深海模式的平衡响应是否减少了模式数据在深海温度和海温空间分布上的错配，重点研究了北大西洋变暖异常和MMCO期间平坦的经向温度梯度。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This Research Infrastructure Improvement Track-4 EPSCoR Research Fellows project will provide a fellowship to a Senior Research Associate and training for a graduate student at Brown University. This work would be conducted in collaboration with researchers at the National Center for Atmospheric Research (NCAR). The Intergovernmental Panel on Climate Change (IPCC 2021) predicts that atmospheric pCO2 will likely reach 940-1490 ppm by the end of the century. Consequently, global temperature is expected to increase by 1.5-4.5°C. This relationship between pCO2 levels and the magnitude of warming estimated from climate models is known as climate sensitivity. Models, however, are built upon an incomplete understanding of Earth’s climate system and, to some extent, upon simplifications of biogeochemical- and physical processes that drive climatic feedback due to our limited computational capability to fully grasp the vast complexity of the system. On the other hand, reconstructions of past climate change using deep-time archives, although sparse in time and space, offer ground truths of equilibrium climatic responses to elevated pCO2, allowing climatologists to benchmark the performance of climate models. In this proposed study, the PI and researchers at NCAR will collaborate to integrate paleoclimate reconstructions of 15 million years ago (aka. the Middle Miocene) with climate model simulations. Current studies suggest that the Middle Miocene was ~6-10°C warmer than today while estimated pCO2 was only 200-300 ppm higher. This suggests a climate sensitivity of 6-10°C warming per doubling of pCO2, much higher than the 1.5-4.5°C suggested by IPCC 2021. The new study aims to explore possible explanations for this model-data discrepancy, with the hope that insights we gain from studying climatic processes under past warm conditions can improve our knowledge about future climate changes.Projections of future global warming depend critically on our knowledge of climate sensitivity, which is continually refined by emerging reconstructions of past warm climates. The mid-Miocene Climate Optimum (MMCO, ~17.5-13.9 Ma) is one such period that has recently inspired the development of paleoclimate simulations, aimed at improving our understanding of climatic dynamics when estimated pCO2 was higher than today. Paleoclimate records, however, pose several challenges to climate models, including high climate sensitivities and flat meridional temperature gradients, both of which are difficult to reproduce in models. This proposal puts forth a hypothesis that a possibly overlooked contributing factor to MMCO warming and model-data disagreement is the equilibrium responses of the deep ocean under elevated CO2 forcing. Unlike SST, Miocene deep-ocean temperature and circulation have not received much attention. Yet increasing evidence suggests that the equilibrium responses of the deep ocean differ from transient responses, and significantly affect the spatial and temporal patterns of surface warming. Evolving feedback with changing SST patterns and the degree of equilibration in turn influences equilibrium climate sensitivity. This proposed study seeks to address this gap by (1) performing long-running MMCO simulations (6000 years) in collaboration with Jiang Zhu at NCAR (host institution) to produce equilibrium responses of the deep ocean; (2) comparing simulated MMCO oceans with paleo-proxies including benthic δ13C and Neodymium isotopes (ɛNd) to explore patterns of Miocene ocean circulation; and (3) examining whether modeled equilibrium responses of the deep ocean reduce model-data mismatches in deep-water temperatures as well as the spatial patterns of SST, with focuses on the warming anomalies in the North Atlantic and the flat meridional temperature gradients during the MMCO.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.