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

研究基础设施改进Track-4 EPSCoR研究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，大气中的二氧化碳可能会达到940-1490ppm。因此，全球气温预计将上升1.5-4.5摄氏度。二氧化碳水平与气候模型估计的升温幅度之间的这种关系称为气候敏感性。然而，模型建立在对地球气候系统的不完全理解基础上，在某种程度上，建立在生物地球化学和物理过程的简化之上，由于我们有限的计算能力，我们无法完全掌握该系统的巨大复杂性，这些过程推动了气候反馈。另一方面，使用深度时间档案重建过去的气候变化，虽然在时间和空间上稀少，但提供了平衡的气候对二氧化碳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(又名。中中新世)的气候模型模拟。目前的研究表明，中中新世比今天高出约6-10摄氏度，而估计的二氧化碳含量仅高出200-300ppm。这表明，二氧化碳每增加一倍，气候敏感度将上升6-10摄氏度，远高于IPCC 2021提出的1.5-4.5摄氏度。这项新的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了解。对未来全球变暖的预测关键取决于我们对气候敏感性的了解，而气候敏感性的不断出现通过对过去温暖气候的重建来不断完善。中新世中期(MMCO，~17.5-13.9 Ma)就是一个这样的时期，最近激发了古气候模拟的发展，目的是在估计的二氧化碳浓度比今天更高的情况下，提高我们对气候动力学的理解。然而，古气候记录对气候模型提出了几个挑战，包括高气候敏感度和平坦的经向温度梯度，这两者都很难在模型中再现。这一提议提出了一个假设，即一个可能被忽视的导致MMCO变暖和模型数据不一致的因素是深海在二氧化碳强迫增加下的平衡反应。与SST不同的是，中新世深海温度和环流并没有受到太多关注。然而，越来越多的证据表明，深海的平衡反应与瞬变反应不同，并显著影响表面变暖的空间和时间模式。随着SST模式的变化和平衡程度的变化而演变的反馈反过来又会影响平衡的气候敏感性。这项拟议的研究试图通过以下方式解决这一差距：(1)与蒋朱在NCAR(东道主机构)合作进行长期的MMCO模拟(6000年)，以产生深海的平衡响应；(2)将模拟的MMCO海洋与包括海底δ13C和钕同位素(ɛ-ND)在内的古代物进行比较，以探索中新世海洋环流的模式；以及(3)检验模拟的深海平衡反应是否减少了深海温度和SST空间模式中的模型数据失配，重点是北大西洋变暖异常和MMCO期间平坦的经向温度梯度。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.