Collaborative Research: P2C2: Re-assessing Pliocene and Miocene warm climates and identifying the 'missing physics' to explain them

在过去6500万年里，气候的许多长期趋势和转变都与大气二氧化碳浓度的重大变化有关。但并非全部如此。持续数十万年或数百万年的时期，例如中新世和上新世，与二氧化碳变化的相关性不大，至少根据气候模型，这些变化太小，无法产生代用记录中所证明的大规模气候变化。本研究的目的是研究中新世和上新世的气候，重点是了解造成广泛变暖、温度梯度降低和明显高气候敏感性的机制。气候模型以前无法重现这些特征，这引起了人们对其正确预测未来变暖能力的担忧。这项研究是新颖而重要的，因为一个多学科的团队结合了古气候数据、模型和理论方面的专业知识，将产生地理上广泛的、多代理的过去气候记录。这些数据将与新的和先进的建模方法进行比较。这种方法的一个好处是，通过与过去的数据进行比较，气候模型将得到改进，从中吸取的经验教训将最终导致对未来气候变化的改进预测。具体而言，虽然在中新世和上新世气候的认识方面取得了重大进展，但缺乏对其基本要素的解释，例如：经向温度梯度减弱，太平洋纬向温度梯度减弱，极地地区，上升流地区和大陆内部的优先变暖。这项研究的目标是通过开发新的地球化学记录，更好地量化这些神秘的特征，并将这些数据与最先进的气候模型进行比较，从而更好地预测地球系统对温室气体和非温室气体的强迫。为了实现这一目标，该项目将：(1)加强对各种古气候指标记录的海表温度（SST）信号的理解，并生成一套新的TEX86和Uk37古海表温度记录，与现有记录相补充，以提供从中新世中期到上新世海洋表面条件演变的综合观点。(2)利用群落地球系统模式（CESM）对中新世中期和上新世中期气候进行模拟，其中CAM5是一个新的大气分量，比以前的模式对云-气溶胶相互作用进行了更复杂的处理。将进行云微物理特性和参数化的敏感性研究，包括混合相云特性和气溶胶（硫酸盐和粉尘）排放，以限制以前未探索的云微物理、气溶胶和过去气候之间的相互作用。(3)利用CESM的“超参数化”（SP）版本模拟中新世和上新世气候。SP-CESM在每个网格单元中集成了一个云分辨模式，以更明确地表示对流和云，而不是将它们参数化，从而更准确地表示对流和云对大尺度大气环流的影响。除了研究目标之外，该项目还包括教育和参与部分。它将资助STEM学科的两名研究生和一名女性早期职业研究科学家。将开展三项公众参与活动，包括公开讲座、面向广大受众的网站和面向高中生的课外活动。

Many of the long-term trends and transitions in climate over the past 65 million years are correlated with major changes in atmospheric carbon dioxide concentrations. But not all. Periods lasting hundreds of thousands or millions of years, for example during the Miocene and Pliocene, are loosely correlated with CO2 changes that are too small-- at least according to climate models-- to generate the large climate changes evidenced in proxy records. The purpose of this study is to investigate Miocene and Pliocene climate, with an emphasis on understanding the mechanisms responsible for widespread warmth, reduced temperature gradients, and apparently high climate sensitivity. Climate models have been unable previously to reproduce these features, raising concerns regarding their ability to correctly predict future warming. This study is novel and significant because a multi-disciplinary team combining expertise in paleoclimate data, models, and theory will generate geographically widespread, multiproxy past climate records. This data will be compared with new and advanced modeling approaches. A benefit of this approach is that climate models will be improved by this comparison with past data and the lessons learned will lead eventually to improved predictions of future climate change. Specifically, although significant progress has been achieved in understanding of Miocene and Pliocene climates, explanations are lacking for some their basic elements, such as: weakened meridional temperature gradients, weakened zonal temperature gradients across the Pacific, and preferential warming in polar regions, in upwelling regions, and in continental interiors. This study's goals are to better quantify these enigmatic features by developing new geochemical records and compare this data with state-of-the art climate modeling that will enable better prediction of the Earth system to greenhouse gas and non-greenhouse gas forcing. To accomplish this, the project will: (1) Enhance understanding of the signal being recorded by various paleoclimate proxies for sea surface temperature (SST) and generate a suite of new TEX86 and Uk37 paleo-SST records, complemented by existing records, to provide a comprehensive view of evolving surface ocean conditions from the mid Miocene through the Pliocene. (2) Conduct simulations of mid-Miocene and mid-Pliocene climate using the Community Earth System Model (CESM), with a new atmospheric component, CAM5, that includes more sophisticated treatment of cloud-aerosol interactions than previous models. Sensitivity studies to cloud microphysical properties and parameterization, including mixed-phase cloud properties and aerosol (sulfate and dust) emissions will be conducted to constrain previously unexplored interactions between cloud microphysics, aerosols and past climates. (3) Simulate Miocene and Pliocene climates using the "Super-parameterized" (SP) version of the CESM. SP-CESM incorporates a cloud-resolving model in each grid cell to represent convection and clouds more explicitly rather than parameterize them, allowing a more accurate representation of convection and cloud effects on the large-scale atmospheric circulation. In addition to the research objectives, the project has education and engagement components. It will fund two graduate students and a female early career research scientist in STEM disciplines. Three public engagement efforts will be conducted, including public lectures, a website for a broad audience, and an afterschool program for high school students.