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

过去6500万年来气候的许多长期趋势和转变与大气二氧化碳浓度的重大变化有关。但不是全部。持续数十万年或数百万年的周期，例如中新世和上新世，与二氧化碳变化的关联很松散，因为二氧化碳变化太小--至少根据气候模型--不能产生代理记录中证明的大规模气候变化。这项研究的目的是调查中新世和上新世的气候，重点是了解造成普遍变暖、温度梯度降低和明显的气候敏感性的机制。气候模型以前一直无法再现这些特征，这引发了人们对它们正确预测未来变暖能力的担忧。这项研究具有新颖性和重要意义，因为一个多学科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，气候模型将通过与过去数据的比较而得到改进，吸取的经验教训最终将导致对未来气候变化的预测得到改善。具体地说，尽管在理解中新世和上新世气候方面取得了重大进展，但对气候的一些基本要素缺乏解释，例如：经向温度梯度减弱，整个太平洋的纬向温度梯度减弱，极地地区、上升流地区和大陆内陆优先变暖。这项研究的目标是通过开发新的地球化学记录来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，后者将能够更好地预测地球系统对温室气体和非温室气体强迫的影响。为实现这一目标，该项目将：(1)加强对各种古气候指标记录的海洋表面温度信号的了解，并生成一套新的TEX86和Uk37古海洋温度记录，并辅之以现有记录，以提供从中新世中期到上新世不断演变的表层海洋状况的综合情况。(2)使用共同体地球系统模式(CESM)对中中新世和中上新世气候进行模拟，其中加入了一个新的大气成分CAM5，它包括了比以前模型更复杂的云-气溶胶相互作用处理。将开展对云微物理特性和参数的敏感性研究，包括混合相云特性和气溶胶(硫酸盐和尘埃)排放，以限制以前未曾探索过的云微物理、气溶胶和过去气候之间的相互作用。(3)使用CESM的“超参数”(SP)版本模拟中新世和上新世的气候。SP-CESM在每个网格单元中加入了一个云分辨模式，以更明确地表示对流和云，而不是将它们参数化，从而可以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后计划。

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.