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

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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.