CAREER: Understanding Cloud Feedback and Natural Aerosol Fingerprints to Interpret Past Warm Climate Forcing and Constrain Tropical Climate Sensitivity

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Equilibrium climate sensitivity refers to as the average surface temperature change of the planet in response to a change in external forcing (e.g., the doubling of the atmospheric CO2 concentration) once the climate system has reached a balanced state. Equilibrium climate sensitivity is a central concept in climate science for estimating the future temperature under the scenario of a continuous increase of atmospheric CO2 concentration using climate models, as a large portion of the total climate warming results from other changes in the climate system in response to the radiative forcing due to anthropogenic greenhouse gas emissions, such as clouds, atmospheric moisture, and snow/ice coverage. The research has broader societal impacts as the central goal of this study is to understand the fundamental physical mechanisms that have controlled tropical climate and how it responds to warming. This is of significant practical value in establishing confidence in the ability of climate models to capture tropical feedbacks under future warming. This award will support the careers and training of the PI (an early career female scientist), a graduate student, and a postdoctoral fellow. The PI will work with Geology graduate students through summer research visits and development of a website for students/researchers from across the country to access and analyze paleoclimate model data. She will mentor local undergraduate and high school students through George Mason's Aspiring Scientists Summer Internship Program and present a "weather and climate" module at a George Mason summer camp designed for females underrepresented in fields of science, technology, engineering, and mathematics. Constraining the range of climate sensitivity estimates beyond those established over the last three decades has proven to be one of the grand challenges in climate science. The limited timespan of the instrumental record makes it difficult to tease out natural, decadal-plus variability from cloud feedbacks in response to greenhouse gas induced warming, with signals further complicated by 20th century aerosol forcing. This project will take a new approach towards constraining the nature of cloud feedbacks under warming by examining climate sensitivities in past warm periods of paleo climate records. The current climate models lack ability to adequately reproduce the reduced meridional surface temperature gradients reconstructed for the past three warm climates, namely, the Pliocene, Miocene and Eocene, as constrained by large-scale surface temperature and hydrological cycle reconstructions. The PI will seek answer about the required strength of the climate sensitivity, particularly from tropical cloud radiative feedback, for climate models to simulate these three very different past warm periods. This multi-warm-climate approach will help establish the extent to which robust tropical cloud feedback mechanisms can consistently explain the patterns of warming, and the hydrological cycle response, seen across the 20th century, Pliocene, Miocene and Eocene. The research project will integrate the latest scientific advances within both the climate modeling and paleoclimate reconstruction communities to: 1) address a fundamental gap in our understanding of the tropical feedbacks needed to simulate distinct past warm climates and the extent to which they are consistent across warm periods, and 2) test the robustness of climate mechanisms put forth within the paleoclimate community to aid in the interpretation and design of climate reconstructions.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.