P2C2: Understanding Mean Patterns, Gradients, Variability and Mechanisms of Early Pliocene Warmth: The Role of Cloud Albedo

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Observational evidence collected over the past decade indicates that five to four million years ago, in the early Pliocene epoch, the Earth experienced a warm, temperate climate characterized by reduced equator-to-pole and east-west temperature contrasts. Temperatures in the Arctic were perhaps 15-20°C higher than today, whereas warm, El Niño-like conditions dominated the tropics. This climate state persisted while carbon dioxide concentrations in the atmosphere were close to 350-400ppm, matching the present-day elevated levels. Thus, understanding this past climate state is critical if we are to make confident projections for the future impacts of contemporary climate change. However, comprehensive climate models are currently unable to reproduce this state, and especially the prevailing spatial patterns in ocean temperatures. One of the potential causes of these difficulties is an incomplete representation of cloud physics in those models. In fact, cloud feedbacks and cloud response to rising concentration of greenhouse gases remain the most uncertain and least constrained elements of the climate models. Accordingly, the goal of this project is to investigate the role of clouds in maintaining the climate of the early Pliocene, including the role of cloud albedo (or reflectivity) that determines poleward heat transport in the system. To address this goal, a team of researchers from Yale University will conduct systematic numerical experiments using a state-of-the-art global climate model (Community Earth System Model, CESM) in which they will modify cloud properties that control albedo in the tropics and beyond. The research team will perform sensitivity experiments and realistic simulations of early Pliocene climate, focusing on the differences in the oceanic mean temperature patterns and gradients between the early Pliocene and present-day climates. They will also study the relationship of the tropical mean state and the El Niño-Southern Oscillation phenomenon (ENSO). The model simulations will be analyzed in close collaboration with paleoceanographers, and new observational data will be integrated in the analysis as they become available, while the modeling results will guide future observational studies. The proposed syntheses of paleoclimate data and modeling results will add to our understanding of the mechanisms driving long-term changes in the climate system.