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

过去十年收集的观测证据表明，500万至400万年前，在上新世早期，地球经历了一个温暖、温和的气候，其特征是赤道到极点的温度差异和东西温度差异减小。北极的温度可能比今天高15-20摄氏度，而温暖的厄尔尼诺Niño-like气候主导了热带地区。当大气中的二氧化碳浓度接近350-400ppm时，这种气候状态持续存在，与目前的升高水平相匹配。因此，如果我们要对当代气候变化的未来影响做出有信心的预测，了解过去的气候状态是至关重要的。然而，综合气候模式目前无法再现这种状态，特别是海洋温度的主要空间格局。这些困难的潜在原因之一是这些模型中云物理的不完整表示。事实上，云反馈和云对温室气体浓度上升的响应仍然是气候模式中最不确定和约束最少的要素。因此，该项目的目标是研究云在维持上新世早期气候中的作用，包括云反照率（或反射率）的作用，它决定了系统中向极地的热输送。为了实现这一目标，耶鲁大学的一组研究人员将使用最先进的全球气候模型（社区地球系统模型，CESM）进行系统的数值实验，在该模型中，他们将修改控制热带及其他地区反照率的云特性。研究小组将对上新世早期气候进行敏感性实验和现实模拟，重点研究上新世早期和现在气候之间海洋平均温度模式和梯度的差异。他们还将研究热带平均状态与El Niño-Southern涛动现象（ENSO）的关系。模型模拟将与古海洋学家密切合作进行分析，新的观测数据将被纳入分析，而模拟结果将指导未来的观测研究。提出的古气候数据和模拟结果的综合将增加我们对驱动气候系统长期变化的机制的理解。

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.