NSFGEO-NERC: Dynamics of Warm Past and Future Climates

化石证据显示，过去的气候比人类历史上任何一次经历的都要温暖得多，而我们对这些气候的了解非常有限。一个特别的挑战是高纬度地区的极端温暖，这一点由高北极地区埃尔斯米尔岛的化石所证明，其中包括短吻鳄，巨龟，蛇，蕨类植物和开花植物，它们不能忍受低于冰点的温度。相比之下，今天埃尔斯米尔岛的冬季气温通常降至零下40摄氏度。Ellesmere化石可以追溯到大约5000万年前的始新世，当时二氧化碳（CO2）浓度较高，可能为百万分之1,000（ppm），而今天约为400 ppm。但是，气候模型与始新世的二氧化碳水平被迫没有成功地再现北极化石记录所指示的冰点以上的最低温度。在上新世，即200万至500万年前的温暖时期，低纬度和高纬度之间的温度差异也急剧减小。上新世之所以令人感兴趣，是因为它的二氧化碳水平与今天大致相同，但全球气温高出2- 3摄氏度，海平面也高出25米。上新世温暖和温度对比小的动力机制尚不清楚，气候模型也没有成功模拟上新世的条件。该奖项下的工作旨在了解在温暖气候中发现的高极地温度和低纬度温度对比的基本机制。 这项工作还研究了这种状态的过渡，特别是从高到低的赤道到极点的温度对比突然过渡的可能性。 该研究特别关注高纬度云反馈和全球海洋热传输在高纬度变暖中的作用。一个云的反馈过程涉及到从今天在北极上空发现的低云到深对流云的转变，这可能发生在变暖用开放的海洋取代海冰时。 深对流云取代低云可能会对地表产生变暖效应，这可能导致海冰进一步减少和对流云进一步增强的循环。 陆地上空的云层也可能在降低寒冷的大陆温度方面发挥作用，因为海洋温度升高会导致向大陆输送的水分增加，从而导致云层阻碍陆地表面的冷却。 关于海洋热输送的研究涉及审查大幅度减少赤道与两极温差所需的热输送量、可能加强热输送的海洋翻转环流的变化以及对高纬度陆地升温最有效的海洋热输送路径。 海洋热输送和云反馈的综合影响也进行了研究。 这项工作主要是通过数值实验进行的，其中大部分是使用Isca，这是一个由PI之一开发的简化和高度可配置的气候模型。随着世界变暖，二氧化碳已经上升到上新世的水平，这项研究具有社会和科学意义。 在过去的三十年里，夏季北极海冰急剧减少，这里考虑的云反馈在当今气候中可能是可能的。 另一个更广泛的影响是Isca模型的开发和传播，该模型可作为广大气候研究人员的研究工具。教育和推广是由PI通过公开讲座和连接波士顿地区的高中学生与夏季研究机会的计划进行。 该项目由美国国家科学基金会地球科学理事会（NSF/GEO）和英国国家环境研究理事会（NERC）通过NSF/GEO-NERC牵头机构协议共同资助。该协议允许美国/英国提交一份联合提案，并由研究者拥有最大预算比例的机构进行同行评审。一旦成功地共同确定了一个奖项，每个机构都会资助与其本国有关的预算和调查人员的比例。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Fossil evidence reveals past climates that were dramatically warmer than any experienced in human history, and our understanding of these climates is quite limited. A particular challenge is the extreme warmth of the high latitudes demonstrated by the fossils of Ellesmere Island in the high Arctic, which include species of alligators, giant tortoises, snakes, ferns, and flowering plants that do not tolerate below-freezing temperatures. By comparison, winter temperatures on Ellesmere Island today commonly drop to -40C. The Ellesmere fossils date from the Eocene, roughly 50 million years ago, at a time when carbon dioxide (CO2) concentration was higher, perhaps 1,000 parts per million (ppm) compared to about 400ppm today. But climate models forced with Eocene levels of CO2 have not successfully reproduced the above-freezing minimum temperatures indicated by the Arctic fossil record. A dramatically reduced temperature contrast between low and high latitudes also occurred during the Pliocene, a warm period from two to five million years ago. The Pliocene is of interest because its CO2 level was about the same as today yet global temperature was 2-3C higher and sea level was considerably higher, perhaps by 25 meters. The dynamical mechanisms responsible for the warmth and small temperature contrast of the Pliocene are not known, and climate models have not succeeded in simulating Pliocene conditions. Work under this award seeks to understand the fundamental mechanisms which give rise to the high polar temperatures and low latitudinal temperature contrasts found in warm climates. The work also examines the transition to such states, in particular the possibility of an abrupt transition from high to low equator-to-pole temperature contrast. The research focuses specifically on the roles of high latitude cloud feedbacks and global ocean heat transport in warming the high latitudes. One cloud feedback process involves a transition from the low clouds found today over the Arctic to deep convective clouds, which could happen when warming replaces sea ice with open ocean. The replacement of low clouds with deep convective clouds would likely have a warming effect on the surface, which could lead to a cycle of further sea ice reduction and further enhancement of convective clouds. Clouds over land could also play a role in reducing cold continental temperatures, as warmer sea temperatures cause increased moisture transport onto the continents, resulting in clouds that block cooling of the land surface. Research on ocean heat transport involves an examination of the amount of heat transport required to substantially reduce the equator-to-pole temperature difference, changes in the ocean overturning circulation that could enhance heat transport, and pathways of ocean heat transport that would be most effective for warming the high latitude landmasses. The combined effect of ocean heat transport and cloud feedbacks is also examined. The work is conducted primarily through numerical experimentation, much of it with Isca, a simplified and highly configurable climate model developed by one of the PIs.The research is of societal as well as scientific interest as the world is warming and CO2 has already risen to the level of the Pliocene. Summer Arctic sea ice has declined dramatically over the last three decades and the cloud feedbacks considered here may be possible in the present-day climate. A further broader impact is the development and dissemination of the Isca model, which could serve as a research tool for a broad community of climate researchers. Education and outreach is conducted by the PIs through public lectures and a program connecting Boston-area high school students with summer research opportunities. In addition, the project supports two graduate students, thereby promoting the future workforce in this research area.This project is jointly funded by the National Science Foundation's Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Agreement. This Agreement allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award, each Agency funds the proportion of the budget and the investigators associated with its own country.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.