NSFGEO-NERC: Dynamics of Warm Past and Future Climates

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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.