Understanding the Influence of Climate Change on Temperature Persistence

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Greenhouse gas emissions cause a variety of climatic effects which go beyond increasing global temperature and many of these, such as increases in the frequency of heat waves and extreme rainfall, are well recognized. Work under this award examines a consequence of global warming that has not been widely appreciated: the tendency for short-term surface temperature fluctuations to persist for longer periods of time. The effect has been seen in comparisons between the last two decades of the 20th and 21st centuries in climate change simulations, with increases of up to 50% in the 5-day persistence of surface temperature anomalies. While the signal has not been identified in the observed record to date it is a robust feature of simulations across a large ensemble of model simulations performed with the Community Earth System Model (CESM) and can be reproduced in a variety of simplified configurations of the model. The pattern of the persistence increase is intriguing as it is more prominent over the oceans than over land, although it extends over Australia, eastern China, and western Canada.Work under this award seeks to understand the change in persistence in terms of a conceptual model in which temperature fluctuations arise from random forcing and decay through linear relaxation. In this model an increase in persistence results from an increase in effective heat capacity, reddening of the stochastic forcing, or a decrease in the damping processes that control relaxation. The work follows two streams, one of which examines reanalysis products and simulations from the Climate Model Intercomparison Project (CMIP), particular the multi-model large ensemble archive. The other stream consists of numerical experiments using the Community Atmosphere Model (CAM, the atmospheric component model of CESM). Some experiments involve techniques to disable relevant feedbacks, for instance "cloud locking" and prescribing water vapor in the radiation code. The project takes advantage of the model hierarchy available as part of the CESM code base.The work has societal relevance as its serves to inform our understanding of how climate change influences the persistence of extreme temperature events, and the impacts of such events depends critically on their duration. The research team also conducts outreach to local public schools, providing a series of outreach lectures on climate science to teachers and students. In addition, the project provides support and training to two graduate students.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.