Attribution of extreme events in the western US to human activities

Abstract:

A project to investigate the role of human activities on the changing nature of extreme events in the western US began as part of a CLIVAR-sponsored Postdocs Applying Climate Expertise (PACE) project. The climate institution was the Oregon State University and the application partner was the Oregon Department of Land Conservation and Development (DLCD). DLCD was interested in the changes in weather extremes in the Pacific Northwest, specifically extreme rainfall, flooding, and droughts. The project employs very large ensembles of regional model simulations through volunteer computing resources and allows for probabilistic event attribution (PEA), an important climate research technique. The model was found to have good representation of atmospheric rivers, a major source of extreme precipitation in the Pacific Northwest. The model domain also encompasses California and Nevada. One of the studies focused on attribution of extreme heat in relation to vulnerable populations in California’s Central Valley, where heat waves have become progressively more severe due to increasing nighttime temperatures. Specifically, we found that that (1) simulations of the hottest summer days during the 2000s were twice as likely to occur using observed levels of greenhouse gases than in a counterfactual world without major human activities, (2) detrimental impacts of heat on public health-relevant variables, such as the number of days above 40°C, can be quantified and attributed to human activities using PEA, and (3) PEA can serve as a tool for addressing climate justice concerns of populations within developed nations. The research conducted through the PACE program has also provided a framework for a pioneering climate attribution study at the Union of Concerned Scientists (UCS). The UCS project takes advantage of new research that shows that nearly two-thirds of carbon pollution released into the atmosphere, reported as carbon dioxide equivalent with hundred-year global warming potentials, can be traced to carbon extracted from the Earth by a subset of producers of coal, oil, natural gas, and cement manufacturers. The approach for this study is to use superensembles of regional model simulations to explore the specific fractional attribution to emissions traceable to 90 major industrial carbon producers.