Attribution of Extreme Weather Events in the Context of Climate Change

The observed frequency, intensity, and duration of some extreme weather events have been changing as the climate system has warmed. Such changes in extreme weather events also have been simulated in climate models, and some of the reasons for them are well understood. For example, warming is expected to increase the likelihood of extremely hot days and nights (Figure S.1). Warming also is expected to lead to more evaporation that may exacerbate droughts and increased atmospheric moisture that can increase the frequency of heavy rainfall and snowfall events. The extent to which climate change influences an individual weather or climate event is more difficult to determine. It involves consideration of a host of possible natural and anthropogenic factors (e.g., large-scale circulation, internal modes of climate variability, anthropogenic climate change, aerosol effects) that combine to produce the specific conditions of an event. By definition, extreme events are rare, meaning that typically there are only a few examples of past events at any given location. Nonetheless, this relatively new area of science—often called event attribution—is rapidly advancing. The advances have come about for two main reasons: one, the understanding of the climate and weather mechanisms that produce extreme events is improving, and two, rapid progress is being made in the methods that are used for event attribution. This emerging area of science also has drawn the interest of the public because of the frequently devastating impacts of the events that are studied. This is reflected in the strong media interest in the connection between climate change and extreme events, and it occurs in part because of the potential value of attribution for informing choices about assessing and managing risk and in guiding climate adaptation strategies. For example, in the wake of a devastating event, communities may need to make a decision about whether to rebuild or to relocate. Such a decision could hinge on whether the occurrence of an event is expected to become more likely or severe in the future—and, if so, by how much. The ultimate challenge for the science of event attribution is to estimate how much climate change has affected an individual event’s magnitude¹ or probability² of occurrence. While some studies now attempt to do this, most consider classes of events that are similar to the event that has been observed. Irrespective of whether a specific event or a class of events is studied, results remain subject to substantial uncertainty, with greater levels of uncertainty for events that are not directly temperature related. The conclusions drawn also depend, in general, on choices made when selecting the events, framing the questions asked about the role of climate change, designing the modeling setup, and selecting statistical tools to quantify uncertainty. More and more event attribution studies are being published every year, and study results are increasingly requested very quickly after events occur. Some of the study methods are still relatively novel, however, and there are a range of views about how to conduct and interpret the analyses. This report examines the science of attribution of specific extreme weather events to human-caused climate change and natural variability³ by reviewing current understanding and capabilities. It assesses the robustness of the methods for different classes of events and attribution approaches, provides guidance for interpreting analyses, and identifies priority research needs (the full statement of task can be found in Appendix A). This study is sponsored by the David and Lucile Packard Foundation, the Heising-Simons Foundation, the Litterman Family Foundation, the National Aeronautics and Space Administration (NASA), the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Department of Energy (DOE), with additional support from the Arthur L. Day Fund of the National Academy of Sciences.