Extreme events are occurrences of unusually severe weather or climate conditions that can cause devastating impacts on communities and agricultural and natural ecosystems. Weather-related extreme events are often short-lived and include heat waves, freezes, heavy downpours, tornadoes, tropical cyclones and floods. Climate-related extreme events either persist longer than weather events or emerge from the accumulation of weather or climate events that persist over a longer period of time. Examples include drought resulting from long periods of below-normal precipitation or wildfire outbreaks when a prolonged dry, warm period follows an abnormally wet and productive growing season.
Scientists usually define an extreme event using either of two approaches. The first approach examines the probability or chance of an event of a given magnitude occurring within a certain reference period (e.g., 1961 - 1990); here an extreme event has a low probability of occurring at a given location (< 10%) and is typically of a high intensity. This type of probabilistic approach is applied in extreme event attribution to determine whether global warming is driving changes in the frequency and intensity of extreme events.
The second approach is more widely applied by the climate adaptation community and uses impact-related thresholds to determine if an event is extreme, and to inform adaptive solutions. A common example here is the number of consecutive days over 100˚F, which can be used to quantify heat waves. Of course, impacts associated with a 100˚F threshold will vary according to location – what might be an extreme event in one place (e.g., Burlington, VT) may be within the normal range somewhere else (e.g., Phoenix, AZ). Thus, thresholds are often location-specific.
This portion of the Climate Hubs website expands on the types of extreme events that impact the different climate hub regions. For example, strong tropical cyclones (hurricanes or typhoons) pound Atlantic and Pacific coastal regions and islands with torrential rain, powerful winds and flooding while drought and wildfire regularly impact Western U.S. States.
The resources listed below also provide more details on the definition and monitoring of extreme climate and weather events, extreme event attribution, and extreme weather events in the context of climate change.
Resources:
Herring, S.C., Christidis, N., Hoell. A., Kossin, J.P., Schreck C.J., Stott, P.A. (Eds.), 2018. Explaining Extreme Events of 2016 from a Climate Perspective. Special Supplement to the Bulletin of the American Meteorological Society, Vol. 99, No. 1.
IPCC, 2012: Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation. A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change [C.B. Field, V. Barros, T.F. Stocker, D. Qin, D.J. Dokken, K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen, M. Tignor, and P.M. Midgley (eds.)], Cambridge University Press, Cambridge, UK, and New York, NY, USA, 582 pp.
Lindsey, R., 2016. Extreme event attribution: the climate versus weather blame game, https://www.climate.gov/news-features/understanding-climate/extreme-event-attribution-climate-versus-weather-blame-game, ClimateWatch Magazine, NOAA.
National Academies of Sciences, Engineering, and Medicine, 2016. Attribution of Extreme Weather Events in the Context of Climate Change, Washington, DC: The National Academies Press. doi: 10.17226/21852.
Walsh, J., et al., 2014: Ch. 2: Our Changing Climate. Climate Change Impacts in the United States: The Third National Climate Assessment, [J. M. Melillo, T.C. Richmond, and G. W. Yohe (Eds.)], U.S. Global Change Research Program, 19-67. doi:10.7930/J0KW5CXT.