What is an "extreme event"? Is there evidence that global warming has caused or contributed to any particular extreme event?

January 23, 2014

An extreme event is a time and place in which weather, climate, or environmental conditions—such as temperature, precipitation, drought, or flooding—rank above a threshold value near the upper or lower ends of the range of historical measurements. Though the threshold is subjective, some scientists define extreme events as those that occur in the highest or lowest 5% or 10% of historical measurements.

Human-caused climate change is not the sole cause of any single extreme event. However, changes in the intensity or frequency of extremes may be influenced by human-caused climate change. Heat waves will tend to be a bit hotter—both the daily high and daily low temperatures. And, because a warmer atmosphere holds more water vapor, precipitation events will tend to be heavier (as measured by total rainfall or snowfall). These are just two examples of how extreme events are becoming more extreme.

Boat pushed inland and sitting on high, dry ground

Some of the aftermath of storm surge from Hurricane Florence: a boat pushed inland onto high ground. Photo courtesy the Morehead City National Weather Service Forecast Office.

Establishing causes of a specific extreme event can be difficult and requires case-specific methods. Scientists can assess whether a specific event (e.g., the 2012 U.S. drought, or the storm surge from Superstorm Sandy) has become more or less likely, or stronger or weaker, as a consequence of human-caused climate change. In nineteen recent analyses of twelve extreme events in 2012, scientists found that some events had direct ties to climate change, while others did not. For more details, see Climate.gov's Q&A with Thomas Peterson, lead editor of the report.


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