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 arbitrary, 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.
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.