North American trends in extreme precipitation

An analysis of extreme precipitation events indicates that there has been a sizable increase in their frequency since the 1920s/1930s in the U. S. There has been no discernible trend in the frequency of the most extreme events in Canada, but the frequency of less extreme events has increased in some parts of Canada, notably in the Arctic. In the U. S., frequencies in the late 1800s/early 1900s were about as high as in the 1980s/1990s. This suggests that natural variability of the climate system could be the cause of the recent increase, although anthropogenic forcing due to increasing greenhouse gas concentrations cannot be discounted as another cause. It is likely that anthropogenic forcing will eventually cause global increases in extreme precipitation, primarily because of probable increases in atmospheric water vapor content and destabilization of the atmosphere. However, the location, timing, and magnitude of local and regional changes remain unknown because of uncertainties about future changes in the frequency/intensity of meteorological systems that cause extreme precipitation.