Are recent heavy rain events due to climate change?
Answer: Across the northern hemisphere, a trend for increased frequency of extreme rainfall events has been observed over the past few decades, and climate models project that this trend is likely to continue. (See the Intergovernmental Panel on Climate Change report on extreme events.) Even in regions where total annual rainfall is decreasing, there is a trend for more of the rain that we do get to come packaged in heavy downpours.
In the Northeast U.S., a “heavy downpour” is often defined as more than 2 inches in 48 hours. This trend is associated with climate change because warming of the earth increases evaporation and transpiration by vegetation, and thus the water vapor concentration in the atmosphere tends to be higher.
With more water vapor in the air, when there is an upper atmosphere cooling event that causes condensation (i.e., rain) there is potential for more to come down. The Northeast U.S. is one of the most substantially hit by this phenomenon, with a 74% increase in the frequency of high rainfall events in the past few decades. (See figure right and the Northeast chapter of the National Climate Assessment.)
Category: Climate Change Q&A