Warmer air increases precipitation extremes because it can hold more moisture. According to the Clausius–Clapeyron equation, for each 1°C of warming, saturated air contains 7 percent more water vapor, which may rain out if conditions are right. The average moisture content of the atmosphere has increased by about 4 percent since the 1970s, as expected from the Clausius–Clapeyron law.
Storms are fueled by available heat. Increasing surface land and ocean temperatures are increasing the potential energy available to passing storms.
Storms reach out and gather water vapor over regions that are 10-25 times as large as the precipitation area, thus multiplying the effect of increased atmospheric moisture. As water vapor condenses to form clouds and rain, the conversion releases heat that add buoyancy to the air and further fuels the storm. This increases the gathering of moisture into storm clouds and further intensifies precipitation.
The northern hemisphere is tending toward increasingly warmer and more humid summers, and the global area covered by extreme water vapor is increasing significantly, increasing the limit for extreme precipitation.
Extreme precipitation trends
The number of record-breaking rainfall events globally has significantly increased in recent decades, and the fingerprint of global warming has been documented in this pattern.
… nearly a fifth of precipitation extremes can be attributed to global warming. Not one of these events is solely the direct result of warming, but warming increases their frequency. And the less common and more extreme the...heavy rainfall event, the more this can be attributed to a man-made contribution.
Dr. Erich Fischer, ETH Zürich
About 18 percent of the "moderate" (i.e. 1-in-3 year) daily precipitation extremes over land are attributable to warming, while a much larger share of very extreme events is attributed to warming.
An April 2017 study found that, from 1961 to 2010, global warming increased the likelihood of occurrence of the wettest five-day periods on record in 41 percent of the observed areas of the world.
“It is likely that since about 1950 the number of heavy precipitation events over land has increased in more regions than it has decreased. Confidence is highest for North America and Europe where there have been likely increases in either the frequency or intensity of heavy precipitation with some seasonal and regional variations. It is very likely that there have been trends towards heavier precipitation events in central North America.”
According to the US Third National Climate Assessment, the number and intensity of very heavy precipitation events (defined as the heaviest 1 percent of all daily events) have been increasing significantly across most of the United States. From 1958 to 2012, the amount of precipitation falling in very heavy events increased by 71 percent in the Northeast, 27 percent in the Southeast, 37 percent in the Midwest, 16 percent in the Great Plains, 5 percent in the Southwest, and 12 percent in the Northwest.
The report also states that the heaviest rainfall events have become more frequent across most of the country, and projects that heavy precipitation events that historically occurred once in 20 years will occur as frequently as every 5 to 15 years by late this century.
In the past century, the US has witnessed a 20 percent increase in the amount of precipitation falling in the heaviest downpours.