In New York, downpours from Hurricane Henri set rainfall records this weekend. At the same time in Tennessee, an astonishing amount of rain led to flash flooding that killed at least 21 people.
Though separated by nearly 1,000 miles, these two extreme weather incidents are examples of the same phenomenon: human-caused climate change supercharging extreme rainfall events. And these types of extreme rainfall events are likely to become more common, too, as long as the planet continues to get hotter.
The reason is based on a physics principle known as the Clausius-Clapeyron relationship, which relates temperature, pressure and water vapor. The principle shows that warmer air can hold more water vapor – about 7% more water vapor per 1 degree Celsius. More water vapor in the atmosphere means more moisture available to fall as rain, which leads to higher rainfall rates.
On average, the planet has already warmed more than 1 degree Celsius since pre-industrial times, according to a major UN climate change report published earlier this month. Over land areas there has been even more warming, and particularly in the Eastern US, which has led to a noticeable increase in heavy downpours that lead to flash flooding, according to the most recent National Climate Assessment.
This dynamic played out in Tennessee this weekend as more than 17 inches of rain fell in just 24 hours around McEwen and Centerville – a third of the region’s typical annual rainfall – which would set a record in the state of Tennessee once made official.
Krissy Hurley, a meteorologist for the National Weather Service, told CNN that one of the biggest factors contributing to this amount of rain was that the atmosphere had ample moisture to work with.
When meteorologists launched a weather balloon over the weekend to collect data, they found the moisture in the air was at record levels – a “recipe for disaster,” according to Hurley.
The flooding was caused by several storms that developed one after the other over the same area, which took advantage of that moisture and led to extreme rainfall rates.
“We had a stationary boundary set up over western part of middle Tennessee that provided the perfect set up conditions,” said Mark Rose, a meteorologist with the National Weather Service. “A constant training of [storms] over that one area for several hours right along that boundary.”
In pictures: Deadly flooding in Middle Tennessee
Meanwhile, in New York City on Saturday night, residents saw extreme rainfall rates unmatched in the city’s history.
Heavy rainbands associated with the leading edge of then-Hurricane Henri resulted in a torrential downpour in New York City. Between 10 and 11 p.m., 1.94 inches of rain fell in Central Park, setting the all-time record there for the largest amount of rain in a single hour, according to the National Weather Service.
In total, 4.45 inches of rain fell in the city on Saturday night, also setting a record for the date – the old record of 4.19 inches had stood since 1888.
Hurricanes are essentially massive engines of wind and rain that are fueled by warm ocean water and air. Over the last two-plus centuries, human activity – mainly the burning of fossil fuels – has added lots of heat to the oceans and air where hurricanes are spawned.
These extreme rainfall rates are becoming more common because of human-caused global warming, scientists say. According to the UN’s report on climate change, “the frequency and intensity of heavy precipitation events have increased since the 1950s over most land area.”
Deadly flash floods have made headlines around the world this summer, including in Western Europe in July. In that incident, several months-worth of rain fell in hours, turning city streets in Belgium and Germany into torrents of water that pushed cars and debris into homes and businesses and killed hundreds of people.
Heavy rainfall also caused floods in Central China last month that led to over 300 deaths. Zhengzhou, the provincial capital of 12 million people, was one of the hardest hit areas, with entire neighborhoods submerged and passengers trapped in flooded subway cars.
CNN’s Eric Levenson contributed to this report.