Climate Change Made Hurricane Maria’s Heavy Rains Nearly 5 Times More Likely to Occur

Hurricane Maria dropped more rain on Puerto Rico than any storm to hit the island since 1956—a feat that was made more likely by nearly a factor of five due to human-caused climate change, new research says. Maria smashed into Puerto Rico on September 20, 2017, as a high-end Category 4 storm with 155 mph winds. Maria’s winds, extreme rains, and storm surge did catastrophic damage estimated at $92 billion, and was America’s deadliest disaster of the past fifty years, with the official estimate of the direct-plus-indirect death toll at 2975.

Hurricane Maria’s torrential rains triggered massive flash flooding and landslides, with over 80% of the island receiving at least ten inches of rain in 48 hours. Widespread near-record and record river flooding was observed across the island, and streamflow data from the U.S. Geological Survey showed that 53 out of 65 river gauges in Puerto Rico met or exceeded flood stage. Among these rivers, 30 exceeded major flood stage, while 13 reached or exceeded their all-time record level.

Figure 1. A group of people walk over debris on the Vivi River to get access to the Rio Abajo community in Utuado, Puerto Rico, on October 17, 2017. The improvised pulley system was the only way to obtain supplies for the community, since flooding rains from Hurricane Maria caused the river to wash away the only bridge that gave access to the neighborhood. Image credit: RICARDO ARDUENGO/AFP/Getty Images.

Using statistical techniques, researchers led by David Keellings, a geographer at the University of Alabama in Tuscaloosa, analyzed rainfall from the 129 hurricanes and tropical storms that have struck Puerto Rico since 1956--the earliest year with reliable rainfall records. The study, published on Wednesday in AGU’s journal Geophysical Research Letters (press release here), found that Hurricane Maria produced the largest maximum daily rainfall of those 129 storms: a remarkable 1,029 millimeters (40.51 inches) of rain, equivalent to over one fourth of the average total annual rainfall at the wettest location on the island, El Yunque National Forest. Maria was among the top 10 wettest hurricanes to ever to hit the United States.

Since Maria was the most powerful hurricane to strike Puerto Rico since 1928, it is no shock that it produced the heaviest rain during the study period, but there is more to the story. At the beginning of the observational record in the 1950s, a storm like Maria was likely to drop the peak observed rain of 40.51" once every 300 years. But in 2017, that recurrence interval dropped to about once every 100 years, according to the study.

Figure 2. Total rainfall from Hurricane Maria in Puerto Rico. Image credit: NWS San Juan (@NWSSanJuan).

“What we found was that Maria’s magnitude of peak precipitation is much more likely in the climate of 2017 when it happened versus the beginning of the record in 1950,” said Keellings. “Due to anthropogenic climate change it is now much more likely that we get these hurricanes that drop huge amounts of precipitation.” Their study did not look at Hurricane Maria specifically using a detailed dynamical computer model, and the researchers recommended further studies based on dynamical modeling to confirm all of their findings.

Video 1. This video taken at Yabucoa, Puerto Rico, near Hurricane Maria’s landfall point shows an extraordinary reversal of Maria’s eyewall winds multiple times. The first huge gust happens around 0:50, throwing cars around. The reversal of the winds multiple times could be due to tornado-scale mesovortices embedded in the eyewall, in combination with the funneling effect of the high rise building across the street. Interestingly, a video shot by storm chaser Josh Morgerman of iCyclone just a few miles away shows a flow that is smoother and steadier, without wild directional shifts. He writes: “This makes sense, because I was right at the coast, where the wind was coming off the open ocean with very little friction.” (The weird reversing wind video was shot a couple of miles inland, where there was lots of friction.) Maria made landfall in Yabucoa, Puerto Rico as a strong category 4 hurricane with maximum sustained winds of 155 mph.

Climate change is making extreme hurricane rains more likely

As discussed in detail in our June 2018 post, Extreme Hurricane Rainfall Expected to Increase in a Warmer World, there is a growing body of literature showing that heavy precipitation events of all kinds—including those from tropical cyclones (which include all hurricanes, tropical storms, and tropical depressions)—have already grown more common. In fact, the three highest-volume U.S. precipitation events have occurred in the past three years.

Global warming increases the rate at which ocean water evaporates into the air, and increases the amount of water vapor the atmosphere contains when fully saturated. This result is about 7% more water vapor in saturated air for every 1°C of ocean warming. This increase in atmospheric water vapor can cause a much larger increase in hurricane rainfall than one might surmise, since water vapor retains the extra heat energy required to evaporate the water, and when the water vapor condenses into rain, this “latent heat” is released. The extra heat helps power the hurricane, making it larger and more intense, allowing it to pull in water vapor from an even larger area and thus dump more rain.

The 2015 assessment by eleven hurricane scientists concluded that typical model results show tropical cyclone rainfall increasing by 20% within 100 km of the center by the end of the century. Three papers in the past two years have been published finding that human-caused global warming significantly increased the odds of the heavy rains of Hurricane Harvey in Texas/Louisiana in 2017. Climate change made Hurricane Florence’s most intense rains over North Carolina in 2018 more than 50% greater in magnitude than they would have been otherwise, according to a first-of-its-kind experimental “pre-attribution” study done as the storm was approaching landfall.

Bob Henson contributed to this post.

Related:

Climate Change Made Hurricane Maria’s Heavy Rains Nearly 5 Times More Likely to Occur, our April 16, 2019 post.
Highly Unusual Upward Trends in Rapidly Intensifying Atlantic Hurricanes Blamed on Global Warming, our February 2019 post.
The 3 Highest-Volume U.S. Rainfall Events on Record Have Happened in the Past 3 Years, our December 2018 post.
Dangerous Rapidly Intensifying Landfalling Hurricanes Like Michael and Harvey May Grow More Common, our October 2018 post.
Extreme Hurricane Rainfall Expected to Increase in a Warmer World, our June 2018 post.
Observed Slowdown in Tropical Cyclone Motion May Portend More Harvey-Like Rainstorms, our June 2018 post
Will Global Warming Make Larger Hurricanes?, our April 2018 post.
Will Global Warming Make Hurricane Forecasting More Difficult?, our January 2017 post.
Hurricane Patricia's 215 mph Winds: A Warning Shot Across Our Bow, our 2016 post.
Katrina-Level Storm Surges Have More Than Doubled Due to Global Warming, our 2013 post.
Damaging Katrina-Level Storm Surges are Twice as Likely in Warm Years, our 2012 post.
Big Money for Hurricane Research, our October 2006 post.

Hurricane scientists Kerry Emanuel, Jim Kossin, Michael Mann and Stephan Rahmstorf wrote an excellent May 30, 2018 realclimate.org post, Does global warming make tropical cyclones stronger?