Climate change at the origin of heavy rainfall of Hurricane Maria

Hurricane Maria has rained more rain in Puerto Rico than any storm on the island since 1956, a feat largely due to the effects of global warming caused by humans, according to a new study.

A new study analyzing the history of hurricanes in Puerto Rico revealed that Maria in 2017 had the highest average rainfall of the 129 storms to have hit the island in the last 60 years. According to the authors of the study, a storm of Maria's magnitude is almost five times more likely to form now than in the 1950s, largely due to the effects of human-induced warming.

"What we have discovered is that the magnitude of maximum rainfall recorded by Maria is much more likely in 2017 than at the beginning of the record in 1950," said David Keellings, a geographer at the University of California. Alabama University of Tuscaloosa and senior author of the new study in the AGU Newspaper Geophysical Research Letters.

Previous studies have attributed Harvey Hurricane's record rainfall to climate change, but no one had thoroughly examined Maria's rainfall, which hit Puerto Rico less than a month after that. Harvey devastated Houston and the Gulf Coast. The extreme rainfall recorded during the two storms caused unprecedented floods that placed them among the three hottest hurricanes ever recorded (the second being Hurricane Katrina in 2005).

The new study adds to the growing evidence that human-induced warming is making extreme weather events of this type more and more common, according to the authors.

"Some things that change in the long run are associated with climate change – such as warming of the atmosphere, rising sea surface temperature and increased availability of moisture in the atmosphere. Together, they give Maria a greater likelihood of its magnitude, precipitation, "said Keellings.

Build a story of rain

José Javier Hernández Ayala, a climate scientist at Sonoma State University in California and co-author of the new study, is from Puerto Rico and his family was directly affected by Hurricane Maria. After the storm, Hernández Ayala decided to team up with Keellings to see how unusual Maria was compared to previous storms that had hit the island.

The researchers analyzed rainfall on the 129 hurricanes that hit Puerto Rico in 1956, the oldest year they could count on. They found that Hurricane Maria produced the greatest amount of maximum daily rainfall among these 129 storms: 1,029 millimeters (41 inches) of rain. This places Maria among the 10 wettest cyclones of all time in the United States.

"Maria is more extreme in her rainfall than anything the island has ever seen," Keellings said. "I just did not think it was going to be so much more than anything that has happened in the last 60 years."

Keellings and Hernández Ayala also wanted to know if Maria's extreme rainfall was a result of natural climate variability or longer-term trends, such as man-made warming. To do this, they analyzed the likelihood that an event like Maria would occur in the 1950s as compared to today.

They discovered that an extreme event like Maria was 4.85 times more likely to occur with the 2017 climate than in 1956, and this variation in probability can not be explained by the cycles natural climatic conditions.

At the start of the record-keeping in the 1950s, a storm like Maria was likely to rain as much as 300 years ago. But by 2017, this probability has dropped to about once every 100 years, according to the study.

"Due to anthropogenic climate change, it is now much more likely that we are getting those hurricanes that cause tremendous amounts of precipitation," Keellings said.

The results show that the human influence on hurricane precipitation has already begun to become evident, according to Michael Wehner, a climatologist at the Berkeley National Laboratory, Berkeley, California, who did not participate in the new study. As much of the damage caused by Maria was due to floods caused by extreme rains, it's safe to say that some of this damage was exacerbated by climate change, Wehner said.

"Climate change has led to an increase in extreme rainfall during tropical cyclones," he said. "Not all storms cause major land floods, freshwater floods, but those that do, floods are exacerbated to some extent by climate change."