Now we know why Hurricane Harvey's rainfall was so intense


Houston is a city accustomed to flooding. There have been 30 major floods there since the 1940s, with three massive surges in the past three years alone. But when Hurricane Harvey hit the Houston, Texas area in August, it dealt a blow much harder than expected. It lingered on the coast dumping massive quantities of water on the region. The storm ultimately killed 82 people, displaced tens of thousands of households, and caused an estimated $180 billion in damage.

In total, between 24 trillion and 34 trillion gallons of water fell on the Gulf Coast during Harvey, equivalent to the amount of water melted off the West Antarctic Ice Shelf in a single year. According to Adrian Borsa, a geophysicist with The Scripps Research Institute, that's enough to push down the crust of the earth itself by over half an inch in some areas.

32 PHOTOS
Houston, Texas post-Hurricane Harvey
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Houston, Texas post-Hurricane Harvey
Ginger Benfield works to save family photos in the aftermath of tropical storm Harvey in west Houston, Texas, U.S. September 11, 2017. "Memories are the hardest thing, but at least they are in your heart," said Benfield. Benfield's home flooded after controlled releases from Addicks Reservoir and neighboring Barker reservoir. REUTERS/Chris Aluka Berry
Mechanic Sebastian Ramirez pours new oil into a truck that was flooded by tropical storm Harvey in Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. Ramirez has worked on more than 100 flooded vehicles since the storm, but always tells the automobile owners that he can't guarantee how long the vehicle will run if he's able to fix the immediate problem. REUTERS/Chris Aluka Berry
James Giles pauses for a moment as Do'nie Murphy gets a breath of fresh air as they clean out a Mexican restaurant that was completely immersed in water in the aftermath of tropical storm Harvey in Kingwood, Houston, Texas, U.S. September 9, 2017. "We never thought it would come to this point," said Giles. REUTERS/Chris Aluka Berry
From left, Kameron Smith, 4, Darius Smith, 9, and Deandre Green, 10, play with toys that they found in the piles of destroyed property at Crofton Place Apartments in the aftermath of Hurricane Harvey in Houston, Texas, U.S. September 8, 2017. The children's apartment was destroyed by the flood waters. Picture taken September 8, 2017. REUTERS/Chris Aluka Berry
Carlos Crane, 54, of Crane's Service Center, cleans a padlock so he can lock up tools after the shop was completely immersed in water in the aftermath of tropical storm Harvey in Kingwood, Houston, Texas, U.S. September 9, 2017. "Just cleaning and keep going, we take it one day at a time," he said. REUTERS/Chris Aluka Berry
Vainer Fredrick, 26, cleans out a convenience store that was completely immersed in water in the aftermath of tropical storm Harvey in Kingwood, Houston, Texas, U.S. September 9, 2017. "I'm glad I'm working and making good money," said Fredrick. REUTERS/Chris Aluka Berry
Janice Young, 63, waits for FEMA outside of her Crofton Place Apartment, north Houston, in the aftermath of Hurricane Harvey, Texas, U.S. September 8, 2017. "I lost everything I got, I thank God I didn't loose my life," she said. Picture taken September 8, 2017. REUTERS/Chris Aluka Berry
Kalacedtra Smith, eight months pregnant, is joined by her son Kameron Smith, 9, as she rests for a moment after inspecting the water damage in her Crofton Place Apartment in north Houston during the aftermath of Hurricane Harvey, Texas, U.S. September 8, 2017. Picture taken September 8, 2017. REUTERS/Chris Aluka Berry
Jacob Chaisson, 9, and his brother Joseph Chaisson, 10, play with items that they found in the piles of destroyed property at Crofton Place Apartments in the aftermath of Hurricane Harvey in Houston, Texas, U.S. September 8, 2017. Picture taken September 8, 2017. REUTERS/Chris Aluka Berry
A pile of destroyed property surrounds a pillow with the word "Hope" inscribed on it in the aftermath of tropical storm Harvey in west Houston, Texas, U.S. September 11, 2017. This neighborhood flooded after controlled releases from Addicks Reservoir and neighboring Barker reservoir. REUTERS/Chris Aluka Berry
Isom Horace, 61, sits on the from porch of his north Houston apartment in the aftermath of Hurricane Harvey in Houston, Texas, U.S. September 8, 2017. Although he still has to pay rent, Horace doesn't know where he will stay the night. He can't stay in his apartment because it is so badly damaged and lined with mold and mildew. "It was like a river was running through the apartment...home sweet home," he said. REUTERS/Chris Aluka Berry
Vera Hsiung cleans off her husband, Elliot Wu's, neck and face as they clean out their home which was flooded with water for twelve days in the aftermath of tropical storm Harvey in west Houston, Texas, U.S. September 11, 2017. "We're really worried about contacting disease from exposure to mold," said Hsiung. Their home flooded after controlled releases from Addicks Reservoir and neighboring Barker reservoir. REUTERS/Chris Aluka Berry
Pamela Shaffer photographs a portrait from her 1984 wedding in the aftermath of tropical storm Harvey in west Houston, Texas, U.S. September 11, 2017. "It's hard to let go of these things, just pack stuff up and hope for the best," said Pamela. Shaffer's home flooded after controlled releases from Addicks Reservoir and neighboring Barker reservoir. REUTERS/Chris Aluka Berry
Savannah Shaffer (L) hugs her mother Pamela Shaffer after finding a pair of boots that weren't damaged by the flooding in the aftermath of tropical storm Harvey in west Houston, Texas, U.S. September 11, 2017. "We celebrate every victory," said Pamela. The Shaffer's home flooded after controlled releases from Addicks Reservoir and neighboring Barker reservoir. REUTERS/Chris Aluka Berry
Jon Shaffer salvages what he can from his home, which was flooded for twelve days, in the aftermath of tropical storm Harvey in west Houston, Texas, U.S. September 11, 2017. Shaffer's home flooded after controlled releases from Addicks Reservoir and neighboring Barker reservoir. REUTERS/Chris Aluka Berry
Francile Lovings, 52, sits on her front porch to avoid the odour of mould and mildew in her home during the aftermath of tropical storm Harvey in Acres Homes, Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. Lovings, who is waiting for assistance from FEMA, is still sleeping in the home although it doesn't have electricity and the mould gets worst everyday. "Im just praying and hoping I can survive until I get out of this situation," said Lovings. REUTERS/Chris Aluka Berry
Cynthia Cochran plays with her granddaughter Elizabeth Thomas, 2, as her husband Edward Stanton sits by the window in their FEMA provided hotel room in the aftermath of tropical storm Harvey in Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. Cochran, who lost all of her belongings in the flood said, "This is just another stumbling block. I don't know how I'm going to step over it, but I'm going to step." REUTERS/Chris Aluka Berry
Joderrica Cornealius, 18, reads to her cousin, Elizabeth Thomas, 2, in a FEMA provided hotel room in the aftermath of tropical storm Harvey in Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. Cornealius' home didn't flood, but she came to the hotel to show support to her family members who lost everything in the flood. REUTERS/Chris Aluka Berry
Dameon Horton, (L) and his father Paul Horton grill ribs outside their FEMA provided hotel room in the aftermath of tropical storm Harvey in Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. Talking about the the flooding, Paul said, "I didn't know what to do, but I couldn't crack under pressure, I got kids, I had to go into survival mode. Texas is strong, for real, we gonna get ourselves together and get back to work." REUTERS/Chris Aluka Berry
Elizabeth Thomas, 2, watches videos on her mother's phone outside their FEMA provided hotel room in the aftermath of tropical storm Harvey in Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. Thomas' family lost all of their belongings in the flood and are living in a hotel room temporarily. REUTERS/Chris Aluka Berry
Quincy Smith, 59, and his wife Francile Lovings, 52, sit outside their home to avoid the odour of mould and mildew in their home during the aftermath of tropical storm Harvey in Acres Homes, Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. The couple is waiting for assistance from FEMA. They are still sleeping in the home although it doesn't have electricity and the mould gets worst everyday. "Im just praying and hoping I can survive until I get out of this situation," said Lovings. REUTERS/Chris Aluka Berry
Mike Taylor, 59, drains water from the gas line in his car in the aftermath of tropical storm Harvey in Acres Homes, Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. "I got it bad, I'm so hot and tired," said Taylor. REUTERS/Chris Aluka Berry
Coby Cochran, plays peek-a-boo with her daughter, Melanie Thomas, 7, outside their FEMA provided hotel room in the aftermath of tropical storm Harvey in Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. "I'm a single mom and it's hard losing everything, but God is going to take care of us no matter what. Just live and love," said Cochran. "Momma I'm happy as long as we're together," said Melanie Thomas. REUTERS/Chris Aluka Berry
Coby Cochran, (L) and her daughter Elizabeth Thomas, 2, receive a visit in their FEMA provided hotel room from family member, Joderrica Cornealius, in the aftermath of tropical storm Harvey in Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. "I'm a single mom and it's hard losing everything, but God is going to take care of us no matter what. Just live and love," said Cochran. REUTERS/Chris Aluka Berry
Quincy Smith, 59, is seen inside his bathroom during the aftermath of tropical storm Harvey in Acres Homes, Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. Smith and his wife are still sleeping in the home although it doesn't have electricity and the mould gets worse everyday. "It's rough trying to live day by day, especially when you don't have any money, we're just trying to make it through until FEMA comes," said Smith. REUTERS/Chris Aluka Berry
Coby Cochran, (L) and her daughters Elizabeth Thomas, 2, and Melanie Thomas, 7, receive a visit in their FEMA provided hotel room from family members in the aftermath of tropical storm Harvey in Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. "I'm a single mom and it's hard losing everything, but God is going to take care of us no matter what. Just live and love," said Cochran. "Momma I'm happy as long as we're together," said Melanie Thomas. REUTERS/Chris Aluka Berry
Evelyn Teague, 88, heads home after Sunday service at True Vine Missionary Baptist Church in the aftermath of tropical storm Harvey in Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. Although no water came into Teague's home parts of her ceiling caved in. "God's taking care of me," said Teague. REUTERS/Chris Aluka Berry
Damaged property is piled up on the streets as mechanic, Sebastian Ramirez, prepares to put new oil into a truck that was flooded by tropical storm Harvey in Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. Ramirez has worked on more than 100 flooded vehicles since the storm, but always tells the automobile owners that he can't guarantee how long the vehicle will run if he's able to fix the immediate problem. REUTERS/Chris Aluka Berry
Paris Thomas, 3, sings along during the Sunday service at True Vine Missionary Baptist Church in the aftermath of tropical storm Harvey in Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. Thomas' family lost all of their belongings in the flood and are living at a hotel with the assistance of FEMA. REUTERS/Chris Aluka Berry
Coby Cochran, tries to figure out how to get her daughter, Melanie Thomas, 7, to school in the morning as they spend time outside their FEMA provided hotel room in the aftermath of tropical storm Harvey in Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. "I'm a single mom and it's hard losing everything, but God is going to take care of us no matter what. Just live and love," said Cochran. "Momma I'm happy as long as we're together," said Melanie Thomas. REUTERS/Chris Aluka Berry TPX IMAGES OF THE DAY
Melanie Thomas, 7, wears a donated dress and shoes at the Sunday service at True Vine Missionary Baptist Church in the aftermath of tropical storm Harvey in Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. Her family lost all of their belongings in the flood and are living at a hotel with the assistance of FEMA. "Momma I'm happy as long as we're together," said Thomas. REUTERS/Chris Aluka Berry
Paris Thomas, 3, thumbs through the Bible as she attends Sunday service at True Vine Missionary Baptist Church with her sister Elizabeth Thomas, 2, and mother, Coby Cochran, in the aftermath of tropical storm Harvey in Houston, Texas, U.S. September 10, 2017. Picture taken September 10, 2017. The family lost all of their belongings in the flood and are living at a hotel with the assistance of FEMA. "I'm a single mom and it's hard losing everything, but God is going to take care of us no matter what. Just live and love," said Cochran. REUTERS/Chris Aluka Berry
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This week, results from two new studies on Harvey's rainfall were announced at the American Geophysical Union annual meeting in New Orleans. They back up early predictions that human-caused climate change increased the intensity of the precipitation.

In the first study, published in Geophysical Research Letters, authors looked at the hurricane's total rainfall and calculated the chances of that volume of rain falling under present climate conditions. Then, they ran their calculations again, but this time looked at the likelihood of that same storm occurring during the 1950s, when there were lower levels of greenhouse gases in the environment.

They thought the increase in rainfall attributable to climate change might be somewhere around six percent. Instead, their analysis suggests that global warming increased the precipitation by at least 19 percent, with around 38 percent being the more likely figure. They also found that anthropogenic climate change likely increased the chances of the observed rainfall by a factor of at least 3.5, with a best estimate of about 10. That means that Harvey’s rainfall was 10 times more likely to happen thanks to anthropogenic climate change.

But even without human greenhouse gas emissions, Harvey would still a massive, rare, and extreme event.

“Harvey was an essentially unforeseen, classic black swan event,” says Michael Wehner of Lawrence Berkeley National Laboratory in Berkeley, California, a co-author of the Geophysical Research Letters study. Even in 2017, he notes, the chances of that kind of rain dump on Texas are only 1 in 3,000.

But we might have to get more used to such massive events. The Geophysical Research Letters study wasn’t the only one to take a look at Harvey and come to this conclusion.

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Precious items found as floodwaters from Hurricane Harvey recede
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Precious items found as floodwaters from Hurricane Harvey recede
Jon and Pamela Shaffer's 1984 wedding album is seen infested with mold in the aftermath of tropical storm Harvey in west Houston, Texas, U.S. September 11, 2017. "It's hard to let go of these things, just pack stuff up and hope for the best," said Pamela. The Shaffer's home flooded after controlled releases from Addicks Reservoir and neighboring Barker reservoir. REUTERS/Chris Aluka Berry
Jon Shaffer salvages what he can from his home, which was flooded for twelve days, in the aftermath of tropical storm Harvey in west Houston, Texas, U.S. September 11, 2017. Shaffer's home flooded after controlled releases from Addicks Reservoir and neighboring Barker reservoir. REUTERS/Chris Aluka Berry
A volunteer helps clean out the home of Jon and Pamela Shaffer in the aftermath of tropical storm Harvey in west Houston, Texas, U.S. September 11, 2017. The Shaffer's home flooded after controlled releases from Addicks Reservoir and neighboring Barker reservoir. REUTERS/Chris Aluka Berry
Pamela Shaffer photographs a portrait from her 1984 wedding in the aftermath of tropical storm Harvey in west Houston, Texas, U.S. September 11, 2017. "It's hard to let go of these things, just pack stuff up and hope for the best," said Pamela. Shaffer's home flooded after controlled releases from Addicks Reservoir and neighboring Barker reservoir. REUTERS/Chris Aluka Berry
A pile of dolls rest in a chair in Ginger Benfield's backyard in the aftermath of tropical storm Harvey in west Houston, Texas, U.S. September 11, 2017. Benfield's home flooded after controlled releases from Addicks Reservoir and neighboring Barker reservoir. REUTERS/Chris Aluka Berry
Ginger Benfield works to save family photos in the aftermath of tropical storm Harvey in west Houston, Texas, U.S. September 11, 2017. "Memories are the hardest thing, but at least they are in your heart," said Benfield. Benfield's home flooded after controlled releases from Addicks Reservoir and neighboring Barker reservoir. REUTERS/Chris Aluka Berry
Savannah Shaffer (L) hugs her mother Pamela Shaffer after finding a pair of boots that weren't damaged by the flooding in the aftermath of tropical storm Harvey in west Houston, Texas, U.S. September 11, 2017. "We celebrate every victory," said Pamela. The Shaffer's home flooded after controlled releases from Addicks Reservoir and neighboring Barker reservoir. REUTERS/Chris Aluka Berry
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In the study published in Environmental Research Letters, researchers from the Netherlands looked at rainfall intensity levels along the Gulf Coast from the start of record keeping (around 1880), and two global climate change models.

“We were trying to figure out how extreme or rare was this event, and did climate change change the odds of this event?” Karin van der Wiel, of the Royal Netherlands Meteorological Institute (KNMI) and a co-author of the new Environmental Research Letters paper says.

They found that Harvey’s rainfall was about three times more likely to happen now than in pre-industrial times, and that it was about 15 percent more intense than it would have been in the past.

“While in pre-industrial times you might only expect to have one of these events in 2,500 years, in the present day within the same period there would be three,” van der Wiel says.

The GRL and ERL studies are the most recent to attribute Harvey’s intensity to anthropogenic climate change, but another group recently came to very similar conclusions. Back in November, MIT meteorologist Kerry Emanuel published a paper in PNAS that found Harvey’s prolific rainfall was about six times more likely thanks to climate change.

“The public wants to know," Wehner says, "Did climate change affect the fact that my house got flooded?” Though the three papers may have come up with slightly different numbers, he says, they're all within one another's range of uncertainty. And that suggests there really was a human influence on the storm.

Researchers are assembling data from the 2017 season's other devastating storms, including Irma and Maria, which caused immense damages that are still being tallied today. Some researchers fear that if we don't stem our flow of greenhouse gases into the ever-warming atmosphere, these storms could continue to become more frequent and intense.

“These storms are a foretaste of our future—if we don’t change our future.” Emanuel says.

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