As Hurricane Harvey continues to bring record-shattering rainfall to the Texas Gulf Coast, Houston’s nearly 2.3 million residents were inundated with more than 20 inches of rain in just one 24 hour period—and the storm is expected to linger for at least three more days, dumping more than 50 inches of rain in some locations, bringing even more misery. So far, five people have died.
The question of whether climate change caused such a staggering storm comes up nearly every time a weather disaster strikes, but scientists say the only way to get an answer is to frame the question properly.
“Over the past few years, we as a community of scientists have been able to say more and more about the role of global warming or climate change on individual weather events,” said Dr. Gabriel Vecchi, a professor of geosciences at Princeton University, acknowledging that the science has not yet drawn the definitive picture of climate change’s role.
Like most scientists, Vecchi explained that climate change increases the likelihood that extreme rainfall will accompany storms like Harvey because a warmer atmosphere holds more water and warmer oceans help pack these storms with even more moisture. Climate change is also increasing the severity of storm-related damages, largely because of rising sea levels.
The cost of those damages is skyrocketing. The impact to the Texas coast is impossible to assess yet, but early guesses put it in the tens of billions of dollars, putting it squarely in the top eight most expensive storms in the U.S. A few insurers are guessing the costs might rise closer to $100 billion, which would begin to rival Hurricane Katrina as the country’s most damaging storm.
On expert believes the residential property damage may exceed Katrina’s impact. Based on a figure of $325 billion in residential property in the Houston area, Kevin M. Simmons, a disaster economist at Austin College told ProPublica that a 10 percent loss would near $35 billion in residential property alone.
It is estimated that only one-sixth of Houston’s residents have federal flood insurance, which is a disaster story in and of itself. Even before Harvey hit, the National Flood Insurance Program was reported to be $24 billion in debt.
Aside from the cataclysmic human cost, the storm also hit the hub of the nation’s oil industry, with more than 130 facilities operating in Houston’s shipping channel, processing 5 million barrels of oil a day.
How the country pays for the escalating costs of these storms is a question that grows more urgent with every major storm.
Michael Burger, executive director of Columbia Law School’s Sabin Center for Climate Change Law, said although the consensus scientific view, as expressed in the draft National Climate Assessment, is that extreme event attribution hasn’t yet evolved to assign blame and potential liability, he said it could reach that level soon.
“At some point the science on attribution of extreme events might advance to the point where courts are able to assign liability for specific harms from specific extreme events to specific emissions, emitters, or other actors—such as the fossil fuel companies,” said Burger, who said no court has yet addressed the issue.
When it does, he said the oil industry could be on the hook.
“The failure to prepare for these types of foreseeable extreme events may hold out its own types of liability—including liability for industrial facilities that wind up violating federal environmental laws, creating nuisances, or causing direct bodily harms as a result of their failure to adapt,” he said.
Climate scientist and Penn State professor Michael Mann said climate change clearly worsened the impact of Hurricane Harvey.
“The combination of coastal flooding and heavy rainfall is responsible for the devastating flooding that Houston is experiencing,” said Mann. “There are certain climate change-related factors that we can, with great confidence, say worsened the flooding.”
Kevin Trenberth, a senior scientist at the U.S. National Center for Atmospheric Research, told The Atlantic: “The human contribution can be up to 30 percent or so of the total rainfall coming out of the storm. It may have been a strong storm, and it may have caused a lot of problems anyway—but [human-caused climate change] amplifies the damage considerably.”
Meanwhile, scientists continue to work on connecting more dots, showing exactly how climate change influences extreme weather events.
“While we cannot say climate change “caused” hurricane Harvey (that is an ill-posed question), we can say that it exacerbate[d] several characteristics of the storm in a way that greatly increased the risk of damage and loss of life,” Mann wrote in a recent post on his Facebook page.
Vecchi said it’s crucial to stop thinking about an individual event being caused by climate change because there are so many variables that go into specific weather events.
“Instead we [scientists] talk about how its probability was effected—that is a question that is actually answerable,” said Vecchi. “We can talk about how the probability of events changes due to climate change and climate phenomena in a very systematic way.”
“We did a study looking at the August of 2016 extreme rainfall in Louisiana and found that its probability had been doubled by global warming since the beginning of the 20th century,” he added.
“The odds of extreme rainfall in the Gulf Coast have been increasing in the past century and that is in part due to global warming,” said Vecchi. He said that because global warming will continue over this coming century, communities must expect that increased rainfall will also continue.
Vecchi said that while scientists can reliably predict an increase in the probability of extreme rainfall events and heatwaves due to climate change, the extent to which global warming influences the increase in the likelihood of hurricanes and other tropical cyclones is not as reliable.
He said tools used to observe hurricanes have improved dramatically in recent decades, which makes it impractical to compare data from collected during Harvey to data collected from storms several decades ago.
“It’s scientifically complicated to say whether or not Atlantic hurricanes have changed over the past century even though we are pretty sure that we expect the intensity of the strongest storms on this planet to increase as the planet warms—and we know the planet has warmed,” said Vecchi.“As the planet warms, the atmosphere can hold more moisture.”
But with hurricanes, Vecchi said it’s not as much about an increase in the probability that they will occur as it is about an increase in impacts when they do.
“A hurricane on a warmer planet will give more rainfall than the same hurricane can in a cooler planet,”said Vecchi.
Rising seas also play a role in increased impacts.
Mann said sea level rise can be attributed to coastal subsidence due to human disturbance like oil drilling and to climate change. He said sea levels have risen more than half a foot in the past few decades.
“That means that the storm surge was a half foot higher than it would have been just decades ago, meaning far more flooding and destruction,” he said.
Mann said warming sea surface temperatures also played a role.
“Sea surface temperatures in the area where Harvey intensified were 0.5-1C warmer than current-day average temperatures, which translates to 1-1.5C warmer than the ‘average’ temperatures a few decades ago,” said Mann.
“That means 3-5% more moisture in the atmosphere,” he said, adding that the moisture contributed to the record-breaking rainfall and catastrophic flooding.
“So Harvey was almost certainly more intense than it would have been in the absence of human- caused warming, which means stronger winds, more wind damage, and a larger storm surge,” said Mann.
Vecchi said these factors all need to be addressed when assessing future risk.
“We need to consider that as we evaluate how exposed we are to natural events,” said Vecchi. “It may be that the records over the last hundred years aren’t a reliable estimate of what our true risk is going forward. We need to have an accurate assessment of what our true risk going forward is in order to make good decisions.”
Burger said the threat of liability could prompt better decisions.
“Hopefully advances in attribution science will persuade reluctant politicians and regulators to move forward with climate mitigation and adaptation efforts,” said Burger. “But if they don’t believe in the basic science of climate change it’s hard to believe they’ll put their faith in the more complex, and more contentious, science of attribution.”
Vecchi said we have a rare opportunity to protect the planet for future generations—if we act now.
He said that while over the next 20 years or so climate change is “baked into the system” by the greenhouse gases already in the atmosphere, what we do today will determine what happens after that.
“What we can change is how much warming is going to happen after that point,” he said, adding that our actions today have the potential to provide the generation being born into right now and subsequent generations with a livable planet.
“It’s rare when such an obvious heritage to a future generation is possible,” he said.