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There is nothing surprising about a hurricane hitting the central Texas coast in late August, as Hurricane Harvey did on Friday night. Hurricanes have been recorded since Christopher Columbus encountered one in 1495. We know they occur with some regularity in the Atlantic at this time of year.
But Harvey soon became an inordinately severe tropical storm, as it pulled water from the atmosphere down to Earth with extraordinary efficiency and intensity. Since Friday, it has dumped an estimated 21 trillion gallons on the Texas coast, and broken the rainfall record for a single tropical storm or hurricane in the continental United States, fulfilling all meteorological predictions that it would be “unprecedented” and “catastrophic.”
Climate scientists have confirmed that climate change made Harvey a worse storm. Because of long-term warming trends, the ocean is warmer, creating more energy for a hurricane to tap. The atmosphere is warmer too, sending more water vapor into the air that can then be pulled back down by a hurricane as rain. Adam Sobel, an atmospheric scientist who directs Columbia University’s Initiative on Extreme Weather and Climate, estimates that 5 to 10 percent of the rainfall was due to global warming (more conservative than the 30 percent estimate from Kevin Trenberth, a senior scientist at the US National Center for Atmospheric Research).
But the biggest factor in Harvey’s gargantuan discharge, according to Sobel, is how it has stuck around the region for several days. The extended stay in Houston was also the most puzzling part of the storm, and global warming’s possible role in this is not well understood. “That could be a climate signal,” Sobel said. “I am not convinced about that yet.” (Scientists will be doing attribution studies to figure out exactly how greenhouse gas emissions influenced the event.)
As Vox’s David Roberts wrote on Monday, the story of Harvey is not centrally about climate change. It’s a human tragedy of a city deluged, of rising waters taking lives and destroying homes and livelihoods, inflicting psychological trauma. It’s about government and citizen response to disaster, and eventually, about a years-long recovery.
Still, a record-breaking event like this forces us to reassess our assumptions about what is possible as the Earth warms, about how bad weather events like storms, heat waves, and droughts can get, about how often we’ll have to confront them. Paolo Bacigalupi, who writes climate dystopia fiction, tweeted Tuesday: “The thing that bothers me most about these unprecedented disasters is that even I imagined they wouldn't happen for a long time yet.”
It’s not that we haven’t been warned. Climate scientists have told us that more frequent extreme weather events, more intense in new ways, were coming. Yet even when they come, and even when meteorologists nail their forecasts as they did with Harvey, it’s still incredibly difficult to prevent disaster. “Crazy events keep happening,” says Sobel. “We are sort of expecting them, but we still get overwhelmed because extreme weather is by its nature peculiar and just tough to predict.”
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Harvey might not have been predictable, but the city of Houston has become increasingly vulnerable to flooding
Certainly local and federal officials were quick to say the storm was outside of anything they’d imagined or planned for. “A flood of this magnitude is an 800-year event, and it exceeds the design specification of our levees,” Robert Hebert, a Fort Bend County judge, said in a statement Monday.
Brock Long, the head of FEMA, said it was impossible to expect a storm as devastating as Harvey: “We have not seen an event like this. You could not draw this forecast up. You could not dream this forecast up.”
Even a local storm expert said Harvey blew his mind. “This was THE historical rainfall event in the history of the United States, in terms of water and the intensity of the water — no other flood comes close,” Phil Bedient, an engineering professor at Rice University in Houston and co-director of the Storm Surge Prediction, Education, and Evacuation from Disasters Center, told Vox. “The storm covered the entire [Harris] county, which is 1,700 square miles. We’ve never had that before. The sheer amount of rain surprised even me, because it wasn’t moving.”
But as my colleague Dara Lind explained, our planning for rare floods like the one caused by Harvey is based not on history, but on outdated probability.
... FEMA has to keep updating its assessments of the floodplains — i.e., which locations should think of flooding as a 1 percent possibility, and which should think of it as something with a 0.2 percent chance of happening in any given year. The FEMA maps for Harris County had just been updated in late 2016. However, the city of Houston itself is working off a Hazard Mitigation Plan it developed in 2012 — based on where FEMA was then saying the city’s 100-year and 500-year floodplains were located.
In the meantime, the reality of Houston’s flooding has already shown the old models to be out of date. An area of West Houston called Memorial City, for example, was outside Houston’s 500-year floodplain but flooded three times in the past decade: in 2009, 2015, and 2016.
It’s now abundantly clear that there have been many terrible land use and zoning decisions made in the Houston area in the last several decades that have turned it into a paved floodplain megacity just waiting for a flooding disaster, as the incredibly timely ProPublica series Hell and High Water shows. Prairie lands and wetlands, which could have absorbed a significant amount of the floodwater caused by Harvey, have been sacrificed over the years to subdivisions and mini-malls.
“There’s no urban area in America that could have survived this without some flooding,” said Bendient. “But given our past history of land use practice, and the concrete surfaces we have here, it was exacerbated.”
Urban planners and engineers have long debated how cities in floodplains could restrict development and manage waterways to lessen the impact of flooding events like Harvey. Here’s how Ian Bogost, writing in the Atlantic, sums up some of the latest thinking:
Reducing impervious surface and improving water conveyance has a role to play, but the most important step in sparing cities from flooding is to reduce the velocity of water when it is channelized, so that it doesn’t deluge other sites. And then to stop moving water away from buildings and structures entirely, and to start finding new uses for it in place.
That can be done by collecting water into cisterns for processing and reuse—in some cases, [Thomas Debo, an emeritus professor of city planning at Georgia Tech who also wrote a popular textbook on stormwater management] explains, the result can even save money by reducing the need to rely on utility-provided water. Adding vegetation, reclaiming stormwater, and building local conveyance systems for delivery of this water offer more promising solutions.
We’re getting much better at assessing the risk of extreme weather
With rapid global warming already underway, human civilization has to prepare for extreme weather. But we also have to reckon with the fact that extreme weather events — especially storms like Harvey — will remain unpredictable in many ways. Any extreme weather event, whether it’s a hurricane, storm surge, heavy rain storm, drought, or tornado, is a collision of accidents in the atmosphere.
At the same time, the study of climate and weather patterns through history is helping scientists understand how patterns might change in the future with greater global warming. They are anxious to improve the science of prediction, to build better models to help us foresee what’s in store in time to lessen the impacts.
“If we really want to be more effective in preventing disasters, we have to learn to understand what the risks are before the events happen,” Sobel said. “And not only understand them, but act on them.”
The good news is that we have an increasingly sophisticated set of government agencies, academic research centers like Sobel’s, and nonprofit organizations dedicated to understanding and predicting these events. Prediction science to assess risk is an exciting and vibrant field.
In January, the National Oceanic and Atmospheric Administration increased its worst-case scenario for the rise of the seas worldwide. In its “extreme” sea-level rise scenario, we’d see a 10- to 12-foot rise in sea level in the US by 2100, incorporating new research that indicates parts of the Antarctic ice sheet may collapse in the next 100 years. We now know that if the Antarctic ice sheet does melt, it could trigger a catastrophic spike in sea levels that would inundate major US cities like New York and Miami. It’s good to know this is possible.
Sometimes, weather events beat the odds for the good. California, for instance, had only a 1 percent chance of overcoming its deep drought, which lasted from 2012 to 2015, within two years. But then it did.
Government agencies are also slowly getting better at working extreme weather risk into planning. FEMA has been updating its guidelines to only approve state disaster plans if they describe how the likelihood and intensity of natural hazards could be affected by the increase in greenhouse gas emissions. Though, Trump has axed several regulations issued by President Obama to limit emissions and reduce the impacts of climate change. The latest to fall: a 2015 directive to federal agencies requiring them to account for sea-level rise and storms when making grants and building infrastructure.
We can’t count on the federal government to lead on climate change mitigation — which still really needs to be our top priority rather than adapting to it. But “after Harvey, those who are inclined to take climate change seriously, along with mitigation and urban planning, resilience, are going to be even more engaged,” said Sobel. “The people who are ignoring it are mostly going to continue to. Maybe there are some people in between.”
Further reading
Vox’s Brian Resnick rounded up the most telling photos of the impact of the storm on the ground.
The US is good at responding to flood damage. We’re terrible at mitigating it.
