The intensity of Hurricane Harvey has almost certainly been worsened by human-made climate changeby Philip Ball / August 30, 2017 / Leave a comment
A word of advice: if you want to deny facts about nature, make them ones that don’t affect our everyday lives. For as Richard Feynman famously quipped (in the context of the Challenger space shuttle disaster in 1986), “Nature cannot be fooled.” It’ll be less embarrassing to have claimed that our planet is just 6,000 years old than to have denied global warming when faced with Houston’s elevated freeways being lapped by waves that have claimed lives and wrought billions of dollars’ worth of destruction.
If you do want to get biblical, the floods in Texas caused by Hurricane Harvey are the place to look. The images are surreal—like something out of JG Ballard’s 1962 novel The Drowned World, showing us once again that the British writer was not imagining but predicting the future.
Whether Donald Trump will parrot the usual climate denier’s line that global warming can say nothing about individual extreme-weather events remains to be seen. It seems unwise to consider any breach of decency, tact or regard for fact to be beyond the US president. Certainly, his bragging tweets about his electoral victories in Missouri while Houston drowned, or his comments about the size of the “turnout” when he spoke in Corpus Christi, Texas, on Tuesday confirmed his sociopathic blindness to public sensibilities, or to basic compassion. But perhaps even he will sense that, purely for the sake of public-opinion ratings, this is not the moment for more dismissal of scientific evidence.
Yes, it’s true that Hurricane Harvey, which over the weekend dumped more rain over the Houston area than has ever been recorded there before, was not caused by global warming. The east coast of the southern US has always been prey to catastrophic tropical cyclones sweeping in from the Atlantic. But the intensity of this storm—a hurricane this big is expected only once every 500 years or so on the basis of past events—seems almost certainly to have been worsened by the warming of the sea surface due to human activity.
“Hurricane Harvey dumped more rain over the Houston area than has ever been recorded there before”
Scientists and other experts have no illusions now about the possible consequences of speaking out against Trump’s assault on climate and environmental science. The White House is now essentially devoid of any reliable advice on scientific issues, and some staff at the US Department of Agriculture (which Trump wants to see headed by a non-scientist) have been told not to use the term “climate change.” Yet there is a weary but defiant devil-may-care attitude in the comments of some climate experts after the inundation of Houston; when the worst comes to pass in so apocalyptic a fashion, fear of offending a discredited administration seems the least of one’s worries.
“We have to be able to honestly discuss these things,” civil and environmental engineer Jim Blackburn of the Severe Storm Prediction, Education and Evacuation from Disasters Center at Rice University in Houston told me via email from his home—the university has been closed by the floods. “Our politicians’ continuing denial to honestly discuss the science that we at least partially understand is, in my mind, a root cause of this problem.”
Part of the difficulty in convincing people that climate change has an influence on things like hurricanes (not exactly warm, are they?)—and therefore part of the reason why deniers can spin webs of doubt—is that the connection seems so obscure. So we put carbon dioxide into the atmosphere and end up with 50 inches of rain over Houston? How does that work?
The crucial link is the temperature of the sea surface water, which is itself linked to the air temperature above it. Tropical cyclones—the gigantic swirling vortices of moist air that spin up hurricane winds—are created by intense evaporation of moisture from warm waters, which is why they are a phenomenon of the near-equatorial oceans where average temperatures are higher. The warmer the water, the more evaporation takes place. And the sea surface where Harvey began in the Atlantic was anomalously warm, by around 0.5-1.0 oC on average, even in comparison to recent temperatures which are themselves a half degree or so up from several decades ago. This warming trend is incontrovertible, and almost certainly due human-induced climate change. So if a natural upward fluctuation in sea surface temperature takes place, it now happens on an elevated baseline, and what might be a stronger-than-average seasonal cyclone becomes a monster like Harvey. “Harvey was almost certainly more intense than it would have been in the absence of human-caused warming,” climatologist Michael Mann of Pennsylvania State University told the Guardian.
The problem is exacerbated by several other factors once Harvey made landfall. First, global warming is creating a deep layer of warmer water in the ocean, so that the stirring of water nearer the coast doesn’t bring up cooler water to lessen the rate of evaporation. “Climate change makes the sea surface temperatures higher,” climate scientist Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Colorado, told me, “but where it really counts is the upper ocean heat content that replenishes the temperature as the water is churned up. That’s where the global warming plays a role: it continues to fuel the warm waters and the moisture evaporation that leads to the huge precipitation and flooding.”
So, far from weakening as it approached land, Harvey got worse. “As Harvey approached Houston last week, surface temperatures of the Gulf near Texas rose to between 2.7°F and 7.2°F [1.5-4.0 oC] above average, making them some of the hottest in the world,” atmospheric scientist Ronald Sass of Rice University said. “Harvey, feeding off this unusual warmth, was able to progress from a tropical depression to a Category 4 hurricane in roughly 48 hours.” Harvey was initially predicted to be a Category 3 hurricane; Category 4 events are the next-to-worst in the five-point scale. The 1900 Galveston hurricane, the worst natural disaster in US history, was such an event; some studies show that their incidence has increased in recent decades.
“Far from weakening as it approached land, Harvey got worse”
Second, global warming has also increased sea level (simply because warmer water expands), so that the flooding caused by the “storm surge” of water pushed ashore by the hurricane is higher. Third, air movements have stalled Harvey over the Houston area, letting it dump all its immense moisture load there. Whether global warming has played a part in creating this confounding circulation pattern isn’t clear—some climate models predict it in a warmer world, but it could just be a weather pattern arising from unfortunate chance.
Regardless of that particular wrinkle, Blackburn thinks Harvey’s severity is a climate-change-related “weird weather” event. “We have seen a skewing of normal climate-event distributions in several storms over the past two years, and Harvey is that issue magnified,” he said. Trenberth said that human-induced climate change can be expected to increase average rainfall during such storms by 5-10 per cent, but a “perfect storm” can arise in which natural variability might double that figure and natural feedbacks double it again. Then you have something like Harvey.
Predicting and understanding such events has never been more vital. But when the Trump administration proposed in March to increase defense funding at the expense of basic science, the University Corporation for Atmospheric Research (of which Trenberth’s NCAR is a part) issued a statement expressing concern that “the proposed funding cuts to Earth system science research would derail the nation’s progress toward improved [weather] prediction.”
This is about far more than better science, though. A severe storm like this comes as no surprise to expert forecasters, but there has been scant preparation for it in Texas. Hurricane Ike in 2008 delivered a wake-up call, but a defensive sea wall suggested at the time (the “Ike Dike”) to ward off the storm surge was never constructed. And property damage and insurance regulations are a mess, with some politicians wanting to weaken building codes. “Insurance companies have put rates up to the [maximum] extent allowed,” says Trenberth. “Flooding is covered by the federal government through a flood insurance program, but rates are not appropriate in many areas, and it is running at deficit. Lobbying from governors of states and local areas means that the rest of the country subsidises bad practices and there is rebuilding in untenable locations on the flood plain.” After Hurricane Katrina ravaged New Orleans in 2005, he said, the Corps of Engineers built the levees back to a standard designed to accommodate only a Category 3 storm. It now seems clear that Category 5 events should be the default assumption.
Despite the extraordinary images from Houston, there have been remarkably few deaths from the floods and hurricane-related damage: so far they are countable on the fingers of two hands, though the devastation of Katrina showed that long-term impacts on health are substantial, particularly among low-income people. As ever, we notice the consequences of climate change most when they are intense and dramatic—one might even say perversely photogenic—and when they affect wealthy nations (if not necessarily wealthy individuals). Other extreme events such as the “Lucifer” heatwave over Europe this summer are likely to claim many lives, but in more distributed fashion and without the disaster-movie imagery.
The time for dismissing all extreme weather as “natural variability” is over—not because it isn’t part of the equation, but because, as Harvey shows, there is already compelling reason to think that climate change can act as an intensifier. And an increase in extreme weather is one of the most secure predictions of climate models, regardless of how we apportion causes and effects for specific events. This stuff is real, not an abstract argument about reason and evidence, statistics and graphs. It’s about lives and livelihoods lost and at risk. And it’s political.