Capital Weather Gang • Perspective
We still don’t know how to talk about floods
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The truth about Harvey and climate change is in the middle

Floodwaters from Tropical Storm Harvey overflow from the bayous around Houston this week. (David J. Phillip/AP)

Did climate change make Hurricane Harvey’s impact significantly worse? There are a lot of opinions on this out there, and it’s okay if you’re confused. The reality is that some scientists say yes; and some, no.

In answering this question, the safest place is the middle ground: Climate change probably made Harvey a little worse. But you’re on shaky ground to say any less or much more.

Before we delve a little deeper into this question, let’s dispense with the idea that climate change or global warming caused Harvey to form. It did not. Climate change does not cause hurricanes. In the tropics, hurricanes require rising air (from converging winds), heat and moisture to form. These ingredients led to Harvey’s formation just as they have led to the genesis of tropical storms as far back as anyone knows.

The real question is whether climate change made Harvey worse than it would have been otherwise and, if so, how much worse.

There are basically four ways climate change could have intensified the hurricane’s effects. I list them here, from high confidence to low confidence:

  1. By raising sea levels, climate change increased the rise in ocean water or storm surge when the storm came ashore and the coastal flooding that resulted.
  2. By warming temperatures in the Gulf of Mexico, climate change intensified the storm’s rainfall.
  3. By warming temperatures in the Gulf of Mexico, climate change intensified the storm’s peak winds.
  4. By slowing down the jet stream, climate change increased the likelihood the storm would stall and unload rainfall over the same areas.

No one disagrees the ocean levels are rising, so it is true that climate change-induced sea level very likely added impact to the area inundated by Harvey’s surge. According to NOAA, sea levels in Rockport, Tex., near where Harvey made landfall, are rising at a rate of about 20 inches per century. So areas that were flooded by storm surge probably have climate change to thank for about a foot and half of extra water.

But of all of Harvey’s hazards, the surge was probably the least damaging because it affected a relatively sparsely populated area of the coastline. In Hurricane Sandy, for example, the surge was a much bigger problem. Disastrous rain, of course, caused the most issues.

Climate change did not cause Superstorm Sandy, but probably intensified its effects

Let’s be perfectly clear: With or without climate change, the rainfall from Harvey would have been catastrophic. But, yes, climate change probably added rainfall for this event.

Our best science indicates there is a 3 percent increase in atmospheric moisture content for every degree (F) of warming in the storm’s environment. As Harvey moved across the Gulf of Mexico, the sea surface temperatures were about 2 degrees warmer than normal, which means rainfall may have been enhanced by 6 percent or so, or a few inches.

The question of whether climate change meaningfully worsened Harvey’s winds is much more ambiguous. According to the latest modeling, more than doubling carbon dioxide in the atmosphere would lead to a 2 percent to 11 percent increase (on average) in hurricane peak winds by the end of this century. So any impact on Harvey’s wind speeds from climate change at this point would theoretically be small. Climate assessments conducted to date have not yet found a “detectable” change in hurricane intensity from rising greenhouse gas emissions.

Harvey is a 1,000-year flood event unprecedented in scale

Finally, there is the question of whether climate change-induced influence on atmospheric steering currents slowed the storm down and enabled it to unload torrential rainfall over the same areas for days. Scientists are uncertain about this.

“There are some ideas in the scientific literature that suggest that global warming may make this situation more probable,” wrote climate scientists Suzana Camargo and Adam Sobel of Columbia University. “However, these ideas are still speculative and not widely agreed upon by scientists.”

Harvey is not the first storm to make landfall in Texas and then stall and dump astronomical amounts of rain. The historical hurricane record is full of examples of storms slowing down and meandering after hitting land masses.

Adding up all of these possible climate change effects on Harvey, how significant were they in reality? That’s where expert opinion diverges.

The forgotten but freakish damage from Hurricane Harvey’s eyewall in Rockport, Tex.

Michael Mann, professor of meteorology at Pennsylvania State University, posted on Facebook that climate change “exacerbate[d] several characteristics of the storm in a way that greatly increased the risk of damage and loss of life.”

But Cliff Mass, a professor of meteorology at the University of Washington, who called global warming a serious issue, concluded that its effects on Harvey were “immaterial.” He also accused some scientists and media of “using hand-waving arguments to push an agenda.”

Where do I come out on this? I take the middle ground between Mann and Mass. Climate change probably made Harvey worse, but I wouldn’t say profoundly worse. This is a storm that, irrespective of climate change, was going to be terrible.

Is the media scaring the public too much about climate change and extreme weather?

Also, and this is an important point, no one should be under the impression that if we slash greenhouse gas emissions radically, these storms are going to stop or become significantly less severe. Devastating storms ravaged our coasts long before human-made climate change was a thing.

While reducing greenhouse gas emissions is a critical strategy for avoiding the worst consequences of climate change, it will take many decades or longer for those reductions to have a detectable impact on hurricane intensity. This is why it is so important to think about how to make our cities more resilient to these storms.

Because climate change most likely is and will continue to make these storms more severe, we can’t ignore the role it plays. But we shouldn’t pretend it’s a bigger deal than it is or that it’s nothing to worry about at all. Instead, we should consider it among all of the factors, including infrastructure and urban planning, that play a critical role in the impact storms have on our society and have constructive conversations about how to deal with them and implement sensible policy.