Even though rain is still falling in Houston, history has already been made: Hurricane Harvey is almost certainly the largest U.S. flooding rainstorm ever. In its rapid intensification and its powerful downpours, it presents many of the symptoms of human-caused climate change, though scientists caution that it will take months for them to distinguish the full effects of global warming on the storm.
Yet Harvey also seems to break with a recent trend. Technically, the storm is the first major hurricane to make landfall in the United States in more than 11 years. The contrast may prompt some to say, well, what gives? Are hurricanes getting any worse, or not?
The answer, in short, is: Yes, they are getting worse. But there’s a twist. At the very frontier of climate science, some evidence now suggests that there’s an “intensity-frequency trade-off”: Though hurricanes are getting more intense overall, weaker storms are also forming less frequently.
“If we just look at frequency, there’s really no theory that says we should see fewer or more storms,” says James Elsner, a climate scientist and geographer at Florida State University. “But if we look at intensity separately, there is theory that says they should get stronger—especially the strongest ones. And we do see some evidence for that in the data.”
Last year, Elsner and his colleague, Nam-Young Kang, published a review of how climate change has affected tropical hurricanes over the last 30 years in Nature Climate Change. It found a slight trend in the data toward more powerful, less frequent storms. As the tropical cyclones that have formed have gotten more intense, fewer have spun into existence.
“If you look at frequency and intensity together, it appears that it’s really the efficiency of intensity that matches the global-warming signal the best. The strongest get stronger, but at the expense of the number of storms,” Elsner says.
He added that this decrease in the number of storms overall doesn’t have much to do with the recent decline in the number of landfalls in the United States. “The coast is kind of a random boundary,” he says. (The 11-year number oft cited on Twitter is also more than a little flukey: Hurricane Sandy is the second-most destructive U.S. tropical cyclone of all time, but it’s not counted as a “major hurricane” since it came ashore as a tropical storm.)
Other researchers are not as sure that this conclusion can be declared viable yet. Unlike global temperature records, which date back to 1870, global hurricane records are much younger and less robust. The oldest global tropical-cyclone record that holds to modern standards is the log of hurricanes that made landfall in the continental United States—and it is both highly irregular and dates back only to 1900.
Thomas Knutson, a research meteorologist at the NOAA fluid-dynamics laboratory in Princeton, New Jersey, told me that a “trade-off” signal still isn’t strong enough to see in the hurricane data. “We haven’t really detected clear changes in the data in the same way we can detect changes in global mean temperature,” he says. “I just think [30 years] is a rather short record to be inferring [human-caused climate] effects, because you can also have natural modes of variability over a period of several decades.”
Most of the research into hurricanes has been done with climate simulations, which can also suggest that a warmer world will have fewer, stronger hurricanes, Knutson says. But above all they suggest that climate-changed hurricanes will be wetter and rainier.
“One of the most robust results we see from model simulations is that hurricanes have higher rainfall rates,” says Knutson. “We interpret that as happening because, in a warming climate, the atmosphere is holding more water in general—and a hurricane brings air into its center and wrings water out of it as precipitation.”
Knutson is one of the authors of the U.S. Climate Science Special Report, a U.S.-government review of the current climatological consensus due out later this year. It summarizes the current high-confidence findings of climate science. An early draft of that report obtained by The New York Times says there is an “observed upward trend in North Atlantic hurricane activity since the 1970s.”
The same report also forecasts “an increase in tropical-cyclone intensity in a warmer world,” and it says that “models generally show an increase in the number of very intense [tropical cyclones].” For hurricanes like Harvey, formed in the North Atlantic, climate models have high confidence that future storms will drop more rain and medium confidence that they will have higher wind speeds.
Hurricane intensity and frequency are one of the more uncertain area of climate science. Climate researchers are much more certain, for instance, that global warming will bleach coral reefs worldwide, cause devastating heat waves, and cause sea level to rise around the world. All these effects will also work together to worsen the human and ecological effects of more intense hurricanes.
“Just about everyone agrees that the amount of rainfall associated with storms and hurricanes is increasing—and the risk of storm surge and coastal flooding is worsening, as well, because of sea-level rise,” says Katharine Hayhoe, a professor of atmospheric science at Texas Tech University and a coauthor of the Climate Science Special Report.
Even if storms weren’t getting more intense, she adds, the relative risk of tropical storms would still be rapidly increasing, because more people are living near the coast while the sea level keeps rising.
“We know population is increasing, and we know vulnerability is increasing because there’s no massive plan to update infrastructure in Houston,” she told me. “There’s really no uncertainty that the risk is going up.”