Powerful hurricanes have been piling on in recent years, creating billions of dollars worth of damage. In September, Hurricane Dorian brewed into a Category 5 storm with winds reaching 185 miles per hour, and proceeded to sit over the Bahamas, devastating the Abaco Islands and Grand Bahama. The year before that, we had Florence and Michael. Another year back: Harvey, Maria, and Irma.
As devastating as these events were, meteorologists don’t all agree on whether storms are indeed growing stronger, or if there’s just more people living on the coast, where they are vulnerable to the powerful winds and rain. But a new study finds that even when accounting for increases of people and wealth in areas hit by the cyclones, the devastation is increasing. “If a storm hits a major city, of course it causes more damage than if it’s in a rural area,” says lead author Aslak Grinsted, climate scientist at the University of Copenhagen. “I wanted to account not only for the change in time but also for the spatial change [in increases of population], and the result is there is a trend: that hurricanes are becoming more destructive.”
In theory, it makes complete climate sense. As temperatures in the atmosphere rise, so does the sea surface as it diligently absorbs heat. The water heats up, causing more of it to evaporate—and the warmer air above can also hold more moisture, which means more rain for storms to dump. In climate models, those conditions have fueled an increase in strong hurricanes.
But in real life, there are a number of variables that confuse the role that climate plays in hurricane damage. One of them is simply that there are more people living on the coast. As Popular Science reported last year, there are 60 million more people living in areas vulnerable to tropical cyclones today than in 1970. And some scientists have said that the increase in storm damage along the Atlantic and Gulf coasts is therefore simply due to there being more people and properties—and thus higher costs reported by insurance companies.
Grinsted used a new method to control for more people and property. Using records of the destruction caused by storms dating back to 1900, he converted the dollar value of damages into a corresponding area of destruction. This way, he focused simply on the area affected by storms, rather than costs, which rise as population rises.
The new method revealed an increase in storm destruction across time, especially during the last two decades. Hurricanes Katrina and Harvey were the most damaging under this new measure; for both, the area of total destruction was over 5,000 square kilometers (or almost 2,000 square miles). Using the area-destroyed measure, Gridstead and his team also included an analysis of the magnitude of hurricanes. Since 1900, the largest hurricanes have become about three times as frequent, they found. “This paper confirms that we’re already seeing measurably more damage from tropical cyclones in the U.S. due to the impact of climate change,” says Andra Garner, a climate scientist at Rowan University who was not involved in the research.
In addition to providing evidence that hurricanes are indeed becoming more destructive, Grinsted says that the new metric of estimating the area of damage can help inform the relationship between hurricane strength and their potential costs. This information could be used by insurance agencies in developing risk assessments.
It remains to be seen what the more skeptical members of the meteorological community think, but Grinsted says this analysis of hurricane frequency jives with other findings by atmospheric scientists. “That part I think will be pretty convincing to a lot of people.”