Hurricanes are becoming so strong we may need a new scale to rate them

Satellite image of Typhoon Surigae over the Pacific Ocean in 2021

European Union/Copernicus Sentinel-3 imagery

Over the past decade, five tropical storms had wind speeds so high they should have been classified as “Category 6,” according to an analysis that suggests the hurricane scale may need to be updated, as rising temperatures fuel stronger storms.

If carbon emissions continue at current rates, we could even see “Category 7” storms. “It’s certainly theoretically possible if we continue to warm the planet,” says one climate scientist. James Kossin at the First Street Foundation, a nonprofit research organization based in New York.

Officially, there is no such thing as a Category 6 or Category 7 hurricane. According to the Saffir-Simpson hurricane scale used by the National Hurricane Center (NHC) in the United States, any storm with sustained winds of a speed of 252 kilometers per hour and above belongs to category 5.

But as wind speeds in the strongest storms accelerate, using this scale becomes increasingly problematic, Kossin and his colleague say. Michael Wehner at the Lawrence Berkeley National Laboratory in California, because it does not account for the growing risks posed by ever stronger storms.

“It’s bad and it’s getting worse,” Kossin says. “These storms are getting stronger and stronger as the climate changes. »

They say there are three lines of evidence that global warming is increasing wind speeds during the strongest storms. First, under the basic theory that hurricanes are a form of heat engine, hotter worlds should generate stronger storms.

Second, high-resolution climate models produce storms with faster winds as global temperatures increase.

And finally, real-world storms are getting stronger and stronger. Of the 197 tropical cyclones classified as Category 5 between 1980 and 2021, half occurred in the 17 years before 2021 and the five with the highest speeds occurred in the last nine years of that period.

If the Saffir-Simpson Hurricane Scale were expanded so that storms with wind speeds exceeding 309 km/h were classified as Category 6, all five of these storms would fall into that category. The five are: Typhoon Haiyan in 2013, Hurricane Patricia in 2015, Typhoon Meranti in 2016, Typhoon Goni in 2020 and Typhoon Surigae in 2021.

However, Kossin and Wehner do not propose that the NHC formally adopt their definition of Category 6. Using a scale based on wind speed is fundamentally flawed given that flooding and storm surge can be a more great threat to lives and buildings, explains Kossin. .

Instead, they believe the NHC needs to introduce an entirely new system that better reflects the overall risk posed by storms. For example, Hurricane Ike in 2008 was a massive storm that caused significant flooding and damage, but it was only a Category 1 or 2 when it made landfall in the United States, Kossin says.

Kerry Emanuel at the Massachusetts Institute of Technology agrees on the need for a new scale. “While I think it is important to recognize the increasing intensity of hurricanes, it is also worth emphasizing that most of the damage, injuries and loss of life caused by hurricanes come from water and not from water. wind,” he said.

“I support replacing the venerable but outdated Saffir-Simpson scale with a new scale that reflects the overall risks associated with a particular storm,” Emanuel says.

Another hurricane expert, Jeff Masters, now semi-retired, doesn't think the NHC could or should change the Saffir-Simpson scale. “However, talking about hypothetical Category 6 storms is a valuable communications strategy for policymakers and the public, because it is important to understand how even more damaging these new superstorms can be,” he says.

Wind damage increases exponentially with wind speed, Masters says, meaning a “Category 6” storm with 190 mph winds can cause four times as much damage as a category 5 with winds of 257 km/h.

The subjects:

  • climate change/
  • extreme weather conditions

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