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

In the past decade, five tropical storms had wind speeds so high that they should have been classified as “category 6” storms, 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 might even see “category 7” storms.  “It certainly is theoretically possible if we keep warming the planet,” says climate scientist James Kossin at the First Street Foundation, a non-profit research organisation 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 US, any storm with sustained wind speeds of 252 kilometres per hour and over is a category 5.

But as the wind speeds of the strongest storms get faster, the use of this scale is increasingly problematic, say Kossin and his colleague Michael Wehner at the Lawrence Berkeley National Laboratory in California, as it doesn’t convey the increasing risks posed by ever stronger storms.

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

They say there are three lines of evidence that global warming is making the wind speeds of the strongest storms get faster. Firstly, according to the basic theory of hurricanes as a form of heat engine, hotter worlds should generate stronger storms.

Secondly, high-resolution climate models produce storms with faster winds as global temperature rises.

And finally, real-world storms are getting 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 this period.

If the Saffir-Simpson hurricane scale were extended so that storms with wind speeds over 309 km/h were ranked as category 6, those five storms would all 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 aren’t proposing that the NHC officially adopts their definition of category 6. The use of a scale based on wind speeds is fundamentally flawed given that flooding and storm surges can be a greater threat to lives and buildings, says Kossin.

Instead, they think the NHC needs to introduce an entirely new system that better conveys the overall risk posed by storms. For instance, Hurricane Ike in 2008 was a massive storm that caused extensive flooding and damage, but was only a category 1 or 2 when it made landfall in the US, says Kossin.

Kerry Emanuel at the Massachusetts Institute of Technology agrees on the need for a new scale. “While I think it is important to acknowledge increasing hurricane intensity, it should also be pointed out that most damage, injury and loss of life in hurricanes comes from water, not wind,” he says.

“I have been in favour of replacing the venerable but out-of-date Saffir-Simpson scale with a new scale that reflects the totality of risks from a particular storm,” says Emanuel.

Another hurricane expert, Jeff Masters, now semi-retired, doesn’t think the NHC would or should change the Saffir-Simpson scale. “However, talking about hypothetical category 6 storms is a valuable communication strategy for policy-makers and the public, since it is important to understand how much more damaging these new super storms can be,” he says.

Wind damage rises exponentially with wind speeds, says Masters, meaning a “category 6” storm with 314 km/h wind can do four times as much damage as a category 5 storm with 257 km/h winds.