As Typhoon Haiyan howled into the Philippines in November 2013, people knew that a powerful storm was coming, but they were not expecting a typhoon with 196 mile per hour winds, driving a storm surge up to 23 feet high. Haiyan killed more than 6,300 people, destroyed or damaged more than 1 million structures and pushed millions of people into poverty. 

In the wreckage, aid workers found that some people had died inside shelters built to withstand the impacts of Category 5 storms—also known as “super typhoons”—which is currently the highest rating on the danger scale. 

But Haiyan’s top wind speeds and storm surge were far stronger than any previously recorded in the Philippines, exceeding the Category 5 level by more than 40 mph. Since 2013, there have been several other storms of similar magnitude, prompting some tropical storm researchers to wonder whether the well-known Saffir-Simpson hurricane danger scale should be updated to better warn of the increasing risks. 

In new research published today in the Proceedings of the National Academies of Sciences, researchers said they’ve compiled evidence that the “open-endedness” of the scale is “increasingly problematic for conveying wind risk in a warming world” because it doesn’t fully reflect the potential for impacts from storms like Haiyan. 

In the paper, James Kossin and Michael Wehner described a potential Category 6 rating for tropical storms with winds stronger than 192 mph. Going back to 1980, there have been five storms with winds that strong, all of them within the last nine years, Kossin said.

A 2018 review of global data on hurricanes showed that, since 1980, the number of storms with winds stronger than 124 mph, or a strong Category 3, have doubled, and those with winds stronger than 155 mph have tripled. That paper also spurred discussions of adding a Category 6 to the Saffir-Simpson scale.

The new analysis showed how just a bit more warming will raise the risk of very strong storms dramatically in places like the Gulf of Mexico and in the Western Pacific, where Haiyan raged. And it reinforced some of the findings of the Intergovernmental Panel on Climate Change’s most recent global assessments of tropical storms and other climate extremes.

Both Kossin and Wehner were also co-authors of Chapter 11 of the IPCC’s 2021 physical climate science report, which focuses on extreme events in a changing climate and concluded that it’s “likely that the global proportion of Category 3–5 tropical cyclone instances has increased over the past four decades.” 

“None of those changes,” the panel concluded, “can be explained by natural variability alone.” 

The IPCC report also found, with high confidence, that the proportion of the most intense tropical storms, and the peak wind speeds of the most intense tropical cyclones “will increase on the global scale with increasing global warming.”

“That inspired me to look at these things,” said Kossin, formerly with the National Oceanic and Atmospheric Administration and now a senior science advisor with First Street, a Brooklyn-based nonprofit focusing on climate risks.

“I don’t remember when the idea of Category 6 came up, but I remember I immediately reacted negatively,” he said. “One example of why that would be a bad idea is I can imagine people saying, ‘I’m not going to evacuate. It’s only a Category 5.’” Normalizing increasing risks is an adaptive strategy for people trying to cope with the growing threats of climate change, he added. But it’s also one that puts people at risk.

As soon as people adjust their thinking to accept the higher risk, “we no longer worry about it,” he said. “Or, we may worry about it, but our actions don’t necessarily bear that out,” which is concerning in a world where climate risks of all kinds are intensifying.

“We have daughters that are college age and they’re really, really worried,” he said. “It’s difficult to imagine what it’s like being in your 20s right now.” 

The increase in the “thermodynamic speed limit for hurricane winds,” has been expected for 35 years, said MIT hurricane expert Kerry Emanuel, who was not an author of the new paper. 

“Now we are seeing this increase in both climate analyses and models,” he said. “Observations of actual hurricanes show that the percentage of storms reaching high intensity has been increasing.”

Emanuel said the new paper makes a strong case for changing the scale but acknowledges that the authorities who determine such matters are unlikely to change it. And problems with the scale aren’t only due to its failure to give the highest wind speeds their own category, but that it does not specifically include water-related threats.

“While I think it is important to acknowledge increasing hurricane intensity, it should also be pointed out that the most damage, injury, and loss of life in hurricanes comes from water, not wind,” he said, noting that Hurricane Sandy in 2012 was only a tropical storm when it caused the worst flooding in New York City’s history. Storm surges are driven by wind but also strongly affected by storm size and the speed at which they travel over an area, he added. 

“The rest of it comes from torrential rain, which can also occur in storms with relatively weak winds,” he said. “I have been in favor 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.”

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Adapting to Super Storms Means ‘Getting Out of the Way’

Thinking about responding to future threats requires understanding “how people respond to warnings and how they respond to risk,” Kossin said. “I think 90 percent of the discussion needs to happen in terms of psychology and sociology.” Any change to the scale needs to be designed by a large group of multidisciplinary scientists, he added.

Discussions about how to improve risk communication have to start somewhere, and talking about Category 6 tropical storms is a starting point, said co-author Michael Wehner, a climate extremes researcher with Lawrence Berkeley National Laboratory. 

“We knew this could be controversial, but we did it anyway,” he said. “We’re not afraid of being controversial. We know there’s potential for a media frenzy, and there are people who are uncomfortable with this kind of thing.”

There is a schism between academic research on climate risks and the institutions that try to operationalize new research and data, he acknowledged. 

“These intense storms are incredibly dangerous. In the modern world such storms indicate you really can’t do anything except get out of the way. To us, it seems like a really important thing for the public to understand.”

Scientists have known for a long time that “climate change is making the worst storms worse,” Wehner said, and describing a new Category 6 would highlight the increasing risk from the most devastating storms. “We thought we would put it on a firm, peer-reviewed, quantitative footing,” he said.

When they used the 192 mph wind speed to mark a Category 6 threshold, they found that there is a “very significant trend in the number of days per year that this hypothetical, category 6 would be exceeded.”

Wehner said he expects at least some dismissive reactions to the research.

“We’re anticipating that some people will say, ‘You’re just being alarmist,’” he said. “This is, of course, alarming, and we don’t think that we’re exaggerating the danger by any means.

“Climate change is causing the ways we describe things to be antiquated.” 

“Yes, they are still rare, but the projections are sobering,” he said of the storms that would exceed the Category 6 threshold. “Even under the Paris Agreement targets, with modest warming compared to what we really expect, the risk in the Gulf of Mexico is doubled and tripled.”