Do We Need a Category 6 Designation for Hurricanes?

With global warming leading to more intense storms, two scientists have investigated the implications of extending the hurricane wind scale to add a sixth category. The Saffir-Simpson hurricane wind scale currently tops out at Category 5—for hurricanes marked by maximum sustained winds of at least 157 miles per hour. Now, the scientists have suggested that any hurricane with sustained winds of 192 miles per hour or more could earn a higher designation.

“Climate change has demonstrably made the strongest storms stronger,” Michael Wehner, a senior scientist at the Lawrence Berkeley National Laboratory and co-author of the study, tells the Washington Post’s Scott Dance. “Introduction of this hypothetical Category 6 would raise awareness of that.”

As ocean temperatures warm, maximum wind speeds will likely rise, and the amount of damage wind can cause increases exponentially with wind speed, the study authors write in a paper published February 5 in the Proceedings of National Academy of Sciences.

Some experts think that changing the scale isn’t necessary.

“Category 5 on the Saffir-Simpson scale already captures ‘catastrophic damage’ from wind so it’s not clear there would be a need for another category even if the storms were to get stronger,” Jamie Rhome, deputy director of the National Hurricane Center, tells Seth Borenstein of the Associated Press (AP).

The National Hurricane Center introduced the Saffron-Simpson scale in the early 1970s. It initially used estimates of peak wind, storm surge and pressure to quantify damage caused by both wind and water, but it was changed in 2010 to just take into account and communicate risk from winds.

The system is widely used in the U.S. for communicating risks from tropical cyclones. But wind plays a relatively small role in deaths from hurricanes. A 2014 paper found that 49 percent of deaths related to tropical cyclones were caused by coastal storm surge, 27 percent by flooding from rain and only 8 percent by winds directly.

“The simplicity of the scale is both a flaw and an advantage,” Brian McNoldy, an atmospheric scientist at the University of Miami who did not contribute to the findings, tells the Atlantic’s Nancy Walecki. He tells people during presentations that “there’s more to the story than the category,” per the publication.

In the new paper, the authors propose the idea of a hypothetical Category 6 for storms with winds of 192 miles or more and lay out an argument for why global warming has and will continue to increase the likelihood for hurricanes with winds this fast.

First, the proportion of intense storms is increasing. Half of the 197 Category 5 storms between 1980 and 2021 occurred in the last 17 years, per the paper. Five of these storms reached the hypothetical Category 6 benchmark, and all occurred within the last nine years.

Theory also suggests that as ocean temperatures warm, the energy available for making more intense storms increases. And climate model simulations show that warmer temperatures increase the risk of intense storms.

“Our results are not meant to propose changes to this scale, but rather to raise awareness that the wind-hazard risk from storms presently designated as Category 5 has increased and will continue to increase under climate change,” the study authors write. In other words, the scientists aren't saying the scale should change, according to the Washington Post, as this decision would require more research into how an update might impact factors like risk perception and storm preparation. But the researchers want to impress upon others how much global warming has impacted hurricane conditions. 

“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,” Kerry Emanuel, an atmospheric scientist at MIT who was not involved in the research, tells New Scientist’s Michael Le Page.

Messages about tropical cyclones also need to better inform the public about the risk of flooding and storm surge, the study authors write.

Adding a Category 6 “doesn’t seem inappropriate,” but combining the wind scale with something like a rating system for the threat of inundation, the water level on normally dry land, could have more of an impact, Deirdre Byrne, an oceanographer with the National Oceanic and Atmospheric Administration who did not contribute to the findings, tells the Washington Post.

“That might save even more lives,” Byrne tells the publication.

Will Sullivan | | READ MORE

Will Sullivan is a science writer based in Washington, D.C. His work has appeared in Inside Science and NOVA Next.