Does the hurricane scale need a Category 6? New climate study found 5 recent storms have met the threshold.
As global temperatures continue to increase, making storms more intense, some researchers say that the Saffir-Simpson scale, which measures a hurricane's wind speeds, doesn't adequately address the hazards associated with extreme storms. In a new study, scientists explored a hypothetical expansion of the scale to include a Category 6, and found that such a designation could help focus people of the worsening risks.
In a new study, published Feb. 5 in Proceedings of the National Academy of Sciences, researchers found that the scale, which was created in the early 1970s and is still used to define hurricane categories, has a "weakness": It tops out at Category 5 even though "the destructive potential of the wind increases exponentially."
The hurricane categories run from 1 to 5, with Category 5 hurricanes having sustained wind speeds higher than 156 mph — enough to produce "catastrophic" damage, which NOAA says can result in "complete roof failure on many residences and industrial buildings," as well as extended power outages.
Michael Wehner, lead author of the latest research, told CBS News that there have been several hurricanes in recent years with winds that far surpass 156 mph — and that it may warrant an entirely new category. He and his co-researcher James Kossin looked at the potential impact of expanding the scale so that Category 5 would be capped at wind speeds of 192 mph, and any hurricanes or cyclones above that be designated Category 6, to better help inform people of the risks.
"We found that five storms had exceeded this hypothetical Category 6, and that all of them were recent, since 2013," Wehner told CBS News.
The most intense of those five storms was Hurricane Patricia, which peaked with wind speeds well over 200 mph before making landfall in Mexico as a Category 4 in October 2015. Patricia "intensified at a rate rarely observed in a tropical cyclone," according to the National Hurricane Center. NOAA reported that the storm hit maximum winds of 215 mph, nearly 60 mph faster than the lower bar of the Category 5 designation.
The other storms of hypothetical Category 6 strength occurred in the Western Pacific, the study says. Typhoon Haiyan, which hit the Philippines in 2013, was "the costliest Philippines storm and the deadliest since the 19th century, long before any significant warning systems," the study says.
The researchers also looked at a "state of the art" climate measurement system that analyzes potential intensity — "basically a speed limit on how fast the highest winds could be in a perfect storm," Wehner said.
"Our motivation here was to draw the connection between climate change — that warming of the atmosphere, the globe, from the burning of fossil fuels — to hurricanes and tropical cyclones," Wehner said, adding that he and Kossin consider themselves "relatively conservative climate scientists."
"Because climate change increases temperature and moisture — which are the sources of the energy for a hurricane or a tropical cyclone — one would expect this speed limit to increase," he said. "And indeed it does."
Wehner and Kossin found a "significant" observed increase in wind speeds since 1982, saying in their study that it's likely wind speed records will "continue to be broken as the planet continues to warm."
They also ran simulations based on various global warming scenarios, and found that the risk of seeing what would be Category 6 storms "has increased dramatically and will continue to increase with climate change."
This study is not the first to look at a hypothetical expansion of the Saffir-Simpson scale. In 2019, former NOAA hurricane hunter and meteorologist Jeff Masters wrote that current hurricane categorizations are "inadequate." He suggested that the scale be expanded with a Category 6, starting with winds of 180 to 185 mph, and a Category 7, to be used for storms with winds of at least 210 mph.
"Any move to expand the Saffir-Simpson scale would have to come from the National Hurricane Center (NHC), though, and there is little support for such a move from the experts there," he wrote in a blog post for Scientific American. "From a public safety/warning standpoint, NHC experts I've heard from believe that including a category 6 would do little good, since a category 5 hurricane is already considered catastrophic."
Wehner acknowledged the Saffir-Simpson scale has been heavily criticized for being the sole determination of a hurricane's category. The scale is based only on wind speed, and while that is a crucial measurement for determining a storm's risk, it doesn't account for the potential destruction caused by the storm's size or the storm surge and flooding it could bring. Wehner's study does not address those other hurricane factors.
Those hurricane risks should be better incorporated when explaining a storm's risk, Wehner said.
"A single number is really not very descriptive of the entire risk of an impending hurricane if you're in the path," he told CBS News. "You really need to know what are the kinds of dangers that you're being exposed to."
The researchers said in their paper that they are not specifically proposing changes to the scale, but are looking to "raise awareness that the wind-hazard risk from storms presently designated as category 5 has increased and will continue to increase." Adding a sixth category to the scale could help raise awareness about expected changes to storms' wind strength in the years to come, and how that, on top of other hazards, could impact communities, they said.
"The stronger the wind, the stronger the storm surge. And there's going to be a lot more rain regardless," Wehner said. "...From the global warming context, trying to put a single number on what the increased risk from the global warming part that's a more long-term kind of danger, we think that this scale is fine."
Another criticism Wehner said an expanded scale has received is the question, "If we add a Cat 6, does a Cat 3 storm not matter anymore?"
"And the answer is, 'Of course it does,'" he said. "...Just because the worst storms are worse, doesn't mean that bad storms aren't bad."