When Herbert Saffir went on commission for the United Nations in 1969, tasked with studying low-cost housing in hurricane-prone regions, he realized that something was missing from our meteorological vocabulary. There was no uniform way to communicate the power and destruction of an oncoming hurricane.
Saffir decided to fix that. In tandem with Robert Simpson, then-director of the National Hurricane Center, he developed the Saffir-Simpson scale: a five-category measurement of a hurricane’s wind speeds.
But what if five categories of hurricanes aren’t enough? What if a sixth—for those with winds at almost 200 mph—is now becoming necessary?
Thanks to climate change, that might be the case. One of the most prominent advocates for a new category is Michael Mann, who believes it is necessary because—as the title of the team’s paper in the journal RealClimate suggests—global warming might be making tropical cyclones stronger. In the paper, researchers reviewed a series of climate studies that have occurred in the past 39 years and concluded that “the strongest storms are getting stronger.”
Michael Mann, professor of atmospheric science at Penn State and one of the four authors of the study, explained it simply: “The heat from the ocean surface [provides] the energy that drives a tropical storm,” he told The Daily Beast via email. “All other things being equal, more ocean heat means more energy to strengthen these storms.”
That means that there’s been a dramatic power increase in the strongest hurricanes. According to the study, “Storms of 200 km/h [124 mph] and more have doubled in number, and those of 250 km/h [155 mph] and more have tripled.”
To support their point, the authors cited a series of recent hurricanes that broke meteorological records. Hurricane Harvey (2017) had more rainfall than any other U.S. hurricane in history. Irma (2017) maintained a wind speed of over 300km/hour for 37 hours, longer than any other storm on record. September 2017—the month that Hurricane Maria struck Puerto Rico—had the highest cumulative energy in the Atlantic in history.
The authors noted that because hurricane measurement tools were less accurate before satellite technology, it’s possible that the strength of earlier storms could have been underestimated.
The implications of these findings are obvious. “Tropical cyclones bring nothing but trouble with them,” Jim Kossin, an atmospheric research scientist at the National Oceanic and Atmospheric Association (NOAA) who was also a co-author on the RealClimate study, said. “Certainly, their strength is directly correlated with the damage that they do, and the mortality rates that are associated with them. Stronger storms, simply, right out of the gate, are going to mean much more risk.”
Under the current Saffir-Simpson hurricane wind scale, hurricanes fall into one of five categories based on their “maximum sustained wind speed,” or the highest average wind speed measured over a 1- or 10-minute period. A Category 1 storm has maximum sustained wind speeds ranging from 74-95 mph. A Category 5 storm, on the other hand, denotes a hurricane with maximum sustained wind speeds above 157 mph.
Given the study’s predictions for the future, Mann told Inside Climate News that a sixth level of hurricane categorization, one that would include any hurricane with wind speeds above 190 mph, is necessary to communicate the expected threat to the public and disaster preparedness officials.
“If you look at the breaks in the scale currently (e.g., 130 mph boundary between [Category] 3 and 4, 156 mph boundary between [Category] 4 and 5), which is based roughly on how intensity varies with wind speed, there should be another boundary somewhere around 185 mph,” Mann noted.
“The argument against introducing another category at that point in the past was that a [Category] 5 storm causes total destruction so there is no need for a higher category. Given the improvements in the robustness or construction and infrastructure, that is no longer necessarily true.
“But more importantly,” Mann added, “the scale is supposed to connote risk, and there is no question that the risk from a 200 mph monster storm is far greater than that for a 157 mph marginal [Category] 5 storm.”
But does actually adding another category make it easier for people on the ground to understand how dangerous an incoming storm is? After all, many residents stubbornly stay put even if meteorologists make repeated pleas to evacuate during Category 5 landfalls.
Kossin sees the merit of Mann’s suggestion, noting that he has “no problem with that idea.” He worries, however, that it would be misinterpreted by the public.
“We’re now at a place where if a storm goes from Category 5 to Category 4 before landfall, people stop paying as much attention,” he told The Daily Beast. “Which is pretty silly, considering how strong a Category 4 storm is, but that’s just human nature. If we add a Category 6, then suddenly Category 5s are gonna be taken less seriously, and I don’t know how good of an idea that is.”
Katherine Fox-Glassman, a professor of psychology at Columbia University who studies the psychology of responses to natural disasters, has a different perspective. Although she acknowledges the risk of the public misinterpreting the difference between Category 5 and Category 6, she believes that the new category could effectively influence public perception of a more dangerous hurricane.
“Generally, people perceive the risk to be higher, when the category is higher, which is good,” she said. “People certainly could pull the wrong information out of this, but my personal opinion is that if hurricane winds are getting stronger, we should probably have a system that represents that.”
She notes, however, that the public’s perception of a hurricane’s danger is not directly related to evacuation or other preventative actions. As many articles have documented, even people who understand the intensity of an oncoming hurricane face additional barriers to preparing properly.
Michael Brennan (PDF), branch chief at the Hurricane Specialist Unit of the National Hurricane Center, who was not affiliated with the study, had yet another perspective. He told The Daily Beast in a statement that a sixth category would distract from what he believes are more dangerous elements of a hurricane.
“At NHC we’ve tried to steer the focus toward the individual hazards, which include storm surge, wind, rainfall, tornadoes, and rip currents instead of the particular category of the storm, which only provides information about the hazard from wind,” he wrote. “Category 5 on the Saffir-Simpson scale already captures ‘Catastrophic Damage’ from wind, so its not clear that there would be a need for another category even if storms were to get stronger.”
Not only that, Brennan noted, but there’s also the fact that 90 percent of deaths from hurricanes are a result of water, whether it be storm surge, flooding, and hazardous surf conditions. Having a Category 6 would potentially de-emphasize water conditions and overemphasize wind conditions, which might be the wrong emphasis in an emergency situation.
Despite the debate, there’s one thing experts agree on: There’s been a spike in intense hurricanes—and it’s likely to get worse.
“This is a risk that we can no longer afford to ignore,” the authors conclude.