When meteorologists began using the five-step Saffir-Simpson scale to measure hurricane intensity in the 1970s, a Category 5 storm represented oblivion. Such a cyclone, with sustained winds of at least 157 mph, could flatten any structure of the era, so there was no reason to give the most ferocious tier of hurricanes an upper bound.
But as the planet warms, storms are increasingly surpassing what was once considered extreme, according to research published Monday. Now, two scientists are proposing a new label they say a growing number of storms already merit: Category 6.
"Climate change has demonstrably made the strongest storms stronger," said Michael Wehner, a senior scientist at the Lawrence Berkeley National Laboratory. "Introduction of this hypothetical Category 6 would raise awareness of that."
Wehner and James Kossin, a distinguished science adviser at the First Street Foundation, suggest the Category 6 label could go to any tropical cyclone with sustained winds of at least 192 mph -- an intensity that five storms have surpassed since 2013.
Meteorologists have for years debated whether the current hurricane scale adequately captures the hazards of today's storms -- it only takes winds into account, not pounding waves or flooding -- and whether a new top-end category is needed. With the new research, the scientists say they are formalizing that discussion, in hopes of spurring more academic debate about the ways climate change is heightening perceived weather hazards.
"Having [Category 5] mean anything above a certain threshold is becoming more and more problematic," Kossin said. "It tends to understate the risk."
There is no sign that government hurricane forecasters will revise their rating scale anytime soon -- and some meteorologists disagree on whether it should be adopted. Still, the proposal underscores how dramatically the potential for extreme storms has surged.
As global temperatures rise, oceanic and atmospheric warming are more often creating a prime environment for storms to rapidly strengthen and swirl more forcefully than ever. The scientists predict that trend will only accelerate, especially in warm basins such as the eastern Pacific and in the Gulf of Mexico.
The scientists predict the trend will only accelerate in such areas, where some sea surface temperature readings surpassed 100 degrees amid record global warmth last summer. Scientists forecast the threat will worsen once planetary temperatures average 3.6 degrees Fahrenheit above preindustrial levels. In that scenario, they say the risk of Category 6 storms in the Gulf will double.
Wehner said that as temperatures rise, the number of days with conditions ripe for potential Category 6 storms in the Gulf of Mexico will grow. Now it's about 10 days a year where the environment could be right for a Category 6, but that could go up to a month if the globe heats to 5.4 degrees above pre-industrial levels.
The research adds to a growing body of understanding -- and proof -- that global warming translates to stronger storms.
Warmer air holds more moisture. And more heat means more energy for storms to feed on and violently unleash. Tropical cyclones effectively serve to even out clashes between high and low pressure and hot and cool temperatures, returning the meteorological environment to equilibrium.
Global warming has already translated to increasing odds of major hurricanes around the world, according to research Kossin led that was published in the Proceedings of the National Academy of Sciences in 2020. Other studies have found that as temperatures rise, more hurricanes are undergoing what meteorologists call rapid intensification, and they are doing so at accelerating rates.
Kossin and Wehner's latest paper adds more detail and scientific rigor to understanding of what climate change means for the most intense hurricanes.
The five storms that hit 192 mph winds or more are:
2013's Typhoon Haiyan, which killed more than 6,300 people in the Philippines.
2015's Hurricane Patricia, which hit 215 mph before weakening and hitting Jalisco, Mexico.
2016's Typhoon Meranti, which reached 195 mph before skirting the Philippines and Taiwan and making landfall in China.
2020's Typhoon Goni, which reached 195 mph before killing dozens in the Philippines as a weaker storm.
2021's Typhoon Surigae, which also reached 195 mph before weakening and skirting several parts of Asia and Russia.
Haiyan stunned meteorologists with its record intensity, yet it "does not appear to be an isolated case," the study said. Two years later, Patricia became even stronger.
Pacific storms are stronger because there's less land to weaken them and more room for storms to grow more intense, unlike the Gulf of Mexico and Caribbean, Kossin said.
So far no Atlantic storm has reached the 192 mph potential threshold, but as the world warms more the environment for such a storm grows more conducive, Kossin and Wehner said.
Though there might be a scientific basis for the idea of a Category 6 storm, not all meteorologists will support adopting it. After all, a Category 5 storm causes "catastrophic" damage that could make an area "uninhabitable for weeks or months," according to the National Hurricane Center's description.
"It's hard for me to envision the need to convey a threat beyond this, even if a hypothetical tropical cyclone had peak winds that would constitute a category 6 (however one defines this)," Michael Fischer, an assistant scientist at NOAA's Atlantic Oceanographic and Meteorological Lab, said in an email.
And there is a risk that the Category 6 designation could backfire, he added.
"If a category 6 were established, would that diminish the threat of a category 5 storm, since that is no longer the most severe rating?" Fischer added.
Even without the introduction of a Category 6, the Saffir-Simpson scale already faces criticism for only considering wind speeds and not dangers from storm surge, flooding or tornadoes. To qualify as hurricanes, tropical cyclones must have sustained winds of at least 74 mph; "major" hurricanes have winds of at least 111 mph.
The National Hurricane Center will soon test a new version of its widely used forecast cone that is intended to communicate that a storm's wind hazards extend far from the spot at which its eye is predicted to make landfall.
But National Oceanic and Atmospheric Administration research shows such water-related hazards are hurricanes' deadliest threats, said Deirdre Byrne, a NOAA oceanographer who studies ocean heat and its role in hurricane intensification. While adding a Category 6 "doesn't seem inappropriate," she said, combining the Saffir-Simpson scale with something like an A through E rating for inundation threats might have a greater impact.
"That might save even more lives," Byrne said.
In a statement, National Hurricane Center Director Michael Brennan seconded those concerns. He said NOAA forecasters have "tried to steer the focus toward the individual hazards," including storm surge, flooding rains and dangerous rip currents, rather than overemphasizing the storm category, and, by extension, the wind threats alone.
"It's not clear that there would be a need for another category even if storms were to get stronger," he said.
Kossin and Wehner said their research doesn't mean to suggest that Category 6 should be added to the Saffir-Simpson scale. That is a decision that would require social science research into how it might affect people's risk perceptions and their actions to prepare for tropical cyclones, they said.
Instead, they said their intention is to convey just how dramatically global warming has changed the environment for hurricanes. The scientists said they hope the discussion raises urgency to better equip coastal communities for new and changing weather extremes.
Wehner compared it to when Australians had to add a new color to heat maps amid unprecedented heat waves, or when, just last month, extreme ocean temperatures prompted NOAA to add three categories to a coral bleaching alert system.
"The ways we considered things in the past are not necessarily good describers of the present, and certainly the future," he said.
Information for this article was contributed by Seth Borenstein of The Associated Press.