Hurricanes are getting so intense, scientists propose a Category 6

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 weather hazards as we know them.

“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 warm basins such as the Gulf of Mexico, 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 2 degrees Celsius (3.6 degrees Fahrenheit) above preindustrial levels. In that scenario, they say the risk of Category 6 storms in the Gulf will double.

Climate change is intensifying hurricanes

The research adds to a growing body of understanding - and proof - that global warming translates to stronger storms.

After all, 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 our understanding of what climate change means for the most intense hurricanes.

They scrutinized observations of past storms to find that five stand as outliers relative to past Category 5 storms: Typhoon Haiyan in 2013, Hurricane Patricia in 2015, Typhoon Meranti in 2016, Typhoon Goni in 2020 and Typhoon Surigae in 2021.

Haiyan killed thousands across the Philippines, stunning meteorologists with its record intensity. Two years later, Patricia became even stronger, with maximum sustained winds of 215 mph, though it weakened before making landfall in Mexico.

They analyzed how often conditions could be ripe for such extreme storms to develop. They found that near the Philippines, risks of a Category 6 storm would rise by 50 percent once global warming reaches 2 degrees Celsius above preindustrial levels and would double at 4 degrees of warming. In the Gulf, the risks would triple if warming reaches 4 degrees above preindustrial levels.

And they used climate models to forecast how often Category 6 storms might form in the future and to be sure the trend is tied to climate change and not natural variability. They found that annual chances of a Category 6 forming somewhere on the planet would climb to 2 percent at 1.5 degrees of global warming, 7 percent at 2 degrees of warming and 10 percent at 3 degrees of warming.

Some worry a new category could backfire

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 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.

Bringing the Saffir-Simpson scale into the future

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.