When Dr. Robert Simpson boarded a military helicopter from Miami on Aug. 19, 1969, bound for the central Gulf Coast, little could prepare him for the horrors he would witness as the first National Hurricane Center director to survey the immediate aftermath of a hurricane.
Camille wasn’t just any hurricane. It was the most intense U.S. hurricane in a generation, with a record-setting 24-foot storm surge that wiped coastal Mississippi homes from their foundations, contributing to the deaths of some 150 people.
It was a seminal event for Bob Simpson that inspired the familiar 1 to 5 Saffir-Simpson hurricane rating scale that would eventually bear his name.
Camille was a chart-topper, one of only four recognized Category 5 hurricanes to strike the U.S. since records began in 1851. Category 5 hurricanes – the most catastrophic storms whose winds top 157 mph – are a rare breed. Of the 962 Atlantic hurricanes in the 172-year historical record, only 40 have reached Category 5 strength – about 4% of all Atlantic storms.
But a new study published this week by two prominent climate scientists concludes storms are increasingly reaching deeper into Category 5 territory due to global warming, with its authors introducing a hypothetical Category 6 extension to the classic five-tiered Saffir-Simpson hurricane wind scale to account for ultrapowerful hurricanes fueled by climate change.
What’s a Category 6?
Though the concept of extending the Saffir-Simpson hurricane wind scale to a 6th category isn’t new, it’s not clear where a Category 6 would begin. In a 2014 study published in the scientific journal Geophysical Research Letters, researchers proposed a Category 6 rating once wind speeds top 178 mph. In their provocative study released this week in Proceedings of the National Academy of Sciences (PNAS), authors Michael Wehner and James Kossin set the Category 6 bar notably higher, with maximum winds starting above 192 mph.
The potential damage from a hurricane increases exponentially with its wind speed, so a 195 mph Category 6 hurricane would be potentially four times more destructive than a 160 mph Category 5 hurricane and nearly 2,000 times more devastating than an entry-level 75 mph hurricane.
Category 6-like storms on the rise
Wehner and Kossin examined all hurricane-strength storms globally from 1980 to 2021, when intensity estimates from global satellite coverage are most consistent and reliable.
During the 42-year period, they found 197 Category 5 equivalent storms worldwide, but only five storms that would meet their hypothetical Category 6 threshold: Typhoon Haiyan (2013), Hurricane Patricia (2015), Typhoon Meranti (2016), Typhoon Goni (2020), and Typhoon Surigae (2021). All but one of these hypothetical Category 6 storms occurred in the active western North Pacific, and each happened in the last 9 years of the period studied.
No Atlantic storms since 1980 met the study’s 192-plus mile Category 6 threshold.
The authors also assess, using state-of-the-art climate models, where and when hypothetical Category 6 hurricanes might be more common today and in the future. They find not only an observed increase in the number of days where the atmosphere and oceans support Category 6 strength hurricanes – most notably in the Gulf of Mexico and Caribbean on the Atlantic side – but also a marked increase in the number of days under future warming when conditions could support Category 6 strength storms.
The upshot, according to the authors, is that human-caused global warming has increased the global risk of high-end Category 5 hurricanes (hypothetical Category 6 hurricanes) over the past 40-plus years.
Does this mean we need a Category 6?
The Saffir-Simpson hurricane wind scale was developed as a communication tool. On the eve of his 100th birthday in 2012, Bob Simpson told me his experience on the ground after Camille inspired the rating scale to convince people it was time to leave.
In fact, the original prototype scale he and renowned Miami-based wind engineer Herb Saffir designed together – known as the Hurricane Disaster Potential Scale – included evacuation recommendations for each scale number to evoke action in threatened populations.
Though intended to simplify hazard communication of hurricanes, the Saffir-Simpson Hurricane Wind Scale has been problematic over the years. Initially based only on wind speed, later versions added other threats like storm surge and flooding, hazards that aren’t only affected by a hurricane’s winds. With a run of destructive hurricanes from 2004-2008, it became evident that a single scale would never accurately convey the destructive potential of a hurricane. The NHC in 2010 stripped the Saffir-Simpson scale back to a wind-only rating system to help reduce public confusion.
“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,” said National Hurricane Center Director Michael Brennan in a statement to WPLG.
He pointed out that water, not wind, is responsible for about 90 percent of tropical cyclone deaths in the United States.
“So, we don’t want to over-emphasize the wind hazard by placing too much emphasis on the category,” Brennan said.
Brennan also noted that a Category 5 rating on the Saffir-Simpson Hurricane Wind Scale already considers “catastrophic damage.”
Including a Category 6 wouldn’t add to the messaging nor would it change precautions taken by the public or officials prior to the arrival of potentially catastrophic winds. Extending the scale to a 6th category could even have the unintended consequence of implying less danger from Category 5 hurricanes.
Simpson opposed adding a Category 6 because, as he noted in a 1991 interview, the potential wind damage above Category 5 strength is “immaterial.”
Moving away from scales
Former Local 10 Hurricane Specialist and NHC Director Max Mayfield once famously said, “I think Camille killed more during Katrina than it did in 1969.” Katrina and Camille struck only miles apart in coastal Mississippi but Category 3 Katrina at landfall was many times deadlier than Category 5 Camille when it came ashore some 36 years earlier. The record-setting 24-foot storm surge Bob Simpson measured after Camille was exceeded only by the 28-foot storm surge wrought by Hurricane Katrina.
Overreliance on scales encourages the comparison of hurricanes that are often more different than alike. In 2022, Hurricane Ian struck at the same spot as Hurricane Charley in 2004 at the same strength and category.
Ian was a $112 billion disaster with over 150 fatalities and a catastrophic 15-foot storm surge. Charley was responsible for $16 billion in damages, over 30 deaths, and a 7-foot storm surge. While the category may have suggested similar impacts, Ian’s large size left a much bigger footprint on southwest Florida.
Regardless of future hurricane strength, treating each storm individually, shining a spotlight on the hazards and avoiding the trap of comparing the next one to the last one is the best way to honor the legacy of Robert Simpson, Herb Saffir, and those lives affected by the destructive storms of the past.