Hurricane experts are debating adding a Category 6 on the current scale to highlight the increasing possibility of stronger storms as ocean waters continue to warm.
Researchers are introducing an extension of the Saffir-Simpson Hurricane Wind Scale, the system used by the National Hurricane Center to rank the strength of hurricanes, to include a sixth category, beyond the Category 5 classification that indicates a storm with sustained winds of 157 mph or more, according to a study published in the Proceedings of the National Academy of Sciences on Monday.
The scale currently rates a tropical system as a Category 1 hurricane once sustained winds reach 70 mph. But the Saffir-Simpson scale is "far from perfect," especially in terms of adequately warning residents in impacted regions of the potential dangers to come, Michael Wehner, a senior scientist at the Lawrence Berkeley National Laboratory and one of the authors of the paper, told ABC News.
"Some Category 3 storms are really deadly, and some Category 5 storms aren't by the time they make landfall," Wehner said.
The authors propose a Category 6 hurricane rating is necessary starting at 193 mph of sustained wind, though Wehner admitted the choice of where to set that limit was somewhat arbitrary.
When the scale was first introduced in the 1970s, it accounted for wind and water-driven destruction from storm surge, the paper highlighted. In 2010, the National Hurricane Center altered the scale to only account for wind.
In addition, only 8% of tropical storm deaths are directly related to wind, while 76% of the mortality is related to storm surge and flooding from heavy rain, the authors noted.
Messaging around hurricane risk is a very active topic among researchers, and adding a sixth category is a way to raise awareness about the increased risks of supersized hurricanes due to global warming, James Kossin, distinguished science adviser at environmental nonprofit First Street Foundation and the other author of the study, said in a statement.
"Our results are not meant to propose changes to this scale, but rather to raise awareness that the wind-hazard risk from storms presently designated as Category 5 has increased and will continue to increase under climate change," Kossin said.
When examining extremes in hurricane wind speeds to determine whether the open-ended Category 5 is sufficient to communicate risk in a warming climate, the researchers found "a new breed of storms" in the last decade, Wehner said.
Within that time frame, there have been multiple storms that may "have exceeded this hypothetical Category 6," Wehner said.
The study looked at all 197 tropical cyclones around the world that were classified as Category 5 from 1980 to 2021, which comprises the period of highest quality and most consistent data, the authors said. Half of the Category 5 storms occurred in the last 17 years of that period, according to the study.
Five of the storms examined exceeded the authors' proposed Category 6, and all of them occurred in the last nine years of the record. Of the five storms achieving the proposed Category 6 status, one of them was Hurricane Patricia in 2015, which had weakened to a Category 4 as it made landfall in western Mexico. The other four storms were typhoons in the Western Pacific.
"If you looked at the beginning of the satellite record around 1980, there was essentially no risk of the Category 6 storm in 1980," Wehner said. "The trend in the speed limit is actually pretty pretty strong."
However, no hurricane in the Atlantic Ocean or the Gulf of Mexico has reached the study's proposed Category 6 level. Typhoons in the Western Pacific do not use the Saffir-Simpson scale for intensity ratings.
Statistical analysis showed the rapidly increasing risk is due to human changes in the atmosphere and not natural variability, Wehner said.
Human-induced climate change and warming waters are fueling tropical cyclones with much windier conditions than the initial Category 5 threshold entailed, at 157 mph, the authors said.
"We expected that the strongest storms will become stronger," Wehner said of how climate change will affect future tropical systems.
It will be necessary to change the "speed limit" of future hurricanes because as global temperatures rise, conditions like warmer ocean temperatures will increase the risk of seeing these so-called "Category 6" storms, Wehner said.
Warmer waters are also fueling rapid intensification as storms approach the coasts, "which is very dangerous because it doesn't give people a lot of time to prepare," Jennifer Collins, a hurricane researcher at the University of South Florida who was not involved in the study, told ABC News.
Other experts have raised concerns that adding a Category 6 to the scale increases the chance of people underestimating the risks from storms that are lower than the highest category. For example, if they chose not to evacuate for a Category 4 or 5 storm because the higher category makes lower-rated hurricanes appear less dangerous.
National Hurricane Center Director Michael Brennan said in a statement that they're focused on communicating the hazards and they don't want to overemphasize the wind hazard by placing too much emphasis on the category system.
"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 said in a statement.
In addition, Category 5 on the Saffir-Simpson scale already captures "catastrophic damage" from wind, so it's not clear that there would be a need for another category even if storms were to get stronger, Brennan said.
In response, Wehner described the hurricane center's comments assessment as "completely appropriate."
"We don't expect that the hurricane center or [World Meteorological Organization] will add this Category 6. It's certainly not for us to tell them what to do," Wehner said. "That's not what we intended."