Five tropical cyclones in the past 9 years have hit wind speeds far above the category 5 threshold, causing thousands of fatalities and billions of dollars of damage. Such ultrastrong, highly destructive hurricanes are becoming more likely as climate change increases the amount of energy available to storms.
“Storms are getting stronger and stronger, so category 5 underestimates actual risk.”
In a new study published in the Proceedings of the National Academy of Sciences of the United States of America, scientists suggest that the growing intensification of tropical cyclones may necessitate adding a sixth category to the Saffir-Simpson Hurricane Wind Scale. Doing so could be one useful tool not only to indicate hurricane risk but also to convey the increasing dangers of climate change.
“Storms are getting stronger and stronger, so category 5 underestimates actual risk,” said James Kossin, an author on the paper and an atmospheric scientist at the University of Wisconsin–Madison.
Warming Winds
The Saffir-Simpson scale is the most widely recognized hurricane intensity scale, ranking storms from “tropical depression,” at wind speeds less than 38 miles per hour (61 kilometers per hour), to “category 5 hurricane,” at wind speeds greater than 157 miles per hour (253 kilometers per hour).
That scale may not capture the risk posed by the most intense storms as the world warms, the authors wrote. They suggest a sixth category that encompasses storms with winds greater than 192 miles per hour (309 kilometers per hour).
The authors used three lines of evidence to support the creation of a sixth category. First, multiple storms have already spilled over into the hypothetical category 6. Typhoon Haiyan, for example, made landfall in the Philippines in 2013 and had winds that reached 195 miles per hour (314 kilometers per hour). Haiyan was the costliest storm ever to hit the country and one of the deadliest, causing more than 6,000 fatalities. In 2015, Hurricane Patricia—considered the strongest hurricane ever recorded—brought winds of up to 215 miles per hour (346 kilometers per hour) to southwest Mexico.
Climate change has likely contributed to the intensification of tropical storms, according to the Intergovernmental Panel on Climate Change, the United Nations body that assesses climate science.
The authors also analyzed the maximum potential intensity of storms in recent decades. That metric refers to the highest wind speeds that are possible on a given day given that day’s weather conditions. They found that in the Gulf of Mexico between 1979 and 2019, conditions were conducive to category 6 hurricanes about 10 days a year.
The number of days conducive to category 6 wind speeds has increased because of climate change, said Kossin.
Last, the authors modeled future hurricanes under various climate change scenarios and found that under each possible scenario, the risk of a category 6 hurricane increased. “Over the next decade, there will be category 6 [hurricanes],” said Michael Wehner, an author on the paper and a climate scientist at the Lawrence Berkeley National Laboratory.
Communicating Climate Change
Communication of risks shouldn’t focus only on the Saffir-Simpson scale, according to Michael Brennan, the director of NOAA’s National Hurricane Center (NHC). Most fatalities caused by hurricanes occur not from wind but from water, including storm surges and rain.
“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,” he wrote in an email. “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.”
“The reality is that hurricanes have changed already. This creates the need to discuss whether the systems that we currently have in place are adequate for the future.”
The question of whether a category 6 would be an effective communication tool requires a larger discussion, with input from social scientists, psychologists, emergency managers, and city planners, Kossin said. He said he hopes the idea of a hypothetical category 6 will spark more discussion of how to warn people about all hurricane-related risks, including wind, storm surge, and rainfall, as hurricanes continue to intensify.
“What we’re trying to highlight is not the immediate danger of an impending storm,” Wehner said. “That kind of thing is already out there. What we’re trying to communicate is that the risk of the most intense storms is increasing because of climate change.”
Kevin Reed, a climate and atmospheric scientist at Stony Brook University who was not involved in the new study, said that expanding the Saffir-Simpson scale would not only indicate increased risks from individual storms but highlight the worsening risks of climate change in general.
“The reality is that hurricanes have changed already,” Reed said. “This creates the need to discuss whether the systems that we currently have in place are adequate for the future.”
—Grace van Deelen (@GVD__), Staff Writer