Scientists Propose New Category 6 for Stronger Hurricanes Linked to Climate Change

The typhoon-ravaged city of Tacloban, Leyte, the Philippines on Nov. 14, 2013, after Typhoon Haiyan hit the city on Nov. 8 as the strongest storm ever recorded at landfall. Jan Hetfleisch / Getty Images

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In a new study, scientists proposed adding a Category 6 on the Saffir-Simpson Hurricane Wind Scale to describe stronger hurricanes linked to global warming.

The study titled “The growing inadequacy of an open-ended Saffir–Simpson hurricane wind scale in a warming world” and published in Proceedings of the National Academy of Sciences noted that global warming and rising ocean temperatures were contributing to stronger hurricanes. 

The climate scientists behind the study, Michael Wehner of Lawrence Berkeley National Laboratory (Berkeley Lab) and James Kossin of First Street Foundation, wrote that tropical cyclones have become so intense amid global warming that a higher category may be necessary to convey the greater dangers of more powerful storms.

The Saffir-Simpson Hurricane Wind Scale has been used for over five decades, ranking hurricanes from Category 1 to Category 5. As the study authors noted, the highest category, Category 5, has no upper limit; instead it encompasses hurricanes with wind speeds of 157 mph or more. According to the scale, a Category 5 hurricane includes a risk of “catastrophic damage” that will destroy framed homes, fell trees and lead to long-lasting power outages.

But the scientists pointed out that greater wind speeds do increase intensity and risk that could be better explained with an additional category.

“Our motivation is to reconsider how the open-endedness of the Saffir-Simpson Scale can lead to underestimation of risk, and, in particular, how this underestimation becomes increasingly problematic in a warming world,” Wehner said in a statement.

The proposal would define Category 5 storms from 70 to 80 m/s (about 157 to 192 mph), then add a Category 6 ranking for storms with winds over 192 mph. The proposed Category 6 could already apply to previous tropical storms, including Typhoon Haiyan, Hurricane Patricia, Typhoon Goni and Typhoon Meranti.

The study revealed that the risk of storms that could be categorized in the proposed Category 6 would increase by 50% around the Philippines and doubles around the Gulf of Mexico if global warming reaches 2°C compared to preindustrial levels. Overall, the study authors found that the risk of storms with an intensity that could rank as Category 6 has more than doubled since 1979.

“Tropical cyclone risk messaging is a very active topic, and changes in messaging are necessary to better inform the public about inland flooding and storm surge, phenomena that a wind-based scale is only tangentially relevant to. While adding a sixth category to the Saffir–Simpson Hurricane Wind Scale would not solve that issue, it could raise awareness about the perils of the increased risk of major hurricanes due to global warming,” Kossin explained. “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.”

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