Climate change appears to be bringing more frequent and stronger storms, and the threat may not be properly communicated to those on the front lines. Now, scientists at Lawrence Berkeley National Laboratory believe Category 6 storms have room to grow in intensity -- five storms have already reached that intensity in the past decade.
Currently, the National Hurricane Center uses a standard called the Saffir-Simpson Windscale to classify the intensity of hurricanes in the Western Hemisphere and reminds people in the region to take appropriate precautions. The Saffir-Simpson scale is based on a storm's average maximum wind speed over a minute - Category 1 hurricanes have winds of 74 mph (119 km/h), and then go through different thresholds all the way up to Category 5, which has winds in excess of 157 mph (252 km/h).
But with winds becoming exponentially more damaging and hurricanes increasing in intensity in recent years, scientists at Berkeley Lab and the First Street Foundation aren't sure the scale reflects the full story.
"Our motivation was to reconsider how the open-ended nature of the Saffir-Simpson scale can lead to underestimation of risk, especially how such underestimation becomes increasingly problematic in a warming world," said Michael Weiner, lead author of the study.
Tropical storms form when warm ocean waters interact with warm, moist air, and these temperatures are rising rapidly due to human-induced climate change. This appears to not only increase the intensity of the hurricane, but also increase the rate at which it becomes more powerful.
In the new study, the team added a hypothetical category to the scale. Based on the scope of the lower categories, they suggested Category 5 would include storms with winds between 157 and 192 mph (252 and 309 kph), while the new Category 6 would include any storms with winds above that limit.
Researchers analyzed historical hurricane data from 1980 to 2021 and found five storms that were strong enough to reach the hypothetical Category 6. These include Hurricane Patricia, the most powerful tropical cyclone on record that hit Central America in 2015 with winds of up to 215 mph (345 km/h).
Worryingly, but perhaps not surprisingly, all five hypothetical Category 6 hurricanes have occurred within the past nine years. This demonstrates the impact of climate change on hurricane intensity. The research team acknowledges that this study is not a formal recommendation to restructure the Saffir-Simpson scale, but is intended to draw attention to its potential shortcomings.
"Tropical cyclone risk messaging is a very active topic, and there is a need to change messaging to better inform the public about inland flooding and storm surge, for which wind-based wind scales are only marginally related," said James Kossin, a co-author of the study. "While adding a Category 6 to the Saffir-Simpson hurricane wind scale will not solve this problem, it will increase awareness of the dangers of increased risk of major hurricanes due to global warming."
The research was published in the Proceedings of the National Academy of Sciences (PNAS).