Above: Satellite image of Hurricane Idalia in August 2023. Image courtesy of CNN.

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As the planet warms, hurricanes are growing more powerful and pushing the limits of the current Saffir–Simpson Hurricane Wind Scale. A recent study by  Dr.Michael Wehner, a senior scientist at Lawrence Berkeley National Laboratory, and James Kossin, a climate scientist at the University of Wisconsin-Madison, introduces a hypothetical sixth category to the scale with the aim of better communicating the increasing dangers of intensified storms.

Above: Infrared satellite image of Hurricane Katrina in 2005. Image courtesy of CIMSS. See this animation: https://cimss.ssec.wisc.edu/satellite-blog/wp-content/uploads/sites/5/2015/08/960x1280_AGOES12_B4_KATRINAIR4_animated_2005234_001500_78_2005242_234500_78_IR4AVHRR4.gif 

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What Is a Hurricane?

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, strong winds, and heavy rainfall. Once a tropical cyclone reaches sustained wind speeds of 74 miles per hour (mph), it qualifies as a hurricane. Tropical cyclones have different names depending on the region in which they form. Although all refer to the same type of storm, they’re known as hurricanes in the North Atlantic, central North Pacific, and eastern North Pacific; cyclones in the South Pacific and Indian Ocean; and typhoons in the Northwest Pacific. 

Above: Structure of a hurricane. Image courtesy of Penn State.

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Broadly, hurricane formation involves the transfer of heat energy from the ocean to the atmosphere. Tropical cyclones form over warm ocean waters near the equator because these heated waters are a plentiful source of thermal ‘fuel’ for the storm. The heat in these regions causes the warm water to evaporate, turning into water vapor and rising into the atmosphere. As the warm, moist air rises it creates an area of low pressure at the ocean's surface. The Coriolis Effect, or the directional circulation of air due to Earth’s rotation, causes the rising air to start spinning. In the Northern Hemisphere, the air spins counterclockwise; in the Southern Hemisphere, it spins clockwise. As the spinning air continues to rise and cool it condenses into clouds and releases more heat, further driving the storm’s growth. The eye of the storm—a circle of calm weather in the middle of intense thunderstorms—forms as warm air is drawn into the cyclone’s center. Once sustained, hurricanes can either make landfall, causing widespread damage with strong winds and heavy rain, or dissipate as they move over cooler waters or encounter wind shear.

Above: (a) Trend in global temperature anomaly. (b) Global sea surface temperature anomaly in 2020. Image courtesy of Reed et al.

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The Effect of Climate Change on Hurricanes

Over the past few decades, climate change has made hurricanes stronger and more destructive than ever before due to global warming and rising sea levels. Hurricanes are fueled by heat energy from warm ocean water. In recent years, anthropogenic climate change has trapped large amounts of heat on Earth—90% of which is harbored in oceans. For example, this past summer, water temperatures off the Florida coast broke 100°F (temperatures comparable to a hot tub). This increasing abundance of heat energy enables the formation of longer-lasting, more powerful storms. Additionally, warmer air can hold more water, which is subsequently released in torrential rain. One study found that nearly 20% more rain fell in Texas during Hurricane Harvey (2017) than would have without the influence of global warming. Researchers found similar metrics for Hurricanes Katrina (2005), Irma (2017), and Rita (2005). 

Rising sea levels caused by climate change have also exacerbated storm surge, the phenomenon in which waters rise above normal levels and are pushed inward by a storm’s wind. Storm surge flooding caused the majority of damage to New Orleans during Hurricane Katrina and $8 billion in damage after Superstorm Sandy (2012) hit New York. Scientists have also found that hurricanes are experiencing slower momentum after formation, causing them to stall over land for longer periods of time while dumping wind and rain. On top of this, scientists have found an increased probability of communities experiencing large storms back-to-back. This development poses great challenges, as limited disaster relief resources make recovery difficult after even just one storm. Climate models have demonstrated that global warming can increase the range of locations and seasons in which hurricanes can form. Warming oceans have allowed storms to form as early as January 2023, while hurricane season in the past has been confined to June 1 to November 30.

Above: The Saffir-Simpson Hurricane Wind Scale. Image courtesy of Forbes.

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The Saffir-Simpson Scale

In the 1970s, the United States National Hurricane Center introduced the Saffir-Simpson Scale as a tool to alert the public about the severity of hurricanes. This scale rates hurricanes from Category 1 to 5 based on the storm’s maximum sustained wind speed. The higher the speed, the higher the category, with storms with winds 157 mph or higher qualifying as Category 5. However, this scale is based solely on wind speeds and does not consider other factors such as storm surge and flooding, which are often the most deadly aspects of land-falling hurricanes. Scientists suggest that the open-endedness of the scale may no longer be adequate as climate change causes storms to become increasingly stronger with each passing season.

Above: Satellite image of Hurricane Patricia, the strongest hurricane in the history of the Western Hemisphere. Image courtesy of Penn State.

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The New Category 6 

The new study by Wehner and Kossin outlined a hypothetical expansion of the Saffir-Simpson Scale to include a Category 6 ranking. On the new scale, Category 5 would be capped at 192 mph and storms with maximum sustained wind speeds exceeding this speed would fall under Category 6. The study found that several recent storms have already achieved this status. Of the 197 Category 5 tropical cyclones that occurred between 1980 and 2021, five reached Category 6 status. This group includes Hurricane Patricia, which hit Texas, Mexico, and Central America in 2015, and is the strongest hurricane in the recorded history of the Western Hemisphere with wind speeds of 215 mph.

The authors expect that as the climate continues to warm, tropical cyclones will become more intense and Category 6 storms more commonplace. While the new scale would underscore the reality that storms are intensifying in terms of wind speed, it still does not address other dangers of hurricanes such as storm surge and flooding which represent the cause of 90% of hurricane-related deaths in the United States. Thus, scientists disagree about creating a new category. Some argue that a new classification in the Saffir-Simpson Scale would place too much focus on wind. Nonetheless, the potential introduction of Category 6 brings attention to an indisputable truth: hurricanes are becoming more intense as the climate changes.