'Monster' hurricanes keep getting even stronger, analysis shows

A satellite loop showing Laura progress from a Tropical Storm over the Caribbean to a Category 4 hurricane that made landfall on the U.S. Gulf Coast. (AccuWeather)

An analysis by AccuWeather meteorologists and scientists shows that while the overall intensity of hurricane seasons appears to be trending downward, the storms that gain hurricane strength have been growing stronger.

The simple answer to what is driving this trend lies in the fact that there is more warm water and warm, moist air in the atmosphere that serve as fuel to help maintain the intensity and strength of the storms. However, the longer answer involves a deeper dive into the data and a look at weather mechanics.

At its core, weather tells the story of a world trying to maintain equilibrium from pole to pole.

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"Nature seeks equilibrium. If you put ice in a glass of water, it will eventually come to room temperature," explained Scott Mackaro, AccuWeather's vice president of science and innovation from 2019 to 2022. "The poles of the Earth are cold, and the equator is warm. Again, nature will seek equilibrium and attempt to mix everything together. This is the basics of why we have weather."

Hurricanes serve as an example of this, he added.

"And so hurricanes serve as an extreme example of nature seeking equilibrium. It's a very natural and very necessary energy exchange mechanism," Mackaro said.

Mackaro gathered data from between 2020 and 1851, when record-keeping on hurricanes began, and plotted out the ACE Index of each season.

The ACE Index, short for the accumulated cyclone energy index, is a measurement that factors in a hurricane's intensity by accounting for both its strength and duration. Stronger, longer-lasting storms will score higher than weaker, shorter-lived storms. When the ACE Index totals of each storm in a season are combined, it tells the intensity of the season.

"ACE tells you the tale of the season," AccuWeather lead hurricane expert Dan Kottlowski said.

If only the ACE Index and time are considered, the resulting graph will show an upward trend in the ACE Index overall. However, this is misleading, Mackaro noted, as developing technology has increased the number of storms scientists and meteorologists are able to observe.

There are typically a handful of storms each season that could have easily been missed or mislabeled before the satellite era due to the fact that they were short-lived or mostly occurred in areas that would have been a blindspot for the available technology, according to Kottlowski.

To account for the possibility that storms in the past could have been missed in the record books, Mackaro took the ACE Index of each season, divided this by the number of named storms then added another line charting the ACE Index of the hurricanes of each season divided by the number of them.

What he found was that the ACE Index of all seasons actually trended downward while the ACE Index of all of the hurricanes separated from the rest of the storms actually trended upward.

"When you add all the ACE units up for a storm, then you have all these weaker storms, it brings down the curve," Kottlowski explained.

Even though the ACE total of a season can be a bit more tricky to calculate for seasons before the 1967 Atlantic hurricane season -- the first with modern-day satellites to track the lifespan of a storm -- the trend remains.

Since at least 1967, and possibly since 1851, hurricanes in the Atlantic Ocean basin have steadily been strengthening and intensifying.

FILE - In this Sept. 28, 2017, file photo, debris scatters a destroyed community in the aftermath of Hurricane Maria in Toa Alta, Puerto Rico. Nasty hurricanes that cause billions of dollars in damage are hitting more often. Laura, which is threatening the U.S. Gulf Coast, is only the latest. (AP Photo/Gerald Herbert, File)

In the story weather creates, one of its chapters highlights the planet's reaction to a warming atmosphere.

"One of the reasons for [strengthening hurricanes] is the fact that again, the climate has been warming -- there's no doubt about that," Kottlowski said. "All the information we're looking at right now suggests that the average temperature continues to rise."

At the start of January 2023, the Copernicus Climate Change Service announced that the past eight years have been the warmest since record-keeping began in 1950, with 2022 ranking as the world's fifth warmest year on record.

Global air temperature at a height of two meters for 2022 shown relative to its 1991-2020 average. (ERA5/Copericus Climate Change Service/ECMWF)

"The atmosphere and the oceans have this synergy, so if the atmosphere gets too warm, some of that warmth goes ... into the ocean. So the ocean's kind of trying to balance out what's going on with respect to the atmosphere." Kottlowski said. "We call this a heat-energy budget."

Kottlowski notes that the Earth is collectively absorbing more heat than its repelling into outer space most likely due to the carbon dioxide build-up. As this happens, the oceans act as a "heat sink," absorbing some of that excess heat.

A secondary reason for the high ocean temperatures could be attributed to a positive phase in the climate pattern in the North Atlantic called the Atlantic Multi-decadal Oscillation (AMO). Positive phases of this cycle usually produce higher ocean temperatures and higher hurricane activity in the Atlantic, and the one we're in began in 1995. The phases of this cycle typically last 25 to 30 years.

However, Kottlowski stressed that that warming of the Earth's atmosphere is a large part of the reason the oceans are still warm.

"Bottom line is the atmosphere is heating up, the oceans are trying to balance that out, and as the oceans absorb that heat, water temperatures, especially sea surface temperatures -- the top layer of the ocean -- are continuing to rise," Kottlowski said. "And that consistent warming may be overshadowing this AMO process."

Linda Smoot, who evacuated from Hurricane Laura in a pickup truck with eight others, reacts as they return to see their homes, in Lake Charles, La., in the aftermath of the hurricane, Sunday, Aug. 30, 2020. (AP Photo/Gerald Herbert)

While it is possible to see a less active hurricane season in a positive AMO year, it isn't worth lowering your guard.

From 2016 to 2020, there were a lot of tropical storms and hurricanes that affected the United States, AccuWeather Founder and CEO Dr. Joel N. Myers said when Mackaro released his findings. That trend continued into 2021 and 2022 with destructive strikes from Hurricane Ida, Ian and Nicole among others. "But most of the previous 10 years [before 2016], you had a lull in hurricanes hitting the coast," Myers said.

Kottlowski pointed out that from 2014 to 2016, there was less activity than normal due to cooler ocean surfaces in "key areas" of the Atlantic. It wasn't necessarily that the entire ocean was cooler -- just the areas important to churning up hurricanes.

Then came 2017.

That season brought forward some heavy hitters: Hurricane Harvey, Hurricane Irma and Hurricane Maria, to name a few. Since then, each season in the Atlantic has churned up at least one Category 5 hurricane.

FILE - This Tuesday, Nov. 10, 2020, satellite image made available by NOAA shows Tropical Storm Eta at 10:40 a.m. EST in the Gulf of Mexico, Theta, right, and a tropical wave to the south that became Tropical Storm Iota. An overheating world obliterated weather records in 2020 — an extreme year for hurricanes, wildfires, heat waves, floods, droughts and ice melt — the United Nations’ weather agency reported Wednesday, Dec. 2, 2020. (NOAA via AP)

"What we have to do is we have to mitigate," Kottlowski said, which would mean building in smarter ways along coastal areas that can withstand higher category storms along with other solutions. That may also mean not building homes or businesses right at the coast, but four or five miles inland to avoid rising storm surge and insurance rates.

Kottlowski warned that now, any storm that develops in the Atlantic or the Gulf of Mexico during the heart of hurricane season shouldn't be underestimated.

"We have to assume," he said, "that these storms could become monsters."

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