A new study provides observational evidence that the odds of major hurricanes around the world — Category 3, 4 and 5 storms — are increasing because of human-caused global warming. The implications of this finding, published Monday in the Proceedings of the National Academy of Sciences, are far-reaching for coastal residents, insurers and policymakers, as the most intense hurricanes cause the most damage.
The study, by a group of researchers at the National Oceanic and Atmospheric Administration and the University of Wisconsin at Madison, builds on previous research that found a trend, though not a statistically robust one, toward stronger tropical cyclones.
Tropical cyclones are a category of storms including hurricanes and typhoons worldwide. The findings are consistent with what scientists expect to happen as the world warms, given that hurricanes get their energy from warm ocean waters and water vapor in the air, among other factors.
Importantly, the observed, statistically significant trends match projections seen in computer model simulations of a warming world.
“We’ve just increased our confidence of our understanding of the link between hurricane intensity and climate change,” said James Kossin, the lead author of the new study and a researcher with NOAA and the University of Wisconsin. “We have high confidence that there is a human fingerprint on these changes.”
The study finds a global increase of about 8 percent per decade of the likelihood that a given tropical cyclone will become a Category 3 or greater storm.
With powerful hurricanes on the increase, one can expect damage costs, in dollar terms and in potential loss of life, to skyrocket. “Hurricane destruction in the United States, in terms of physical damage costs, has historically increased by 10 percent for every 5 mph increase in wind speed,” meteorologist James B. Elsner of Florida State University said in an email. Elsner was not involved in the new research, and he published a study with Kossin and others in 2008 that relied on different evidence to find an initial uptick in the occurrence of major tropical cyclones worldwide.
Answering one of hurricane researchers’ biggest questions
The big question during the past two decades of hurricane research has been whether there is already a detectable trend toward stronger storms, or whether this will emerge in the future, said Suzana Camargo, a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory.
The new study answers that question by showing there is already a significant detectable trend, she said in an interview. Camargo was not associated with the new research.
Kerry Emanuel, a hurricane expert at MIT, said researchers determined that “there’s a robust global increase in the proportion of tropical cyclones that get to a high intensity.” Emanuel was not involved in the new study, but his research has shown that global warming is likely to increase what’s known as a storm’s “maximum potential intensity,” or how powerful it could get based on its surrounding environment.
The new research attempts to overcome the one huge problem that has been vexing hurricane researchers for years: the issues with tropical cyclone data.
Multiple methods have been used over the years to track and gauge the intensity of such storms, which can include flying into them, as is done in the North Atlantic Ocean Basin; using algorithms to deduce storm intensity from satellite imagery; and ship-based reports.
The study grapples with inconsistent data collection methods by creating a 39-year data set (1979 to 2017) of satellite-based storm intensity estimates for tropical cyclones around the world. The researchers then examined their more homogenous record for any clear trends. The work builds upon an earlier study, done by this same group in 2013, that used a 28-year period of data and found an upward trend but fell short of statistical significance.
Now, though, the trends are stronger and more obvious. In addition to seeing stronger, wetter storms, there is also a trend toward hurricanes that suddenly make leaps in their intensity, much as Cyclone Amphan has done this week in the Bay of Bengal.
“We have a significantly building body of evidence that these storms have already changed in very substantial ways, and all of them are dangerous,” Kossin said.
Gabriel Vecchi, a hurricane expert at Princeton University who was not involved in the new research, said the study shows an uptick from 30 percent of storms being major to about 40 percent of storms being major, calling that “a pretty large increase.”
“It’s a gratifying confirmation of what the field was concluding anyway for researchers … a much more robust indicator that now we’re really seeing this in nature,” Emanuel said of the new study.
Emanuel says the hurricane research community is more confident about particular conclusions, namely that storms are bringing more rainfall due to warming oceans and the added moisture content of the atmosphere. Other aspects of tropical cyclones, such as the hypothesis that they are tending to slow down as they go near or travel over land, greatly exacerbating flood severity, are not yet widely agreed upon, he said.
Numerous questions remain. For example, the study found that trends toward stronger storms are clearest in the north Atlantic, southern Indian and eastern north Pacific oceans. The western north Pacific Ocean, where most of the world’s tropical cyclones, including the most intense storms, tend to form, showed little to no trend.
Camargo says this may be because of a poleward shift in where these storms tend to occur, which itself is thought to be tied to climate change. As storms move closer to Japan and away from the Philippines, for example, their potential intensity decreases a bit, which may be masking the trend the new study tried to detect.
Tom Knutson, an NOAA researcher who studies hurricanes and climate change, said the study may be capturing aspects of natural variability in some ocean basins, tempering somewhat the significance of its conclusions. For example, changes in ocean currents and climate cycles that operate on the scale of decades can turn the dial up or down on the number and intensity of storms in the Atlantic.
Vecchi also said that increasing amounts of carbon dioxide from human activities are one piece of the puzzle and that changing ocean currents and other factors could be contributing to the trends identified in the new study.
The bottom line is that dangers are growing for residents of the world’s coastlines. As of 2010, 39 percent of the U.S. population resided in coastal shoreline counties, according to NOAA.
While the study analyzes changes in storm intensity through 2017, record-setting hurricanes have continued erupting up to the present. The past five to 10 years have been a period of exceptionally strong, record-setting tropical cyclones in the Atlantic and several other ocean basins.
Between 2016 and 2019 in the Atlantic, storms attained Category 5 strength in four straight years for the first time on record in the satellite era of storm monitoring (dating to the 1960s). Six of the Atlantic’s 26 Category 5 storms in the satellite era have occurred since 2016, each of which was notable for its intensity:
- Hurricane Dorian, in 2019, which ravaged the Bahamas, tied for the second-strongest storm recorded in the Atlantic Ocean, packing winds of 185 mph. These peak winds were the strongest so far north observed in the Atlantic Ocean east of Florida.
- Hurricane Lorenzo, in 2019, became a Category 5 at the farthest northeastern point on record in the Atlantic Ocean. It became a Category 5 over 600 miles east-northeast of the previously farthest-east Category 5 storms on record, Isabel and Hugo.
- Hurricane Michael in 2018 made landfall on the Florida Panhandle, the latest a storm this strong has ever struck U.S. shores.
- Hurricane Irma, in 2017, with peak winds of 185 mph, tied for the second-strongest Atlantic hurricane on record (with Dorian in 2019, Wilma in 2005, Gilbert in 1988 and the 1935 Florida Keys Hurricane). The storm maintained maximum wind speeds of at least 180 mph longer than any other storm on Earth in recorded history.
- Hurricane Maria, in 2017, saw its peak winds increase 70 mph in only 18 hours, a rate of intensification only exceeded by Hurricanes Wilma (2005), Felix (2007) and Ike (2008).
- Hurricane Matthew, in 2016, became the southernmost Category 5 on record in the Atlantic. As its winds increased 80 mph in 24 hours as it leaped from a Category 1 to 5, its rate of intensification was the third-fastest on record in the Atlantic, exceeded only by Hurricanes Wilma (2005) and Felix (2007).
Storms in several other ocean basins, in both hemispheres, have also demonstrated exceptional might in the past five to 10 years:
- Super typhoon Haiyan, in 2013, with peak winds of 195 mph, became the strongest tropical cyclone in the northwest Pacific and Eastern Hemisphere based on wind speed.
- Hurricane Patricia, in 2015, with peak winds of 215 mph, became the strongest storm on record measured by the National Hurricane Center in the northeast Pacific.
- Tropical cyclone Winston, in 2016, with peak winds of 185 winds, became the strongest storm on record in the South Pacific.
2020 hurricane season raises concerns
This year may prove to be yet another exceptional year for these tempests. Just this week, Cyclone Amphan catapulted to a Category 5 in the Indian Ocean, intensifying by 110 mph in just 36 hours. “That would certainly put it in a rare class of [rapid intensification] events globally,” meteorologist Alex Lamers said via Twitter.
Forecasters are concerned that a much busier-than-normal season may lie ahead for the tropical Atlantic. When Tropical Storm Arthur formed Saturday night, it launched the Atlantic hurricane season before its official June 1 start for the sixth straight year.
A noted uptick in early-season storms — probably linked to warming waters fueled by climate change — has been observed in recent years.
Looking ahead, warmer-than-normal ocean temperatures in the Atlantic and record-setting waters in parts of the Gulf of Mexico and Caribbean could fuel more extremely potent storms.