The federal government counts 52 tropical cyclones since 1980 that, with the cost adjusted for inflation, have caused, on average, $20 billion in damages in the United States. There were a record seven so-called “billion dollar” cyclones just last year.

With climate change helping produce a global horror film of extreme weather disasters this summer, and the peak Atlantic hurricane and Western fire seasons just arriving, a new landmark United Nations climate report by the Intergovernmental Panel On Climate Change offers the most thorough evaluation of the physical science associated with global warming so far, including the climate science of tropical cyclones.

Made public Monday, the IPCC report was written by 234 scientists from 66 countries and approved by 195 member governments of the IPCC.

To meteorologists, tropical cyclones are rotating, organized systems of clouds and thunderstorms that originate over tropical or subtropical waters. In the North Atlantic, central North Pacific and eastern North Pacific they are called hurricanes once wind speeds reach 74 miles per hour. Major hurricanes, categories 3 to 5, pack wind speeds of 111 miles per hour to more than 157 miles per hour, and cause damage that ranges from devastating to catastrophic.

The IPCC authors did not do original research for their report. Rather, they assessed the state of knowledge and the confidence in it based on previously published scientific reports.

Hugh Willoughby, a professor at Florida International University in the Department of Earth and Environment who has been studying hurricanes for decades, said the IPCC report’s assessment of hurricanes reveals an unsurprising picture of evolving science.

“It is consistent with what we have thought for 40 years,” said Willoughby, who flew more than 400 missions into hurricanes as a meteorologist with the federal government and led the National Oceanic and Atmospheric Administration’s Hurricane Research Center from 1995 to 2002. “What is new is more and more (hurricane) events that fit the pattern, and that is what you would expect.” 

To determine six, key takeaways about tropical cyclones and climate change from the IPCC report, Inside Climate News interviewed Willoughby and two academic researchers whose work was cited by the report’s authors: Colin Zarzycki, an assistant professor of meteorology and climate dynamics at Pennsylvania State University and Phil Klotzbach, a hurricane expert and research scientist at Colorado State University’s Department of Atmospheric Science.

More Major Tropical Cyclones

Scientists cannot yet make claims with a high level of confidence about long-term trends in the frequency of all tropical cyclones.

But it is likely, the report found, that the percentage of major tropical cyclones—those reaching categories 3 to 5—increased over the last four decades. Scientists are confident in saying that there will be more frequent storms in the highest intensity categories of 4 and 5.

The Southeast United States, Central America and the Caribbean Islands have been hammered by very active hurricane seasons in recent seasons, including last year. In 2020, there were 30 named storms—the most on record and almost three times the typical number—fueled by warmer sea surfaces that scientists say were, at least in part, caused by climate change. This season, which runs through October, is also predicted to be very active.

The IPCC authors acknowledged the 2019 IPCC Special Report on Ocean and Cryosphere in a Changing Climate, which attributed “medium confidence” to the assertion of a human contribution to an increase in Atlantic hurricane activity since the 1970s. But, they concluded, “There still is no consensus.”

Looking back in time at tropical cyclone activity is tricky, in part because technology to identify and study cyclones hasn’t always been as good as it is now, Zarzycki said. Scientists probably missed some cyclones entirely before the 1970s, when the first satellites began monitoring the Earth, he added. 

Wetter, Windier Tropical Storms

Warmer air holds more moisture, and human-driven climate change is making tropical cyclones wetter. The authors found that, generally, storms with extreme daily precipitation are projected to intensify by about 7 percent for each 1 degree Celsius of global temperature warming.

The authors also expressed high confidence in projections that peak wind speeds in the most intense tropical cyclones—categories 4 and 5—will increase with increasing global warming.

Tropical cyclone intensity and the rates of rainfall act to further elevate storm surges, and rising seas from global warming will likely exacerbate the storm surge from future tropical cyclones.

Tropical Storms are Shifting North

In the North Pacific, tropical cyclones are reaching their maximum intensity farther north than before, potentially exposing areas that have no experience with tropical cyclones to a risk of dealing with them in the future. The report said this is partly explained by changes in global-scale tropical atmospheric circulation. 

Zarzycki also said that warmer sea surface temperatures would be a contributing factor, because warmer water fuels hurricanes, keeping them going. 

More Explosive Tropical Cyclones

With warmer sea-surface temperatures, the frequency of storms that rapidly intensify has increased. The authors based that conclusion on research that examined tropical cyclone activity globally, while also citing the 2017 North Atlantic hurricane season specifically, which included three major hurricanes—Irma, Maria and Harvey—that went through a period of rapid intensification. “These types of seasonal events will intensify” with human-aided warming, the IPCC authors wrote.

Slower Tropical Cyclones that Can Do More Damage

Scientists call the speed at which cyclones travel across the ocean and then over land as “translation speed,” and the IPCC said that globally there has been a slowdown in all areas except the Northern Indian Ocean.

When slower moving cyclones make landfall, they can drop more rain, cause more wind damage and sustain a larger storm surge simply because they hang around longer. The IPCC authors cited research in 2019 that provided evidence that translation speed for Atlantic storms decreased 17 percent from 1900 to 2017. In addition, what scientists call “meanders” and “stalls” in tropical cyclone paths have become increasingly common, along with the slower speeds.

The cause for slower cyclone speeds is not yet clear, the authors found, but “it is more likely than not” to be related in some way to human-caused climate change.

How Climate Change Affects Individual Cyclones

The report sketched out how some researchers are beginning to assess the role of climate change in a single tropical cyclone, with mixed results. It’s tricky work, Willoughby said, because you can find current hurricanes that have acted like those of the past.

Still, the authors reported case studies of tropical cyclones using what they called “event attribution”—linking the causality of individual weather events to climate change—to test whether the severity of recent intense storms can be explained without human-caused effects. For example, they cited 2018 research that found little evidence of an attributable change in the intensity of Hurricane Katrina in 2005, Irma in 2017 or Maria in 2017, but increases in heavy rains in those storms that they could pin on climate change. When Harvey stalled over the Houston area in 2017, it dropped more than 60 inches of rain over parts of southwest Texas, for example.

The dominant factor in the extreme rainfall amounts during Harvey was the storm’s slow speed, the authors wrote. But studies published after Harvey have argued that human-caused climate change contributed to an increase in the rate of rainfall, which compounded the extreme local rainfall, according to the IPCC report.

Event attribution is a rapidly growing field within climate science, and one with increasingly sophisticated tools, Zarzycki said.

He added, “A lot of stakeholders have an interest in this,” from flood managers designing building codes to the insurance industry and environmental justice advocates. “We want to be very thorough and cautious.”