Katrina. Maria. Ian. Sandy. Those once-ordinary names are now associated with grim memories. For many, just hearing them produces a sense of foreboding—which storm will be the next to join the long line of hurricanes that have altered countless lives and helped shape the course of U.S. history?
Nobody knows where and when that next big storm will hit. But a new study, published Thursday in Scientific Reports, found that in recent decades Atlantic hurricanes were far more likely to dial up from weak Category 1 to major Category 3 or higher storms in only 24 hours. Storms from 2001 to 2020 did so at more than twice the rate as the same types of storms between 1970 and 1990.
Exactly what might be fueling this trend, and whether it will continue into the future, are two questions with major implications for millions of people throughout the Americas.
Andra Garner, a meteorologist and climate scientist at Rowan University, analyzed wind speeds and how they changed over the lifespan of every Atlantic hurricane between 1970 and 2020. She identified each storm’s greatest increase in maximum wind speed during a 24-hour period—the maximum intensification rate. She then grouped the storms into three periods (1970 to 1990, 1986 to 2005, and 2001 to 2020) and created an “average storm” for each.
Comparing those averages shows that recent hurricanes have intensified to significant strengths more quickly. While the probability of a weak storm becoming a major one in 24 hours was only 3.23 percent between 1970 and 1990, it was 8.12 percent between 2001 and 2020. Between 1970 and 1990, 42.3 percent of storms had wind speeds intensify by 23 miles per hour or more over the span of 24 hours. Between 2001 and 2020, 56.7 percent of the storms grew more powerful by that same measure.
The data also showed that geography played a role in maximum intensification rates. Hurricanes strengthened more quickly in the Caribbean Sea and off the U.S. Atlantic coast than they did in the Gulf of Mexico.
While the study did not include an analysis that attempts to firmly identify the cause or causes of the more quickly intensifying storms, Garner interprets the results as a warning call on how climate change is raising sea surface temperatures. “More work would need to be done to see exactly how these changes might carry through in the future, and firmly identify all the variables that are playing a role here,” she says. “But we’ve already seen in just the last 50 years, this change in the rate at which storms are intensifying, while we’ve seen the ocean get warmer over that same time period.”
Hurricanes are fueled by warm surface water. A cluster of thunderstorms at sea can suck heat energy from evaporating surface water up into the air, which creates a ton of humidity. When that water vapor condenses into clouds and rain, its stored heat is released and can result in hurricanes, which convert energy into powerful winds.
“So I think it’s fair to say that if we don’t take action to limit future climate change,” Garner says, “this is a trend that I think we would continue to see worsen in the future.”
While Garner’s position serves as a warning, currently the National Oceanic and Atmospheric Administration (NOAA) asserts that “a detectable greenhouse gas influence on hurricane activity has not been identified with high confidence.” That’s because other hurricane fueling factors are in play, from atmospheric aerosol levels to long term shifts in ocean circulation patterns.
Rapid intensification is associated with particularly powerful, destructive storms, and Garner’s study isn’t the first to note that it has become more common. “Their results are consistent with those of several other recent studies, in that they demonstrate that the number of rapidly intensifying tropical cyclones in the Atlantic Basin has been increasing in the 2000s compared with previous decades,” says Sharan Majumdar, an atmospheric scientist at the University of Miami. Majumdar does caution that all such studies have some uncertainties in their estimates of tropical cyclone intensity, especially in earlier decades, because satellite and aircraft flight data weren’t as robust in the 1970s and 1980s.
Majumdar recently co-wrote a study that found that rapid intensifications—wind speed boosts of more than 34 mph in 24 hours—increased significantly between the 1980-2000 period and that of 2001-2021. And the regional differences he documented in hurricane intensification rates largely matched those seen in Garner’s research. Rapid intensifications increased by 68 percent in the Yucatan and Caribbean Sea region, and by 50 percent in the southern North Atlantic region. The Gulf of Mexico saw only a 21 percent increase.
“We found that these increases in intensification appear to be more closely related to the increase in sea surface temperature than to changes in the vertical wind shear or relative humidity,” he explains.
But the story of intensifying hurricanes isn’t a simple matter of heating up the sea’s surface.
Tom Knutson, a climate scientist at NOAA’s Geophysical Fluid Dynamics Laboratory, is an expert on how climate change is influencing hurricanes. Knutson notes that much research shows various different measures of Atlantic hurricane strength have increased from the 1970s and ’80s to recent years. Any of three major factors during that period, or combinations of them, might be responsible: greenhouse gas warming; a major decrease in aerosol air pollution, which previously reflected sunlight to help cool ocean waters; or natural variability related to changes in ocean circulation.
“Greenhouse gas-induced warming may have done it all, it may have contributed to it, or it may have contributed very little,” he says of the escalating hurricane intensification identified in the new study. “My personal opinion is that these results are basically showing the difference between two periods, and they aren’t making a very strong case that what they are seeing is actually due to greenhouse gas-induced warming.” If the observed Atlantic hurricane changes are due to factors like aerosol levels or ocean circulation changes, the intensification trend likely won’t continue into the future, he notes.
Worldwide, since reliable global satellite data began to be collected in the 1980s, there has been a remarkably consistent average of about 80 tropical cyclones each year. Most current research suggests that Atlantic hurricanes won’t become more frequent, and they may actually become less frequent with climate change. That sounds like good news, but it shouldn’t be entirely reassuring. Research also shows that the hurricanes we are experiencing are becoming more destructive, and that trend also seems likely to continue—because of climate change and human behavior.
Climate change is causing hurricanes to produce more rainfall, according to reports from the Intergovernmental Panel on Climate Change. For every degree Celsius of warming, studies suggest, rainfall rates may rise by about 7 percent. So under a 2-degree Celsius warming scenario, hurricanes will be expected to saturate impacted communities with 14 percent more rainfall.
Sea-level rise, which drives the frequency and severity of coastal flooding, is another destructive factor that’s projected to worsen by perhaps two feet by the year 2100. Since 1960, sea levels have already risen in many parts of the U.S. The coasts of Texas and Louisiana have seen a surge of more than eight inches over that time.
And as coastal development continues to grow, more people and buildings are placed squarely in harm’s way. Because of this, damages from tropical storms have increased significantly over time and will continue to do so even if storm behavior remains the same.
Garner suspects, given projections of future ocean warming, that hurricanes won’t remain the same and will continue to intensify, making the changes we’ve already seen “a bit of an urgent warning” about what may be to come.
Knutson stresses that the science remains unsettled on how to interpret what’s gone on with hurricane behavior in the Atlantic basin over the last half-century. He notes we don’t know exactly how factors like aerosols and ocean circulation have contributed to what has been observed.
“It’s an open question, and an important question, because it does have implications for the future,” he says. “If it actually is greenhouse gas-dominated, then we would indeed expect this trend to continue into the future with some pretty dire consequences.”
Editor’s note, October 20, 2023: This story has been updated to more accurately summarize Tom Knutson’s comments about external factors affecting the strength of hurricanes.