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For over a year, global ocean temperatures have been consistently shattering records, shocking scientists. Now hurricane watchers are getting even more worried, given that ocean heat is what fuels the biggest, most destructive cyclones. Researchers at the University of Arizona just predicted an extremely active North Atlantic season—which runs from June 1 to the end of November—with an estimated 11 hurricanes, five of them being major (meaning Category 3 or higher, with sustained wind speeds of at least 111 miles per hour). That would dwarf the 2023 season—itself the fourth-most-active season on record—which saw seven hurricanes, three of which intensified into major ones.
“Part of the reason is very warm ocean surface temperatures in the tropical Atlantic Ocean,” says Xubin Zeng, director of the Climate Dynamics and Hydrometeorology Center at the University of Arizona. The other reason is that the Pacific Ocean is transitioning from a warm El Niño, which discourages the formation of Atlantic hurricanes, into cold La Niña, which encourages them. “So those two factors together give us a very active hurricane season prediction for this year.”
As a tropical cyclone grows, scientists measure sustained wind speeds to get an idea of how it’s intensifying. (“Tropical cyclone” is the general term for these storms. The ones that hit the coasts of the US are known as hurricanes.) When the speeds increase by 30 knots (35 miles per hour) or more in 24 hours, that’s considered “rapid intensification.” Last year’s Hurricane Lee, for instance, grew from 70 knots to 116 knots over just 12 hours. Previous research has found a huge increase in this sort of rapid intensification near coastlines since 1980.
And it’ll only get worse from here. A study from another group of scientists, published today in Earth’s Future, finds that across the planet, hurricanes have been intensifying faster and faster. It looked at hurricane behavior near coasts, as opposed to when they’re traveling across the open ocean, and measured intensification generally instead of rapid intensification specifically.
The study found that globally in the last four decades, the tropical cyclone intensification rate has grown by 3 knots per 24 hours. Put another way: We can expect a hurricane today to intensify by 3 extra knots over the course of a day, on average. Between 1979 and 2000, the average rate of intensification increased by 0.37 knots every six hours of a hurricane’s lifespan, rising to 1.15 knots every six hours in the period between 2000 and 2020.
The authors warn that climate change is creating the conditions for plenty more coastal intensification going forward. That, in turn, is making hurricanes more dangerous than ever, as the storm can suddenly intensify close to shore into something fiercer than what emergency agencies were preparing for. “This increase in intensification near the coast is supported by changes in the environment,” says Pacific Northwest National Laboratory climate scientist Karthik Balaguru, lead author of the paper in the journal Earth’s Future. “The projected changes also show increasing intensification of tropical cyclones in a future climate.”
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Three main factors converge to intensify hurricanes. The first is that as the world in general warms, so too do the oceans. Water evaporating off the surface rises, releasing heat that fuels the developing hurricane. The warmer a patch of ocean water is, the more energy a cyclone has to exploit. If a hurricane like Lee forms off the coast of Africa, it’s got a lot of Atlantic ocean to feed on as it moves toward the East Coast of the United States. As we approach this year’s hurricane season, tropical Atlantic temperatures remain very high.
The second factor is humidity. As the atmosphere warms, it can hold more water vapor, so some parts of the world are getting more humid. Hurricanes love that, as drier air can lead to cooling and downdrafts that counteract the updrafts that drive the storm. “So long as it remains moist, the storm can strengthen, or maintain its intensity,” says Balaguru. “However, once the core enters into a dry environment or becomes less moist, then the storm will start weakening.”
And lastly, hurricanes hate wind shear, or winds of different speeds and directions at different altitudes. (Think of it like layers of a cake, only made of air.) Instead, cyclones like a stable atmosphere, which allows their winds to get swirling and intensifying. Wind shear can also inject drier air from outside the storm into the core of the hurricane, further weakening it. As the world warms, wind shear is decreasing along the US East Coast and East and South Asia, providing the ideal atmospheric conditions for cyclones to form and intensify. “Under climate change, the upper troposphere is expected to warm up at a higher pace than the surface,” says Balaguru. “This can enhance the stability of the atmosphere and also weaken the circulation in the tropics.”
Nearer term, La Niña conditions in the Pacific could help form and intensify hurricanes this summer. Even though La Niña’s in a different ocean, it tends to suppress winds over the Atlantic, meaning there’s less of the wind shear that hurricanes hate. Hence the University of Arizona’s prediction for an extremely active hurricane season, combined with very high sea surface temperatures in the Atlantic to fuel the storms. By contrast, last year’s El Niño created wind conditions in the Atlantic that discouraged the formation of cyclones.
Even then, Hurricane Lee developed into a monster storm last September. A week before that, Hurricane Idalia rapidly intensified just before slamming into Florida. That sort of intensification close to shore is extraordinarily dangerous. “When the storm is very close to the coast—let's say it's a day or two out—if it then suddenly intensifies rapidly, then it can throw you off guard in terms of preparations,” says Balaguru. A town may have planned its evacuations expecting winds of 100 mph, and suddenly it’s more like 130 mph.
Unfortunately, Balaguru’s new study finds that climatic conditions, particularly near the coast, are becoming more conducive for storm intensification. It’s up to teams like Zeng’s at the University of Arizona to further hone their forecasts to manage that growing risk to coastal populations. “For scientists, seasonal forecasting is a reality check of our understanding,” says Zeng. “We have done pretty well over the past few years, and it's going to give us more confidence.”
