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Hurricane Helene devastated the southeastern United States at the end of September 2024, dumping unprecedented levels of rain. Then, just two weeks later, Hurricane Milton rapidly revved up to a Category 5 (before weakening), and shattered more rainfall records. Experts agree that typhoons, cyclones, and hurricanes are growing more intense with climate change, yet how these weather events—known collectively as cyclonic storms—build and amplify remains somewhat mysterious. A new paper proffers a possible answer: the airborne water cycle drives the power of these storms. If correct, deforestation may play a part in increasing storm intensity.

Healthy, intact forests play an outsized role in the world’s climate. They store carbon, absorb floods, and stabilize the water cycle—which, research shows, helps stabilize the climate.

It’s widely recognized that healthy forests stabilize the water cycle, in part, by pulling up groundwater and releasing vapor into the air, generating rain. Atmospheric physicists Anastassia Makarieva and her late mentor, Victor Gorshkov, both of Russia’s Petersburg Nuclear Physics Institute, took that idea one step further. First proposed in 2006, their theory, known as the biotic pump, argues that megaforests like the Amazon don’t just exhale vapor that becomes rain, they also generate wind, which actively pulls more vapor inland from the ocean to produce more rain.

The process starts when trees photosynthesize and release water vapor into the air. These vapor particles rush about as if inside an inflated balloon, increasing pressure. The gas then rises to cooler air, where it transforms into rainwater. This change shrinks the water’s volume, which in turn reduces pressure close to Earth’s surface, creating a partial vacuum that sucks in air and generates wind.

In a January 2024 study that’s under revision, Makarieva and coauthor Andrei Nefiodov, who is also a researcher at the Petersburg Nuclear Physics Institute, analyze historic and modeled cyclones in an attempt to prove that cyclonic winds are similarly driven by water’s change from gas to liquid. They find that atmospheric pressure drops (the signature of intensifying cyclonic storms) at approximately the same rate that rain falls. This is a clue that rain may be a hidden force behind storm intensification.

The two physicists believe that, as rain falls, the resulting pressure vacuum pulls in wind, giving a cyclone its power. Rain “absolutely drives the hurricane,” Makarieva says. As Hurricane Milton made landfall in Florida, she found that its intensification rate and rainfall conformed with the study’s model.

Douglas Sheil, a forest ecologist at Wageningen University in the Netherlands who has coauthored papers with Makarieva, says both the biotic pump and the new cyclone theory invert the conventional view of wind formation, which is that temperature differences drive airflows that carry water vapor. Instead, Makarieva thinks that water’s change from gas to liquid plays a large role in driving airflows.

According to the prevailing theory of cyclones being driven by heat, a storm can begin to form when water evaporates off a warm ocean. With each degree Celsius of warming, the atmosphere can hold about seven percent more water vapor. When that moisture condenses into clouds higher in the sky, it releases heat, which pushes air away from a storm’s core, dropping pressure in the center. That decreased pressure acts like a vacuum, drawing in air from along the sea’s surface. But warm seawater and air won’t necessarily form a cyclone, which experts are still trying to explain.

Makarieva agrees with scientific evidence that climate change is making hurricanes more intense. But she thinks the torrential rainfall dumping from the warmer atmosphere is the bigger driver—not ocean heat.

In their new study, Makarieva and Nefiodov test the traditional heat-focused hypothesis and find that heat alone will not generate a cyclone. When they remove the energy from falling rain, cyclones form more slowly or not at all. “We show if you take [the existing] model and switch the precipitation off, the hurricane doesn’t develop,” Makarieva says.

Kerry Emanuel, professor emeritus and hurricane researcher at the Massachusetts Institute of Technology who designed the still-dominant model of cyclone formation in 1988, has acknowledged the need for further study. As he wrote on his blog in 2006, “Very few atmospheric processes are as poorly understood as tropical cyclogenesis [how cyclones form]. In spite of years of study, it remains largely a matter of guesswork.”

Makarieva and meteorologists who predict cyclonic storms agree that real-time evaporation from oceans doesn’t account for the majority of the rainwater that accompanies hurricanes. So, where does the rain come from? Answering this question is vital if rain is the key to storm intensification.

In a 2017 paper—“Fuel for Cyclones,” published in Atmospheric Research—Makarieva, Sheil, and coauthors propose that cyclones collect preexisting water vapor from the atmosphere they travel through. “We showed that the amount of rain that the hurricane produces is related to its movement,” Makarieva says.

This is where forests come back in. Earth contains a finite amount of water. If cyclones harvest water along the path they travel, and if more water is indeed pulled inland by intact forests, then less water would remain over the ocean to fuel hurricanes. Conversely, if industry chops down forests in the Amazon and elsewhere and weakens the biotic pump that pulls water vapor over continents, inland drought is more likely, and more moisture will linger over oceans, thus supercharging cyclones. (Makarieva notes that this record season for Atlantic hurricane intensity is also an extraordinary drought year in the Amazon.)

Anecdotally, she and Sheil point to the fact that few cyclones form off the large Amazon and Congo forests. However, Emanuel and other scientists say the lack of cyclones in these areas is likely due to cooler oceans, which evaporate less water, or winds that push against a cyclone’s prevailing spin.

In an email, Emanuel wrote that the rainfall mechanism Makarieva describes is small. “The effect of evaporation and condensation driving airflow is correct, but constitutes a truly tiny part of what actually drives wind systems.”

This criticism is similar to what many climate scientists have said about the biotic pump: it’s a real mechanism, but its effect is small. Yet in the 17 years since the publication of that theory, it has not been disproven, and growing evidence from Makarieva and other independent researchers suggests it may be correct—and more important than most people think for stabilizing the climate.

How cyclones intensify also seems destined for years of debate. But with meteorologists admitting there’s still much to learn, the dynamic role of water—and forests—may deserve a closer look.