Ample research shows how surging greenhouse gas concentrations intensify deadly storms, droughts and heat waves, but cutting them, along with other industrial pollution emissions, will also affect global weather.

The effects of emissions reductions are less studied than increases, but understanding how the climate responds to such declines is also critical to protecting people from climate extremes like flooding, heat waves, dry spells and cyclones.

New research by NOAA published today in Science Advances shows that the rise and fall of industrial soot—aerosols—plays a critical role in tropical storm activity all over the world. The research identified some twists that should be of interest to policy makers, said study author Hiroyuki Murakami, a tropical storm researcher at NOAA’s Geophysical Fluid Dynamics Laboratory. 

Aerosols often form a reflective shield in the atmosphere that can trap warmth higher up, but reduces the amount of heat reaching the planet’s surface. Murakami’s modeling study suggests that an estimated 50 percent drop in atmospheric aerosol pollution in Europe and North America between 1980 and 2020 led to surface warming of the tropical Atlantic Ocean, where cyclones have increased 33 percent during the same 40-year period, he said.

“Reducing air pollution is not always decreasing the risk of hazards from tropical cyclones,” he said, adding that climate policy needs to consider the pros and cons of different impacts from reducing industrial pollution. If policies to reduce greenhouse gas emissions rapidly succeed, it could further reduce aerosol pollution, leading to more heating and more tropical storm activity. 

Princeton University climate researcher and tropical storm expert Gabriel Vecchi, who was not involved in the study, said the research helps provide a more coherent global picture of how changes in aerosol levels affect tropical storms. 

“Given the role that has been identified for aerosols in global and regional temperature, hydroclimate and Atlantic tropical cyclones, it makes sense that they could have had a detectable impact on global tropical cyclone activity,” he said. “The study nicely shows…that aerosols have contributed to a redistribution of tropical cyclone activity—increasing it in some places and reducing it in others.”

The study shows that the impact of aerosols is not isolated to the Atlantic, he said, but involves a global shift. Aerosols, he noted, are some of the most volatile elements of the climate system, so he anticipates more studies to explore the uncertainties associated with their impacts.

Research on aerosols and tropical storms helps explain when and where to expect these storms to cause significant damage, allowing citizens and governments to prepare accordingly, he said.

Murakami said he found the aerosol fingerprint by changing the levels and patterns of human emissions in his model while leaving the other climate factors unchanged. The study did not calculate exactly how much the reduction of aerosols increased ocean warming and tropical storms, he said.

The decline of pollution in the Northern Hemisphere also contributed to the increase in Atlantic tropical storm activity by shifting key wind patterns, he said.

As aerosols decline, overall surface warming in mid- and high latitudes pushes the jet stream, a band of strong winds blowing west to east, about 5 to 9 miles up in the atmosphere, poleward. When those westerly winds weaken above the tropical Atlantic, where many major hurricanes form, he said, conditions at the surface are more conducive for tropical cyclones to develop and strengthen.

Aerosols have other effects on hurricanes. A 2020 paper in Geophysical Research Letters found that they increased precipitation and lightning by a factor of two  in the urban area around Houston during Hurricane Harvey, which flooded the area with record rains.

And a 2021 study in Nature Communications looked into the past and found that the drop in hurricane activity from the 1960s to 1980s was probably linked with the increase in aerosols during a time when industrial pollution of the atmosphere peaked.

Vecchi said that, in the future, he expects the warming effect of greenhouse gases to become more dominant relative to aerosols.

“Given the relatively short lifetime of aerosols in the atmosphere, as aerosol emissions are reduced the impact of greenhouse gases should become more and more evident,” he said.

Regionally Nuanced Impacts

At the same time, the decrease of pollution in Europe and North America had the opposite effect on the Southern Hemisphere, where it dampened tropical storm activity. The Northern Hemisphere is warming faster than the Southern Hemisphere, and, since warm air rises, that growing contrast is causing more air to rise north of the Equator, Murakami said that causes a downward flow of air in the Southern Hemisphere, which inhibits tropical cyclone formation.

The rapid growth of industrial pollution in Asia, particularly China and India, has likely also played a significant role in reducing tropical storm activity in the Western North Pacific, where typhoons threaten vulnerable island nations, as well as Okinawa, Japan and China. 

“One Important thing is, that as aerosols were increasing in China and India, people didn’t pay much attention to the effect,” Murakami said. “We found clearly that aerosols reduce tropical cyclones in the Western North Pacific.” The effect is the “flip side” of what’s happening in the Atlantic, he said, with air pollution as the “key ingredient” for the 14 percent decrease of tropical cyclones in the North Pacific from 1980 to 2020.

“This study indicates that decreasing air pollution leads to an increased risk of tropical cyclones, which is happening in the North Atlantic,” he said. “It could also happen, if air pollution is rapidly reduced, in Asia.” 

Deep dives into the connection between aerosols and hurricanes are useful, said Jim Kossin, a hurricane expert and senior scientist with The Climate Service, which provides financial climate risk analyses for governments and businesses. 

“At this point, any change we make is going to have some kind of effect on the climate and local tropical cyclones,” he said. “We know reducing aerosols in the Atlantic has contributed to increased frequency and maximum intensity. It’s increased a lot of things that are not good to increase.” 

Climate extremes fueled by global warming are increasing globally, and in the United States, hurricanes and tropical storms were by far the deadliest and most damaging between 1980 and 2021, killing 6,697 people and causing more than $1.1 trillion in damages, more than half the $2.1 trillion in total damages from all weather and climate disasters in that span, according to the National Oceanic and Atmospheric Administration. 

Kossin said that, for decades, the cooling aerosols masked the warming effect of greenhouse gases, which intensifies tropical storms. 

“But there is no offset anymore,” he said. “Now we’re paying the piper. It’s coming back on us now.”