The shifting relationships between atmospheric and oceanic climate change, air pollution, and the patterns of tropical cyclone (TC) activity have long been of interest to atmospheric scientists.
TCs form only over tropical oceans in the summer, when both the air and water temperatures are higher than normal. Besides high sea surface temperatures, other factors contributing to TC genesis and activity are low vertical wind shear, natural or internal variations due to El Niño or the Atlantic Multidecadal Oscillation, and external forcings such as influxes of greenhouse gases (such as carbon dioxide or methane) and aerosols (including pollutants like black carbon).
A new study has examined the frequency and spatial patterns of TCs over the past 40 years and has found an intriguing relationship between aerosol pollution and the frequency of the storms.
Kerry Emanuel, professor emeritus of atmospheric science at the Massachusetts Institute of Technology, posited that “aerosol forcing…is probably, at least in the short term, more important [to tropical cyclone formation] than the effect of greenhouse gases.”
Emanuel, who was not involved in the new research, explained that tropical cyclones respond much more powerfully to regional change than to global change. Because aerosols are concentrated near their sources, aerosol forcing has a very regional effect. “That’s really, really important for TCs,” he said.
Gabriel Vecchi, a professor and tropical storm expert at Princeton University also not involved in the study, agreed. “Aerosols have been one of the main drivers of the decadal changes in TCs.”
Both scientists predict, however, that the long-term effect of greenhouse gas forcing will eclipse the impact of aerosols on tropical cyclone patterns.
Waxing and Waning Aerosol Inputs
Aerosols are tiny particles and droplets floating in the atmosphere. Natural sources include volcanic eruptions, whereas human-made sources include vehicular exhaust, industrial activity, and fossil fuel burning. Aerosols cool the ocean directly by blocking sunlight—an umbrella-like effect called dimming, explained James Kossin, a senior scientist with the risk firm The Climate Service.
The new study, published in Science Advances, found that levels of aerosols associated with regional air pollution were linked to varying rates of tropical cyclone activity around the globe. Although the North Atlantic Ocean has been besieged by a spate of hurricanes in recent decades, the number of typhoons in the western North Pacific basin has nosedived, the study revealed. These rates of cyclone activity correspond to reduced and increased amounts of aerosol forcing.
In the past, high aerosol levels in the United States and Europe offset the warming effect produced by greenhouse gas emissions, leading to relatively infrequent Atlantic hurricanes. Lower aerosol levels (cleaner air) in the region today block less sunlight and drive up sea surface temperatures, turbocharging more North Atlantic hurricanes.
The research was conducted by NOAA tropical storm expert Hiroyuki Murakami. By imposing different levels of human-induced aerosol emissions between 1980–2000 and 2001–2020 on a sophisticated high-resolution climate model simulation, Murakami quantified the impact of these aerosols on TC activity.
Vecchi pointed out that “it seems that the modeling evidence and the observational evidence [based on satellite data] are converging in indicating that aerosols have had a discernible impact on TCs.”
Suzana Camargo, a climate scientist at Columbia University, said Murakami’s research has “very clean, very clear, and robust results, which confirms the importance of aerosols on TC activity, especially in the Atlantic.”
Emanuel, who had previously proposed a link between aerosols and hurricane activity, added, “What’s new about the Murakami paper is it goes more into the modeling world and covers the whole planet, not just the Atlantic.”
Besides this global standpoint, the study links changes in the Northern Hemisphere to TC activity in the Southern Hemisphere.
“This teleconnection or link between the Northern and Southern Hemispheres [that Murakami has uncovered],” Kossin added, “is entirely novel, as far as I know.”
There are other mechanisms contributing to the precipitous rise in Atlantic hurricanes, explained Murakami. Low vertical wind shear, for example, favors hurricane formation, and warming of the tropical North Atlantic has (through a poleward shift of the jet stream) reduced wind shear and increased instances of Atlantic hurricanes.
Difficult to Message
Kossin expressed concern that the correlation between decreased particulate pollution and the increase in hurricane activity in the Atlantic was “a difficult one to message.”
The knee-jerk reaction from some people, he feared, might be that if by cleaning up the air we are increasing the number of hurricanes, why are we cleaning up the air? “That’s a very bad way to think about this. The last thing we want to do is to have people say that we need to stop cleaning up the air…that would be dumb,” he said. We still need to clean up the air because “air pollution is far more detrimental to human health than isolated and singular hurricane events.”
Aerosols as a Blip on the Radar
Putting matters into perspective, Vecchi said that “going into the future, the relative impact of greenhouse gases will, I think, overwhelm the relative impact of aerosols on TCs.” Although aerosols will wane as we clean the air, the greenhouse gas signal will continue to grow.
Kossin concurred. “Aerosol pollution…is a bit of a blip [overlaid on the long-term greenhouse gas effect].”
In the final analysis, all the scientists said the relationship between climate change, tropical cyclone activity, and air pollution is complex and difficult to isolate. “It is challenging to detect and attribute past changes in TCs to anthropogenic climate change,” said Murakami. The observed trend in TCs is orchestrated by a symphony of factors, with aerosols being just one player—albeit an important one.
—Alakananda Dasgupta (@AlakanandaDasg1), Science Writer