The past half-century has seen remarkable improvements in air quality in many parts of the world, thanks largely to legislation like the U.S. Clean Air Act. Efforts like these took aim at pollutants like the group of chemicals known as aerosols, which include sulfur dioxide, nitrogen dioxide, and other compounds that are harmful to human health.
Like greenhouse gases, aerosols are produced by cars, factories, and power plants — but unlike greenhouse gases, they make the earth cooler rather than warmer. This is because aerosols reflect the sun’s rays, rather than trapping its heat like carbon. Some studies estimate that, without aerosol pollution, the world might have already warmed by another half a degree Celsius.
This creates a tricky paradox, which renowned climate scientist James Hansen has called a “Faustian bargain.” If you remove aerosols from the air, you reduce the health impacts of pollution, saving thousands of people from lung and heart disease, but you might also make global warming worse. This powerful relationship has been on display over the past few years in the maritime shipping industry: As freight ships have stopped using dirty bunker fuel since 2020, they’ve also stopped emitting trails of sulfur dioxide, which has caused world temperatures to jump by an additional .05 degrees C.
Now, new research shows that the interaction between aerosols and greenhouse gases also has implications for flooding, which is one of the costliest climate disasters. A peer-reviewed paper published this week in Nature Communications finds that the presence of toxic aerosols in the atmosphere over the United States helped suppress the impacts of climate change on rainfall for decades, postponing a surge in rainfall and flood risk driven by climate change. The passage of clean air laws, which removed these aerosols from the atmosphere, ironically unleashed a trend of worsening floods.
The paper’s results help solve what had been something of a mystery in climate science: Even though warmer air holds more moisture, rainfall in the United States hasn’t been increasing in the way scientists expected as temperatures rise.
“This paper highlights that the counteraction between aerosols and greenhouse gases has likely masked a lot of climate hazards over the last few decades,” said Geeta Persad, an assistant professor of Earth sciences at the University of Texas at Austin and an expert on aerosols. (Persad wasn’t involved in the study.)
“If aerosol emissions drop drastically over the next few decades and greenhouse gases don’t, a lot of those unanticipated climate hazards could be revealed,” added Persad.
The paper uses data from thousands of rain gauges to tease out how aerosols and greenhouse gases have influenced rainfall averages and the frequency of extreme rain events. The use of rain gauges allowed researchers to trace how the two types of human-caused pollution balance each other out in different regions of the country.
Greenhouse gases have been stacking up in the atmosphere for more than a century, and they have a pretty simple impact on rainfall. The more carbon dioxide is in the atmosphere, the hotter it gets; the hotter it gets, the more moisture the atmosphere can hold. Aerosols are more complicated: They react differently with different types of clouds, and as a result their impact on rainfall varies from region to region and from season to season. In most of the U.S., they made things drier.
The passage of the landmark Clean Air Act in 1970 caused a rapid decline in aerosol pollution as factories installed “scrubber” devices to clean up their smokestacks and automakers updated their cars to comply with emission limits. The disappearance of these aerosols left greenhouse gases to dominate in the atmosphere, which started to ratchet up rainfall totals. If those aerosols hadn’t been there, the paper argues, rainfall and flooding might have started worsening in the United States several decades earlier.
Separating out the effect of these aerosols also allows the researchers to make predictions about how flood risk will change over the next decade. It’s not good news: Now that there’s nothing to offset the heat-trapping effect of carbon dioxide and methane, much of the country is about to get a lot wetter and see a lot more monster storms.
“This somewhat rapid intensification of rainfall extremes is the new normal, at least for the next five years,” said Mark Risser, a research scientist at Lawrence Berkeley National Lab and one of the paper’s lead authors.
The effect is most pronounced in the southeastern United States, where a slew of hurricanes and rainstorms have caused billions of dollars of flood damage in recent years. The authors find that aerosol pollution tamped down summer and fall precipitation until the late 20th century, when the effect of greenhouse gases started to dominate in the region. That led to both an increase in annual rainfall totals and an increase in the frequency of big rainstorms. (Previous research has shown that aerosols can also suppress the emergence of tropical storms by disrupting cloud formation.)
The paper’s findings could have big implications for the next few decades of environmental regulation. President Biden’s Environmental Protection Agency is racing to finalize strict regulations on industrial pollution that could slash emissions of key aerosol pollutants such as sulfur dioxide. If these regulations take effect, they would apply to numerous facilities in the Southeast, including the petrochemical facilities in the Louisiana region known as “Cancer Alley.”
These regulations would protect residents who live near industrial facilities from asthma, heart disease, and cancer, but a further decline in aerosols could also make hurricane season worse by allowing big storms to hold moisture — meaning more events like Hurricane Harvey, which struck in 2017 and stunned climate scientists by dropping more than 50 inches of rain over Houston, Texas.
Persad, the aerosols expert, says the paper offers a grim warning about future climate risk. If air pollution declines in the United States over the next few decades, many more Americans in regions such as the Southeast could see stronger storms and more severe flooding.
“We’re looking at a situation where over the next 30 years, you could either keep masking, or you could reveal 50 percent more warming,” she said. “Up until now, there has not been very much recognition of how much the evolution of this aerosol signal, over the lifetime of a mortgage of a house that somebody buys today, is going to affect the climate hazards they’re exposed to.”
