The past half century has seen remarkable improvements in air quality in many parts of the world, thanks in large part to legislation such as the US Clean Air Act. Efforts like these have targeted pollutants such as the group of chemicals known as aerosols, which include sulfur dioxide, nitrogen dioxide and other compounds 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 treasure that, without aerosol pollution, the world might already have warmed another half degree Celsius.
This creates a difficult paradox that noted climate scientist James Hansen 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 can also exacerbate global warming. This powerful relationship has been in the past few years the shipping industry: As cargo ships have stopped using dirty bunker fuel since 2020, they have also stopped emitting traces of sulfur dioxide, which has raised global temperatures by an additional 0.05 degrees Celsius.
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 find that the presence of toxic aerosols in the atmosphere over the United States helped suppress the impact of climate change on rainfall for decades, delaying 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 set off a trend of worsening floods.
The paper’s results help solve something of a mystery in climate science: even as warmer air holds more moisture, rainfall in the United States has not increased as scientists expect as temperatures rise.
“This paper highlights that the counteraction between aerosols and greenhouse gases has likely masked many climate hazards over the past several decades,” said Geeta Persad, an assistant professor of earth sciences at the University of Texas at Austin and an expert on aerosols. (Persad was not involved in the study.)
“If aerosol emissions drop drastically over the next few decades and greenhouse gases don’t, many of those unforeseen climate hazards could be revealed,” Persad added.
The paper uses data from thousands of rain gauges to find out how aerosols and greenhouse gases have affected rainfall averages and the frequency of extreme rain events. The use of rain gauges has enabled researchers to track how the two types of human-caused pollution balance each other in different regions of the country.
Greenhouse gases have been piling up in the atmosphere for over a century, and they have a fairly straightforward impact on rainfall. The more carbon dioxide there is in the atmosphere, the warmer it gets; the warmer it gets, the more moisture the atmosphere can hold. Aerosols are more complicated: They react differently with different types of clouds, and consequently their impact on rainfall varies from region to region and from season to season. In most of the US 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 clear their smoking blocks and car manufacturers updated their cars to meet emission limits. The disappearance of these aerosols allowed greenhouse gases to dominate the atmosphere, which began to increase rainfall totals. If those aerosols weren’t there, the paper argues, rainfall and flooding would have started to worsen in the United States several decades earlier.
By isolating the effect of these aerosols, the researchers can also 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 evident in the southeastern United States, where a series of hurricanes and rainstorms have caused billions of dollars in flood damage in recent years. The authors find that aerosol pollution moderated summer and fall precipitation until the late twentieth century, when the effect of greenhouse gases began to dominate the region. This has led to both an increase in annual rainfall totals and an increase in the frequency of large rainstorms. (Previous research has shown that aerosols can too suppress the emergence of tropical storms by disrupting cloud formation.)
The paper’s findings could have major implications for the next few decades of environmental regulation. President Biden’s Environmental Protection Agency is racing to finalize strict regulations on industrial pollution which can reduce emissions of important aerosol pollutants such as sulfur dioxide. If these regulations come into effect, they will apply to numerous facilities in the Southeast, including the petrochemical facilities in the Louisiana region known as “Cancer Alley.”
These regulations will protect residents living near industrial facilities from asthma, heart disease and cancer, but a further decrease in aerosols could also worsen hurricane season by allowing large storms to hold moisture — meaning more events like Hurricane Harvey, which in 2017 hit. and stunned climate scientists by dropping more than 50 inches of rain on Houston, Texas.
Persad, the aerosol expert, says the paper offers a grim warning about future climate risk. If air pollution in the United States declines over the next few decades, many more Americans in regions like the Southeast could see stronger storms and worse flooding.
“We’re looking at a situation where over the next 30 years you can either continue to mask, or you can reveal 50 percent more warming,” she said. “Until now, there hasn’t been much recognition of how much the evolution of this aerosol signal, over the lifetime of a mortgage on a home someone buys today, is going to affect the climate hazards they’re exposed to.”
