In a new study, researchers examined how human-caused greenhouse gas and aerosol emissions affect rainfall in the United States. Greenhouse gas emissions enhance rainfall, whereas aerosols have both long-term drying effects and seasonal consequences. As aerosols decrease, their long-term drying influence is projected to reduce, causing rainfall averages and extremes to rise dramatically.
We understand that greenhouse gas emissions, such as carbon dioxide, should increase rainfall. The emissions heat the atmosphere, resulting in a one-two punch: warmer oceans make it easier for water to evaporate, and warmer air can store more water vapor, allowing more moisture to fall as rain. However, for long of the twentieth century, the data did not demonstrate the increase in precipitation.
A new study led by researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) finds that the expected increase in the rain has been largely offset by the drying effect of aerosols — emissions like sulfur dioxide that are produced by burning fossil fuels and commonly thought of as air pollution or smog. The research is published today in the journal Nature Communications.
The seasonality piece is really important. For rainfall, the nature of climate change depends on what season you’re talking about since different kinds of weather systems create precipitation in different parts of the year.
Mark Risser
“This is the first time that we can understand what’s causing extreme rainfall to change within the continental U.S.,” said Mark Risser, a research scientist at Berkeley Lab and one of the lead authors for the study. He noted that until the 1970s, the expected increases in extreme rainfall were offset by aerosols. However, the Clean Air Act caused a drastic reduction in air pollution in the United States. “The aerosol masking was turned off quite suddenly. That means rainfall might ramp up much more quickly than we would have otherwise predicted.”
Traditional climate models have struggled to accurately anticipate the human impact on rainfall at sizes smaller than a continent, even though this is where the majority of climate change adaptations and mitigations occur. Researchers were able to better estimate how human activities have influenced rainfall in the United States by employing a new method and relying significantly on rain gauge observations from 1900 to 2020.
“Before our study, the Intergovernmental Panel on Climate Change [IPCC] concluded that the evidence for changes in U.S. precipitation due to global warming was mixed and inconclusive,” said Bill Collins, associate laboratory director for the Earth and Environmental Sciences Area at Berkeley Lab and study co-lead author. “We have now provided conclusive evidence for higher rainfall and also helped explain why past studies assessed by the IPCC reached conflicting conclusions.”
Specifically, the study isolates how greenhouse gas and aerosol emissions affect both average and extreme rainfall. Researchers confirmed that increased greenhouse gas emissions, which quickly disperse over the whole planet, cause an increase in rainfall. The impact of aerosols is more nuanced. Over the long term, aerosols cool the planet, which causes a drying effect. But they also have a faster, more local response. That fast impact depends on the season, with aerosols generally reducing rainfall in the winter and spring, and amplifying it in summer and fall over much of the United States.
“The seasonality piece is really important,” Risser said. “For rainfall, the nature of climate change depends on what season you’re talking about since different kinds of weather systems create precipitation in different parts of the year.”
Some of the contrasting research on precipitation patterns over the previous century can be explained by how aerosols offset greenhouse gas effects, as well as how models and simulations account for these two driving forces. The researchers stated that tracking aerosols and incorporating them more thoroughly into models and simulations will be critical for improving forecasts used in infrastructure design and water resource management.
In recent years, the United States has seen an upsurge in extreme precipitation, with numerous intense, record-setting storms.
“Thanks to improvements in air quality, the aerosols that shielded us from the worst effects of global warming are declining worldwide,” Collins said. “Our work shows that the increases in extreme precipitation driven by elevated ocean temperatures will become increasingly obvious during this decade.”
