How air pollution hides impact of greenhouse gases on rainfall

A new study reveals that the expected rise in rainfall due to greenhouse gas emissions was largely masked by the drying effect of air pollution in the United States for much of the 20th century.

Human impact on US rainfall

The study, led by researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), used a novel method and historical data from rain gauges to determine how human activities have influenced rainfall patterns in the United States from 1900 to 2020.

The study, published in the journal Nature Communications, found that greenhouse gas emissions, which heat the atmosphere and increase the amount of water vapor in the air, have caused an increase in both average and extreme rainfall across all seasons.

However, this increase was offset by the cooling and drying effect of aerosols, which are tiny particles emitted by burning fossil fuels, such as sulfur dioxide. Aerosols reflect sunlight and reduce cloud formation, leading to less rainfall.

The study also found that aerosols have a faster and more local impact on rainfall, depending on the season and the location. For example, aerosols tend to reduce rainfall in the winter and spring, but enhance it in the summer and fall over much of the United States.

Rainfall linked to greenhouse gases

The researchers said that their study is the first to provide conclusive evidence for higher rainfall due to greenhouse gas emissions in the United States, and to explain why previous studies reached conflicting results.

"Prior to our study, the Intergovernmental Panel on Climate Change had concluded that the evidence was mixed and inconclusive for changes in U.S. precipitation due to global warming," said Bill Collins, associate laboratory director for the Earth and Environmental Sciences Area at Berkeley Lab and co-lead author on the study. "We have now provided conclusive evidence for higher rainfall and also helped explain why past studies assessed by the IPCC reached conflicting conclusions."

They said that their study also has implications for future climate change, as the effect of aerosols has diminished since the 1970s due to the implementation of the Clean Air Act, which reduced 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,” said Mark Risser, a research scientist at Berkeley Lab and one of the lead authors for the study.

He added that the seasonality of rainfall is also important to consider, as different weather systems create precipitation in different parts of the year. “For rainfall, the nature of climate change depends on what season you’re talking about,” he said.

Study enhances rainfall predictions

The researchers said that their study can help improve the predictions of climate models and simulations, which have struggled to capture the regional and seasonal variations of rainfall in the United States.

They also said that their study can help inform the adaptation and mitigation strategies for water resource management and infrastructure design, as the United States faces more frequent and intense storms and floods due to climate change.

"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."

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