We know that emissions of greenhouse gases such as carbon dioxide should increase precipitation. Emissions warm the atmosphere, causing a one-two punch: Warmer oceans make it easier for water to evaporate, and warmer air can hold more water vapor, meaning more moisture to fall as rain. is available. But for most of the 20Th century, the increase in rainfall did not show up clearly in the data.

A new study led by researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) finds that the projected increase in precipitation is largely offset by the drying effect of aerosols — emissions like sulfur dioxide. which are produced by burning fossil fuels, and are commonly thought of as air pollution or smog. The research is published today in the journal Nature Communications.

“This is the first time we can really understand what’s driving extreme rainfall changes over the continental United States,” said Mark Reser, a research scientist at Berkeley Lab and one of the study’s lead authors. He noted that by the 1970s, the expected increase in heavy rainfall was offset by aerosols. But the Clean Air Act drastically reduced air pollution in the United States. “Aerosol masking was stopped quite suddenly. This means that precipitation may increase much faster than we would have otherwise predicted.”

Traditional climate models have struggled to confidently predict human impacts on rainfall at scales smaller than a continent – ​​and that the regional scale is exactly where most climate change adaptation and mitigation takes place. Using a new method and relying heavily on rain gauge measurements from 1900 to 2020, the researchers were able to more firmly determine how human activity has affected rainfall in the United States.

“Prior to our study, the Intergovernmental Panel on Climate Change [IPCC] concluded that the evidence for changes in U.S. precipitation caused by global warming was mixed and inconclusive,” said Associate Laboratory Director of the Earth and Environmental Sciences Area at Berkeley Lab and co-lead author of the study. Bill Collins said. provided conclusive evidence for more precipitation and also helped explain why past studies reviewed by the IPCC came to conflicting conclusions.”

Specifically, the study isolates how greenhouse gas and aerosol emissions affect both average and extreme precipitation. The researchers confirmed that the increase in greenhouse gas emissions, which spread rapidly across the planet, caused an increase in rainfall. The aerosol effect is more important. Over long periods of time, aerosols cool the planet, causing it to dry out. But they also have a faster, more localized response. This rapid effect depends on the season, with aerosols typically reducing precipitation in winter and spring, and increasing it in summer and falling over much of the United States.

“The weather piece is really important,” Risser said. “For precipitation, the nature of climate change depends on what season you’re talking about, because different types of weather systems produce precipitation during different parts of the year.”

Looking at precipitation trends over the past century can explain some of the conflicting studies on how the aerosol effect complements the greenhouse gas effect, and how models and simulations factor into these two driving forces. Tracking aerosols and incorporating them more fully into models and simulations will be critical to improving predictions used for infrastructure design and water resource management, the researchers noted.

The United States has already seen examples of recent increases in extreme rainfall, with several intense, record-setting storms over the past few years.

“Thanks to improvements in air quality, the aerosols that protect us from the worst effects of global warming are declining worldwide,” Collins said. “Our work suggests that increases in extreme precipitation driven by higher ocean temperatures will become increasingly evident during this decade.”