Research uncovers how air pollution masks rises in precipitation levels

It has long been understood that greenhouse gas emissions, such as carbon dioxide, are expected to lead to an increase in rainfall. These emissions warm the atmosphere, creating conditions where water evaporates more readily from oceans due to warmer temperatures, and where warmer air can hold more moisture, resulting in increased rainfall. However, throughout much of the twentieth century, data did not indicate a rise in precipitation levels.

A recent study, led by researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), sheds light on this phenomenon. The study, published in the journal Nature Communications, reveals that the anticipated rise in rainfall has been largely counteracted by the drying effect of aerosols – emissions like sulfur dioxide, produced primarily from burning fossil fuels, commonly known as air pollution or smog.

Mark Risser, a research scientist at Berkeley Lab and one of the lead authors of the study, stated, “This is the first time that we can truly comprehend what’s driving changes in extreme rainfall within the continental U.S.” He explained that until the 1970s, the anticipated increases in extreme rainfall were offset by aerosols. However, the implementation of the Clean Air Act led to a significant reduction in air pollution in the United States. “The aerosol masking effect was essentially turned off quite suddenly. This means that rainfall might increase much more rapidly than previously predicted.”

Conventional climate models have struggled to accurately predict the human impact on rainfall at regional scales, precisely where most climate change adaptation and mitigation efforts are focused. By employing a new methodology and heavily relying on rainfall measurements from 1900 to 2020, researchers were able to more confidently determine how human activities have influenced rainfall patterns in the United States.

Bill Collins, associate laboratory director for the Earth and Environmental Sciences Area at Berkeley Lab and co-lead author of the study, highlighted, “Prior to our research, the Intergovernmental Panel on Climate Change [IPCC] had reached mixed and inconclusive conclusions regarding changes in U.S. precipitation due to global warming. We have now provided definitive evidence for increased rainfall and also clarified why previous studies evaluated by the IPCC yielded conflicting results.”

The study specifically examines the impact of greenhouse gas and aerosol emissions on both average and extreme rainfall. It confirms that increased greenhouse gas emissions, which disperse globally, lead to increased rainfall. The influence of aerosols is more complex. While aerosols have a long-term cooling effect on the planet, resulting in a drying effect, they also have a more immediate, localized impact. This immediate effect varies by season, with aerosols generally reducing rainfall in winter and spring while amplifying it in summer and fall across much of the United States.

Risser emphasized the importance of seasonality in understanding climate change’s effects on rainfall, stating, “The nature of climate change’s impact on rainfall varies depending on the season, as different weather systems generate precipitation in different parts of the year.”

The study suggests that the conflicting findings of previous studies examining precipitation trends over the last century can be attributed to how aerosols counteract the effects of greenhouse gases, and how models and simulations incorporate these two influencing factors. The researchers noted the significance of tracking aerosols and integrating them more comprehensively into models and simulations to enhance predictions used for infrastructure planning and water resource management.

Collins pointed out that the United States has already experienced instances of increased extreme precipitation, with several intense, record-breaking storms occurring in recent years. He concluded, “Thanks to improvements in air quality, the aerosols that previously mitigated the most severe effects of global warming are decreasing worldwide. Our research indicates that the increases in extreme precipitation driven by rising ocean temperatures will become increasingly evident during this decade.”