Various studies on the effects of climate change have indicated in recent years that the emissions of gases such as carbon dioxide warm the atmosphere, causing, among other effects, the seas and oceans to increase in temperature, facilitating the evaporation of water. In the same vein, warmer air can hold more water vapor, meaning there is potentially more moisture available to fall as rain. That's it for the theory. The reality, however, is that for much of the 20th century, this possible increase in precipitation has not become a reality in many of the countries where systematic meteorological studies are carried out, such as the United States.
A new study led by researchers at the Lawrence Berkeley National Laboratory (Berkeley Lab) of the United States Department of Energy concludes that what is happening in practice is that the expected increase in rainfall has been offset - negatively - by the desiccant effect. from aerosols: emissions such as sulfur dioxide that are produced by burning fossil fuels and are commonly considered air pollution or smog. The results of this research, focused on climate and meteorology in the United States (although perhaps extrapolated to other parts of the planet), have been published in the journal Nature Communications (February 22).
"This is the first time we can really understand what is causing extreme precipitation to change in the continental United States," said Mark Risser, a research scientist at Berkeley Laboratory and one of the lead authors of the study. The team responsible for the work observed that until the 1970s, the expected increases in extreme precipitation were offset by aerosols. But the Clean Air Act caused a drastic reduction in air pollution in the United States. "Aerosol masking suddenly turned off. This means that precipitation could increase much more quickly than we would have predicted," says Professor Risser in a note released by his university.
Traditional climate models have struggled to reliably predict human impacts on precipitation at scales smaller than a continent, and that regional level is precisely where most climate change adaptations and mitigations take place. By using a new method and relying heavily on rain gauge measurements from 1900 to 2020, researchers were able to more robustly determine how human activities have influenced precipitation in the United States.
"Prior to our study, the Intergovernmental Panel on Climate Change [IPCC] had concluded that the evidence was conflicting and inconclusive about changes in US 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 of the study. "We have now provided conclusive evidence of increased rainfall and have also helped explain why previous studies evaluated by the IPCC reached contradictory conclusions."
Specifically, the study isolates how aerosol and greenhouse gas emissions affect both average and extreme precipitation. Researchers have confirmed that increased greenhouse gas emissions, which are rapidly dispersed across the planet, cause increased precipitation. The impact of aerosols is more nuanced. In the long term, aerosols cool the planet, causing a drying effect. But they also have a faster and more local response. That rapid impact depends on the season, as aerosols typically reduce precipitation in winter and spring and amplify it in summer and fall across much of the United States.
"Seasonality is really important," Risser said. "In terms of precipitation, the nature of climate change depends on the season in question, as different types of climate systems generate precipitation in different parts of the year."
Some of the conflicting studies looking at precipitation trends over the last century can be explained by how the effect of aerosols offsets the effect of greenhouse gases and how models and simulations take these two driving forces into account. The researchers noted that tracking aerosols and incorporating them more fully into models and simulations will be important for improving predictions used for infrastructure design and water resource management.
The United States has already seen examples of recent increases in extreme precipitation, with several intense and unprecedented storms in recent years.
"Thanks to improvements in air quality, the aerosols that protected us from the worst effects of global warming are declining around the world," Collins said. "Our work shows that increases in extreme precipitation driven by rising ocean temperatures will become increasingly evident during this decade."
