Extreme rainfall gets an unpredictable boost from air pollution

According to a recent study, emissions of greenhouse gases such as carbon dioxide are expected to amplify extreme rainfall due to their heat-trapping nature, which warms the atmosphere. 

This warming effect leads to a dual consequence: the oceans heat up, facilitating the evaporation of water, and the warmer atmosphere can retain more water vapor, thereby increasing the potential for rain.

Despite these expectations, this uptick in precipitation was not clearly reflected in the data throughout much of the 20th century.

The role of aerosols 

Aerosols — tiny particles emitted through the burning of fossil fuels, such as sulfur dioxide – are known for contributing to air pollution or smog. 

Recent research conducted by the Lawrence Berkeley National Laboratory (Berkeley Lab) reveals that aerosols have significantly counteracted the anticipated increase in rainfall.

The findings highlight how the drying influence of aerosols has largely neutralized the expected rise in precipitation.

Shifts in extreme rainfall

Mark Risser, a research scientist at Berkeley Lab and one of the study’s primary authors, emphasized the novelty of their findings in understanding the drivers behind shifts in extreme rainfall across the continental United States. 

Risser pointed out that aerosols had been masking the increase in extreme rainfall up until the 1970s, a situation altered by the enactment of the Clean Air Act, which significantly curtailed air pollution levels 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,” explained Risser.

Human influences on rainfall 

The research sheds light on the challenges faced by traditional climate models in accurately predicting human influences on rainfall, especially at regional scales critical for planning climate adaptations and mitigation strategies. 

Employing a novel approach and leveraging data from rain gauges spanning from 1900 to 2020, the team was able to more confidently assess the impact of human activity on rainfall trends in the United States.

Bill Collins, co-lead author of the study and associate laboratory director for the Earth and Environmental Sciences Area at Berkeley Lab, reflected on the prior ambiguity surrounding changes in U.S. precipitation patterns attributed to global warming. 

Conclusive evidence

“Prior to our study, the Intergovernmental Panel on Climate Change (IPCC) had concluded that the evidence was mixed and inconclusive for changes in U.S. precipitation due to global warming,” said Collins. 

“We have now provided conclusive evidence for higher rainfall and also helped explain why past studies assessed by the IPCC reached conflicting conclusions.”

Complex impacts of extreme rainfall

The research distinctly identifies the effects of greenhouse gas and aerosol emissions on both average and extreme rainfall patterns.

It was found that while greenhouse gasses uniformly trigger rainfall increases worldwide, aerosols have a more complex impact, cooling the planet and generally causing a drying effect over the long term, but with immediate, seasonally varied effects on rainfall across 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.”

Enhancing future predictions

The study also addresses discrepancies in previous precipitation studies by considering the offsetting influences of aerosols and greenhouse gasses.

Incorporating aerosol data more comprehensively into models is deemed essential for enhancing future predictions crucial for infrastructure and water resource planning.

With the United States experiencing recent surges in extreme precipitation events, Collins warns that the decline in aerosol levels globally, due to air quality improvements, will make the effects of global warming on extreme precipitation increasingly evident in the coming years.

More about aerosols and air pollution

As discussed above, aerosols are tiny particles or droplets suspended in the atmosphere, capable of traveling vast distances. These minuscule substances, invisible to the naked eye, play a significant role in air pollution and climate change.

They originate from both natural sources, such as volcanic eruptions and sea spray, and human activities, including industrial emissions, vehicle exhaust, and the burning of fossil fuels.

Impact on air quality

The presence of aerosols in the air significantly affects air quality. When we burn fossil fuels, for example, we release a large number of fine particles into the air, which can penetrate deep into the lungs and even enter the bloodstream.

These particles can cause a range of health problems, from respiratory issues like asthma and bronchitis to more severe conditions such as heart disease and lung cancer.

In urban areas, the concentration of aerosols is particularly high, leading to smog and reduced visibility. This not only affects human health but also impacts the environment, damaging crops, forests, and ecosystems.

Aerosols, extreme rainfall, and climate change

Aerosols also have a complex role in climate change. Depending on their composition, they can either warm the planet by absorbing sunlight or cool it by reflecting sunlight back into space.

For instance, black carbon, a type of aerosol produced from incomplete combustion, is a potent climate warmer. On the other hand, sulfate aerosols, arising from the burning of coal and oil, have a cooling effect.

However, this cooling effect does not counteract the long-term impacts of greenhouse gases, which remain in the atmosphere for much longer periods. Instead, aerosols add another layer of complexity to understanding and modeling climate change.

Addressing the challenge

Reducing aerosol emissions is crucial for improving air quality and mitigating climate change. This requires concerted efforts at both the local and global levels.

Strategies include transitioning to clean energy sources, improving industrial processes to minimize emissions, and enforcing stricter vehicle emission standards.

Moreover, public awareness and individual actions, such as using public transportation, reducing energy consumption, and supporting policies aimed at cutting emissions, can make a significant difference.

In summary, aerosols are a pivotal factor in air pollution and climate dynamics, with far-reaching impacts on human health, the environment, and the global climate system.

By understanding these particles’ nature and effects, we can take meaningful steps towards cleaner air and a more sustainable future.

Addressing aerosol emissions is not just about combating pollution; it’s about ensuring the well-being of our planet and future generations.

The study is published in the journal Nature Communications.

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