China's success in cleaning up air pollution may have accelerated global warming: Study

Efforts to clean up air pollution in China and across East Asia may have inadvertently contributed to a spike in global warming, a new study has found.

The decline in aerosol emissions — which can cool the planet by absorbing sunlight —  have added about 0.05 degrees Celsius in warming per decade since 2010, according to the study, published on Monday in Communications Earth & Environment.

At that time, China began implementing aggressive air quality policies and was ultimately able to achieve a 75 percent reduction in emissions rate of toxic sulfur dioxide, the authors noted.

Sulfur dioxide gas, harmful pollutants that result from fossil fuel combustion and volcanoes, are precursors of sulfate aerosols, which are the dominant aerosol species that cool the Earth today.

Despite posing health threats to plants, humans and other animals, these particles are among the many types of aerosols that also cool the planet. 

When clouds form around aerosols, such particles can absorb solar energy from the atmosphere and thereby reduce sunlight at ground level. And if clouds are not present, aerosols can reflect sunlight back into outer space.

Before China’s air quality improvement policies took effect, pollution was a leading cause of premature death in the country, the study authors noted.

However, with fewer cooling aerosols now present in the atmosphere, areas of East Asia and around the world have endured intensified warming — and are expected to face even more extreme heat, shifting monsoon patterns and potential disruptions to agriculture, according to the study.

The plunge in sulfate levels “partially unmasks greenhouse-gas driven warming and influences the spatial pattern of surface temperature change,” the researchers observed.

“Reducing air pollution has clear health benefits, but without also cutting CO₂, you’re removing a layer of protection against climate change,” co-author Robert Allen, a climatology professor at the University of California, Riverside, said in statement.

“It highlights the need for parallel efforts to improve air quality and reduce greenhouse gas emissions,” he said.

Allen and his colleagues drew their conclusions based on simulations from major climate models for the years 2015 to 2049, using data from the Regional Aerosol Model Intercomparison Project, which includes contributions from the U.S., Europe and Asia.

They projected a global, annual mean warming of about 0.07 degrees Celsius due to aerosol emissions reductions, with 0.05 degrees Celsius of warming per decade already occurring since 2010.

Emissions reductions applied to their simulations corresponded closely with those realized over the 2010 to 2023 period in East Asia, the authors noted, adding that emissions from the region are expected to continue to decline — albeit at a slower rate.

Although their work focused on sulfate aerosols, the researchers stressed that carbon dioxide and methane emissions remain the biggest drivers of long-term climate change.

“Our study focused on the recent, dramatic speedup in global warming, which is very concerning but still small compared to the overall, long-term amount of warming from increased CO2 and methane,” said lead author Bjørn Samset, a senior researcher at the Center for International Climate and Environmental Research in Norway, in a statement.

Allen, meanwhile, also emphasized that because aerosols are short-lived in the atmosphere, the spike in global temperatures could subside in the near future.

“Sulfur dioxide and sulfate aerosols have lifetimes of about a week,” he said. “Once they’re removed, we’ll eventually settle back into a warming rate that’s more consistent with the long-term trend.”

As other regions across the world, including South Asia, Africa and North America, begin to phase out aerosol emissions, the scientists said they plan to analyze how potential shifts could shape forthcoming climate trends.

“Air quality improvements are a no-brainer for public health,” Allen said. “But if we want to prevent the worst impacts of climate change, we have to cut CO₂ and methane too. The two must go hand in hand.”