A tourist and her daughter wearing masks visit Tiananmen Square at dangerous levels of air pollution on January 23rd, 2013, in Beijing, China. (Photo: Feng Li/Getty Images)

Officials in China have been grappling with some of the world's worst air pollution for years, the result of rapid industrialization and a reliance on coal as a household energy source. The health risks of poor air quality are well-documented and severe; it causes some 1.6 million premature deaths in China every year. But new research, published today in Nature Energy, shows that poor air quality has also reduced the amount of sunlight reaching the country's solar installations, undermining China's efforts to meet more of its energy needs with renewables.

The country has made great strides in reducing pollution in recent years. Beijing declared a "war on air pollution" in 2013, when the capital city faced apocalyptic levels of choking smog, and began rolling out policies to replace smog-producing coal with natural gas and renewables like solar power. Today, China is a world leader in solar energy; more than half of the globe's installed capacity is located there.

That's why the study's lead author Bart Sweerts and his colleagues decided China would be the perfect place to study the relationship between anthropogenic emissions and solar energy.

The sun delivers more energy than the entire global population needs every day right to Earth's surface in the form of solar radiation. The amount of sunlight that reaches Earth's surface—and the solar panels located there—fluctuates over time as atmospheric conditions change. Cloud cover and aerosols—fine particulates like dust or ash, kicked up into the air or coughed out of smokestacks—can scatter sunlight on its way toward the ground.

To find out if China's emissions might be affecting its solar installations, the international team of researchers first looked at a data set tracking decades of solar irradiance, the measure of how much sunlight reaches the Earth, and compared that to emissions of sulfur dioxide and black carbon. They took advantage of a long record of sunlight data, collected between 1960 and 2015 at 119 stations across China, which showed that solar irradiance had decreased by as much as 15 percent over the recorded time. Emissions increased rapidly in China after industrialization began in the 1950s, while cloud cover decreased over nearly the same time period (1955 to 2000), making anthropogenic aerosols a prime suspect for the cause of the dimming.

The long data set, from the Chinese Meteorological Administration, "allowed us to look back very far in time before much of the industrialization of China actually occurred, and then you can look at relatively clean air," Sweerts says. "As such, it provided a very probable answer to the question of whether it is indeed air pollution that causes a reduction in radiation in China."

Sweerts and his colleagues showed that, if the sunlight levels from the 1960s were restored, China's solar installations would have generated 12 percent more electricity in 2016 and cost savings to solar power plants of nearly $2 billion. China is planning to roughly triple its installed solar capacity by 2030, and reducing air pollution to the 1960s levels by then would give China what the study authors call an "electricity bonus" of an additional 51 to 74 terrawatt hours of energy per year more than it would have generated with current air pollution levels.

"I think that, for many countries, the phase of such dire air pollution that it would decrease solar radiation substantially is over," Sweerts says. "So let's say for Europe and America, there are very few places that have air pollution levels so high that they block so much sunlight."

India, which is home to 14 of the world's 15 most polluted cities, could be one of them. "In India, where air pollution levels are also very high and, due to their geographical location, these air pollutants can also really stay there for quite a while without being blown away," he says. "I think this is definitely a study worth replicating there."

For China, the study provides a "very direct monetary, or energy incentive for battling air pollution," according to Sweerts. But he also notes that the health effects of air pollution, which cost the country $1.6 trillion in welfare losses in 2013, are still the best arguments for improving air quality.