Over the past 15 years, Chinese officials saved the lives of an estimated more than 200,000 residents by reducing the air pollution from coal-fired power plants. But this public health campaign has an unfortunate side effect: The drop in pollutants is helping warm the planet.

In fact, China’s push to continue cleaning up its air in the future could warm the entire northern hemisphere by 0.1 degrees Celsius by the end of the century, making it even tougher to keep the Earth’s temperature below the 1.5 Celsius degree (2.7 degrees Fahrenheit) warming limit that scientists say is necessary to avoid severe weather disruptions, increased rainfall, sea level rise, droughts, and other disastrous climate change effects by the year 2100. That’s because the same sulfur dioxide particles that come from coal burning and cause respiratory problems in humans also reflect sunlight, which cools the planet.

In a new study published this week in the journal Environmental Research Letters, a team of researchers from China and the United States analyzed emissions data between 2006 and 2017 during China’s big air cleanup. By installing scrubbers and other new technologies on older coal plants and other factories, the country cut sulfur dioxide emissions by 70 percent. The researchers then developed a computer model to forecast how this change in air quality would affect something called “radiative forcing,” or the amount of the sun’s energy that becomes trapped by the Earth’s atmosphere, minus the energy the Earth reflects back to space.

The scientists ran their model to simulate the passage of 150 years at both the higher 2006 emissions rate and the lower 2017 emissions rate. Then they looked at what the temperature changes would be after about a century. The results show that the emissions reductions will allow more energy to reach the Earth, resulting in a total warming of about 0.1 degrees Celsius, and not just where the pollution reductions occurred in China. That’s because these sulfur dioxide pollutants are dispersed by wind currents, and as a result the warming effect will be widespread throughout the northern hemisphere.

“It’s one of these tradeoffs that people have known about before, but we put some numbers on it,” says Steven J. Davis, a professor of earth system sciences at UC Irvine and a coauthor on the new report.

Of course, there are also benefits to cleaner air. Tiny particles that come from coal emissions, automobile combustion, and industrial plants enter the airways and get deep into the lungs, where they erode tissue and can enter the bloodstream. This can lead to an array of both short-term and chronic health problems from asthma to heart attacks, according to the Environmental Protection Agency. But from 2013 to 2017, China’s pollution abatement helped reduce premature deaths from respiratory disease, stroke and lung cancer by nearly 10 percent, or about 200,000 people, according to this 2019 study by health researchers published in the journal Science China Earth Sciences.

Since China is the 800-pound gorilla of planet-warming greenhouse gases (accounting for 28 percent of the world’s emissions in 2018, compared to the US at 15 percent, according to the Union of Concerned Scientists), how the nation sources its energy and whether it switches to cleaner fuels soon enough may well affect the rest of the world’s population as well. Last week, Chinese President Xi Jinping told the United Nations General Assembly that his country would adopt tougher greenhouse gas targets and become carbon neutral before 2060. “We call on all countries to pursue innovative, coordinated, green and open development for all, seize the historic opportunities presented by the new round of scientific and technological revolution and industrial transformation, achieve a green recovery of the world economy in the post-COVID era and thus create a powerful force driving sustainable development,” Xi said, according to an official transcript of his speech.