Cutting lung-damaging—but planet-cooling—sulphur pollution from China’s coal plants, industry, and vehicle exhaust over the past 15 to 20 years has driven much of the faster global warming seen since 2010, finds new research from Norway’s CICERO climate institute.

The cleanup likely added about 0.07°C to global temperatures—possibly as little as 0.02°C or as much as 0.12°C, researchers estimate in a paper now awaiting peer review.

Prompted by public outcry against lethal levels of air pollution, China began to crack down on sulphur dioxide (SO2) emissions in the mid-2000s, ultimately cutting them by 75%.

It was excellent news from a public health perspective, but the reductions also sped up the rate of warming, since SO2 turns into sulphate aerosols in the atmosphere—and these aerosols help cool the Earth by reflecting sunlight away from the planet.

The cooling effect is two-fold: aerosols are highly reflective, radiating sunlight back into space, and they also encourage formation of long-lived, brighter, more reflective clouds.

While atmospheric cooling through sulphate aerosols might seem like relief, it is misleading. In their presence, the full extent of human-caused global heating was masked. Without them, it is now revealed.

“The warming was always there, we just had some artificial cooling from pollution, and in removing the pollution we are now seeing the full effect of the greenhouse gas-driven warming,” lead author Bjørn Samset told the New Scientist.

This effect has been understood in past research, but the new study is the first to model and quantify the atmospheric impact of China’s pollution-slashing efforts.

“There really isn’t that much air pollution left to remove from China,” Samset said, but other factors may continue to accelerate warming. Rising methane levels, “record-low albedo levels” from melting polar ice and glaciers, and regulation to reduce marine SO2 pollution add to uncertainty, say the researchers.

Rising temperatures may also be reducing the reflectivity of clouds, which is another cause for concern, along with “worries that models may have misjudged how sensitive the climate system is to changes in aerosols,” Samset said.

“The question of how fast the world will keep warming is absolutely crucial now.”