Dramatic cuts in China’s air pollution drove surge in global warming

A recent surge in the rate of global warming has been largely driven by China’s efforts to reduce air pollution, raising questions about how air quality regulations are influencing the climate and whether we fully understand the impact of removing aerosols from the atmosphere. This extra warming, which was being masked by the aerosols, accounts for 5 per cent of global temperature increase since 1850.

In the early 2000s, China had extremely poor air quality as a result of rapid industrialisation, leading to a public outcry in the run-up to the 2008 Beijing Olympics. In response, Chinese authorities fitted scrubbers to coal power plants to curb the dirtiest emissions and tightened rules governing vehicle exhausts, leading to a 75 per cent drop in sulphate emissions.

But there is a sting in the tail of this environmental success story. According to a new analysis, China’s dirty air had inadvertently been cooling the planet, and now that it is gone we are starting to see a greater warming effect.

We know that warming has probably sped up in the last decade or so. Since 1970, the world had been warming at a constant rate of about 0.18°C (0.32°F) per decade, but since 2010, that seems to have increased to around 0.24°C (0.43°F) per decade, once the influence of natural climate variability is stripped out. Researchers have previously pointed the blame for this uptick in warming at efforts to curb air pollution, but until now they had struggled to pin down what contribution individual regions were making to the global trend.

Sulphate aerosols, released by burning fossil fuels, cool the planet in two ways. The particles themselves reflect sunlight back into space, shielding Earth from solar radiation. They also influence the way clouds are formed, increasing the occurrence of whiter, longer-lived clouds that also reflect radiation. Removing these aerosols from the atmosphere therefore eliminates a cooling effect.

To tease out this effect, Bjørn Samset at the CICERO Center for International Climate Research in Norway and his colleagues used newly published emissions data that gives a more accurate picture of Chinese action on aerosol pollution since 2005. They used state-of-the-art models to simulate how the climate system would respond to rapid drops in aerosol levels, specifically in China. They then compared these results with real-world data, such as satellite observations and estimates of sulphate pollution drawn from emissions reports, and found the modelled scenario was consistent with the real-world data signals.

This allowed the team to isolate the global warming impact of reductions in Chinese aerosol pollution, says Samset. “When we started looking at the numbers, it turns out it is definitely macroscopic – it’s not a small effect,” he says. In total, China’s air pollution crackdown is responsible for 80 per cent of the increased rate in global warming seen since 2010, the team concludes, around an extra 0.05°C (0.09°F) per decade. If you look at the full amount of warming since 1850, about 0.07°C (0.13°F) can be attributed to the clean-up in Chinese aerosols, or around 5 per cent of the total, says Samset. The analysis has yet to be peer-reviewed.

Part of this can be explained by the sheer scale of air pollution reductions China has delivered, cutting sulphur dioxide emissions by around 20 million tonnes per year since the mid-2000s. But China’s air quality also has a particularly strong impact globally, says Samset. “When you emit aerosols over China, they are taken by the atmospheric circulation, transported out over the Pacific, so they spread over a large area,” he says. “The same amount of emissions from India would not have had the same effect on global warming.”

Satellite data has picked up a warming trend over the North Pacific in the past few years, which this new work suggests is explained by the reduction in Chinese aerosols. “If you look at the actual observations, the big temperature series… global warming has been accelerating,” says Samset. “If you look at the geographical pattern of that, a major part of it is over these two patches of the North Pacific. So it fits in.”

It is important to note that China’s action hasn’t caused additional warming, Samset stresses. Rather, it has “unmasked” what was already there. “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,” he says.

Despite the impact on global temperatures, the action was worth taking to save lives, says Duncan Watson-Parris at the University of California San Diego. “The consequence for the climate is not great, but it’s not as acute as the number of people that were dying because of air quality,” he says – previous research has suggested the measures have helped avoid 150,000 premature deaths per year.

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The pace of air quality clean-up in China has slowed in recent years. “There really isn’t that much air pollution left to remove from China,” says Samset. That should mean the rate of warming should fall back to near the 0.18°C per decade rate recorded before 2010, he says.

But other factors could disrupt this. Just as reductions from China tapered off, in 2020 the global shipping industry implemented new rules forcing ships to curb their aerosol emissions, prompting a sharp fall in pollution over the open ocean. This might be particularly important in changing cloud cover in those regions, notes Hugh Coe at the University of Manchester, UK. “It’s happening in remote places where clouds are super sensitive to change,” he says.

Scientists also warn that rising temperatures alone could be causing ocean clouds to become less reflective, reducing their cooling effect, while there are also worries that models may have misjudged how sensitive the climate system is to changes in aerosols. “The question of how fast the world will keep warming is absolutely crucial now,” says Samset.

China’s Ministry of Ecology and Environment did not respond to a request for comment.