Has China reducing air pollution been the leading cause of global warming in the 21st century?

Over most of the last six decades, the long-term average rate of global warming has been 0.18°C per decade. Yet, over the last decade and a half, warming has been occurring 50% faster. We’re also just coming off a record-breaking heat streak: beginning in June 2023, every month for 15 straight months set a new global surface temperature record — the first time that’s happened since at least 1850. For air conditioner salesmen, the last 15 years have been a godsend.

Put simply, global warming has been accelerating.

But why?

Over that time, carbon dioxide releases have been concerning, yet pretty normal. Emissions from India and China have increased, though partially offset by reductions from Europe and the US.

But while much of the energy world’s attention has been focused on addressing greenhouse gas emissions, those emissions are not alone in their ability to impact the climate. Much of the recent warming is likely the result of efforts to address air pollution.

The Great Wall of Smog: Since the time of hosting the 2008 Olympics, China has been actively working to reduce smog and air pollution. At the time, cities like Beijing, Tianjin and Shijiazhuang routinely recorded hazardous levels of PM2.5 — fine particulate matter.

Efforts to reduce smog in China have been effective

Following the winter of 2013, which was dubbed the “airpocalypse,” China developed the Air Pollution Prevention and Control Action Plan, which looked to reduce PM2.5 in major cities, cap coal use in urban areas and subsidize natural gas and renewable power.

It worked: By 2019, there were 59,000 fewer deaths per year in the country directly due to improved air quality. And last year, there were just two days of heavy air pollution in Beijing, down from 58 in 2013.

But it came at a price. Included in the now-reduced smog was sulfur dioxide, a gas that forms particles that actively reflect incoming sunlight and cool the planet like an umbrella. Reducing the pollutant that kills also results in more warming. It’s a Faustian bargain. A catch-22.

And the impacts are not small. Researchers at the CICERO Center for International Climate Research in Norway believe China’s air pollution crackdown is responsible for 80% (!) of the increased rate in global warming seen since 2010.

So, what do we know about sulfur dioxide and what does it tell us about where we’re at with climate change?

Sulfur dioxide 101

Sulfur dioxide is a colorless gas that’s often described as having a pungent and irritating odor, similar to a burnt matchstick. Like trying to light all the candles for grandma’s 90th.

Made up of one sulfur atom and two oxygen atoms, it’s the most common sulfur-containing molecule in the atmosphere. And because it reacts easily with other substances, it plays an outsized role in air quality and climate change.

The cooling effect: When coal plants, ships or volcanoes release sulfur dioxide gas, it doesn’t stay a gas for long. Reacting with water vapor, oxygen and other chemicals in the atmosphere, sulfuric acid droplets and sulfate particles are formed and take the less scary sounding name of “aerosols.”

Addressing air pollution has helped bring down sulfur dioxide emissions

Sulfate aerosols both directly reflect sunlight back into space and also help seed clouds, making them brighter and more reflective. Both of these prevent some solar energy from reaching the Earth’s surface and effectively help cool the planet.

Aerosols brightening clouds is known as the Twomey effect in a field of study known as cloud physics. (Don’t laugh, it’s a totally real branch of physics.)

Emission sources: On the man-made side, the burning of both coal and oil produces sulfur dioxide, the largest being bituminous coal to generate electricity. Countries like China have recognized the health impacts related to coal plants, and many are installing technologies like scrubbers to remove up to 99% of the gas from exhausts. This has helped nearly halve global sulfur dioxide emissions.

Mother Nature is also an occasional player in the game. Every once in a while, she rolls the dice and hits whatever combo is needed for a world-changing volcanic eruption. The largest in modern history have been Tambora (1815) and Krakatoa (1883), both in Indonesia and each emitting close to nearly a year’s worth of human-related sulfur dioxide today over the course of an afternoon.

In 1816, writing from across the world, an aging Thomas Jefferson penned a letter to Albert Gallatin about the strange weather that the US faced in the year following the Tambora eruption.

“We have had the most extraordinary year of drought & cold ever known in the history of America… The summer… has been as cold as a moderate winter. In every state north of this, there has been a frost in every month of the year; in this state we had none in June & July. But those of August killed much corn over the mountains. The crop of corn thro’ the Atlantic states will probably be less than 1/3 of an ordinary one, that of tobacco still less, and of mean quality,” wrote Jefferson.

Large volcanic eruptions can cool the planet for years

In many ways, sulfate aerosols are like bizarro greenhouse gases. On top of cooling instead of heating, aerosols don’t stick around in the atmosphere all that long, no more than a few years. Contrast that with carbon dioxide and methane, which can linger for decades. While GHGs are uniformly distributed across the atmosphere, like sugar in coffee, aerosols are highly localized, like the low-fat foam on top. And unlike carbon dioxide, which is not directly harmful at ambient levels, aerosols damage lungs, cause heart disease and kill millions every year.

The bizarro GHG.

Clearing the air

China isn’t alone in its pursuit of reducing sulfur emissions. In 1990, the US created a cap-and-trade system for the gas, known as the Acid Rain Program, which helped lower emissions of the gas by more than 90%. In Canada, the Air Quality Agreement of 1991, which also partnered with the US, helped reduce sulfur dioxide emissions in the country by 70%. And in most other developed nations across the world, there are some regulations on the books designed to curb the pollutant.

Beyond power generation, there are other industries that emit sulfur dioxide.

Shipping: If you track down some old satellite photos near international ports prior to 2020, you’ll notice a series of contrail-looking lines of white clouds. These unnatural formations are called tanker tracks, the result of tankers and cargo ships burning high-sulfur bunker fuel. And now we all know the science: fuel combustion releases sulfur dioxide that forms aerosols and serves to seed clouds.

Bunker fuel shipping tracks off the US West Coast

In 2020, the International Maritime Organization (the IMO) released new rules that look to curb sulfur content used in fuel oil for shipping, known as IMO2020. The directive lowered the limit of sulfur by mass in fuels by ~85% and has decreased sulfur-related emissions by an estimated 8.5 to 8.9 million metric tons since its adoption. As a result, it’s believed 570,000 premature deaths have been avoided.

But, bizarro GHG. A study published in American Geophysical Union estimates that the reduced emissions that stemmed from IMO2020 have increased global annual surface temperatures by two to three years’ worth of warming. And a separate study, led by climate scientist James E. Hansen, found the new fuel rules to be the culprit behind the unexpectedly large temperature jump in 2023 and 2024.

Monthly global average temperature anomaly degrees

This all raises an important question: if aerosols from sulfur dioxide are so effective at accidentally cooling the planet, what if we used them to cool the planet on purpose?

Geoengineering

Startups like Make Sunsets and Stardust are actively working on solutions to commercialize the cooling of the Earth with aerosols.

Their pitches go something like this: Look, before you freak out, we’re not trying to bring smog back, but we can use some of the stuff inside of smog thoughtfully. If we were to put a small amount of aerosols into the stratosphere (~20 kilometers up), they’re not going to make acid rain, they’re not going to be entering people’s lungs and they’re not going to be localized. Instead, we believe we can use them to reduce warming by half a degree within months, and if it doesn’t work, the particles are gone in a year. Two tops. At the very least, we should study this in more detail.

But geoengineering remains extremely controversial, with many of its own unknowns to worry about, like effects on rainfall, the ozone layer and other unknown unknowns out there.

Aerosols have played a large role in keeping global warming in check

And for governments looking to pass popular laws, it’s easier to sell policies that reduce emissions that directly affect people’s health than to advocate for those same molecules to be released into the stratosphere. Chemtrail theorists would have a field day.

Zoom out

The International Panel on Climate Change estimates that aerosols contributed a cooling of 0.0 and 0.8 degrees Celsius, a large error bar. The exact amount of cooling is difficult to pin down and is one of the trickiest questions in climate science.

The uncertainty lies with the many unknowns left to be solved. How much cloudier and floofier are clouds today than they were back in the 1850s because of aerosols? Do brighter clouds impact rainfall, cloud lifetime and height? How do aerosols impact thunderstorm clouds that often produce extreme weather themselves?

The indirect effects of clouds are tough to both model and test. Just ask a cloud physicist. (Again, a real job.)

But addressing sulfur dioxide gives opportunity for scientists to better understand the impact of aerosols on climate. For many, China reducing air pollution and ships nixing tanker tracks are rare real-world climate experiments, like conducting a long-running geoengineering test in reverse.

And to be clear, removing sulfate aerosols from the atmosphere doesn’t cause global warming, it reveals what was hidden but already there. Unfortunately for us, that likely means higher levels of warming than we had expected.

+Bonus reading: How human-caused aerosols are ‘masking’ global warming