Could SAI Replace the Lost Cooling Effect From Cleaning Up Air Pollution?

On 5 December 1952, a yellowish, foul-smelling smog settled over London. When the smog lifted four days later, it had killed about 12,000 people and hospitalised a further 150,000. Air pollution – specifically sulphur dioxide (SO₂) emitted from burning coal containing harmful amounts of sulphur – was responsible.

The 1952 smog in London and other high-profile incidents inspired efforts to clean the air of industrial pollutants, first in Europe and North America, and later in Asia as the location of industrial pollution shifted.¹ Despite these efforts, outdoor particulate air pollution still kills more than 4.5 million people per year.

Sulphur particulate pollution has another side effect: it lowers temperatures, partially masking human-caused global warming.² Could SAI counteract the warming that accompanies cleaning up particulate pollution?

Stratospheric aerosol injection

SAI, the most commonly studied sunlight reflection method, is an idea to lower global temperatures by continuously adding small particles to the stratosphere – the level of the atmosphere that starts 7 km to 20 km above the Earth’s surface.³ The particles would stay here for one to two years, reflect away sunlight, and decrease temperatures.⁴

Sulphate particles are the most well-studied candidates for SAI because of their observed cooling effect in the aftermath of large volcanic eruptions.⁴ There are a smaller number of studies on the cooling potential of other particles, such as calcite.⁵

Scientists have suggested that releasing sulphur particles into the stratosphere could have a similar cooling effect to air pollution, while causing less exposure to dangerous pollutants.⁴ However, releasing sulphur particles would still have side effects, including impacts on human health.⁶

Air pollution and climate change

Human-caused climate change and air pollution are linked. Burning fossil fuels emits greenhouse gases such as carbon dioxide (CO₂) that persist in the atmosphere for centuries⁷ as well as short-lived pollutants that also have impacts on the climate. Some short-lived pollutants such as SO₂ stay in the lower atmosphere for days to weeks,⁸ while others like methane can remain for years to decades.

Different pollutants affect the climate in different ways. For example, methane is a strong greenhouse gas that contributes to warming, while sulphate particles, which form when SO₂ reacts with other compounds in the atmosphere, reflect sunlight and interact with clouds, providing a strong cooling effect.²

Fine particulate matter, such as sulphates, can reach deep into the lungs and potentially the bloodstream, causing issues with breathing and heart function.

Cleaner air, faster climate change

Air quality standards aim to protect people from pollutants, including SO₂ and sulphate particles. Efforts to decrease sulphur pollution vary between different countries, but global emissions have dropped from 141 million tonnes in 1979 to around 73 million tonnes in 2022.

Decreasing sulphate pollution is critical for human health, but eliminating this pollutant removes its cooling effect. As individual countries and the international community work to remove sulphate particulate pollution, some climate models predict that global warming will accelerate this decade as a result.⁹

In 2020, new international regulations introduced stricter limits on sulphur levels in shipping fuel to tackle pollution. The effort succeeded in cutting SO₂ emissions from shipping by more than two thirds.

Decreased SO₂ emissions from ships may have partially unmasked the warming effects of greenhouse gases. Climate models suggest the new regulations could increase global temperatures by 0.05°C by 2050. Some researchers have found evidence the new regulations have contributed to higher temperatures,¹⁰ but others do not see a distinct signal over the short time period.¹¹

Could SAI play a role in slowing warming?

Air pollution has substantial negative health impacts. But cleaning it up will unmask the full extent of warming caused by greenhouse gas emissions, increasing the health and environmental risks posed by climate change.

Paul Crutzen, a Dutch scientist, suggested SAI might be able to replace the cooling effect as sulphate pollution declines – and do so with a fraction of the health and environmental impacts.⁴ He noted that SAI would require a smaller amount of sulphur to provide the same cooling effect as sulphate pollution and would be implemented high above the surface, away from people.

However, subsequent research has highlighted that SAI would also come with risks related to air pollution.¹² SAI could contribute to particulate pollution as stratospheric sulphates eventually make their way to the lower atmosphere. One study found that degraded air quality from SAI could contribute to about 7,400 premature deaths for 1°C of cooling.^6,13

SAI would also change atmospheric chemistry. Specifically, SAI would have impacts on stratospheric ozone, the naturally occurring layer of ozone high in the atmosphere that prevents harmful ultraviolet radiation from reaching the Earth’s surface.⁶

SAI could delay the recovery of the ozone layer, which has been damaged by human activity. Decreased stratospheric ozone can lead to declines in ground-level ozone – a dangerous pollutant.¹⁴ However, deaths from skin cancer from decreased stratospheric ozone would likely increase in some regions.¹⁵

Crutzen, who won a Nobel Prize in 1995 for his work on damage to the stratospheric ozone layer, was aware of the new risks SAI poses. He viewed SAI implementation as a last resort, but believed SAI risks might prove small relative to the climate impacts that would be avoided. This intuition is supported by the results of a 2024 paper that found that the decrease in temperature-related deaths from SAI deployment would considerably outweigh the increase in deaths from SAI’s side effects.¹⁶

Is meeting global climate goals through emissions cuts alone a “pious wish”?

Crutzen emphasised that the best option would be cutting emissions to the point where SAI would never be needed.⁴ However, he worried that “this looks like a pious wish”.⁴

Since the publication of his controversial 2006 article, global annual CO₂ emissions have risen by over 23%, SO₂ emissions have fallen by over 40%, and global temperatures have surged upwards by around 0.6°C.

Countries are currently off track for meeting global climate goals. To reach these goals, countries would need to greatly accelerate emissions cuts and later deploy large-scale methods for removing CO₂ from the atmosphere. SAI could keep temperatures below the target level while these other approaches are pursued, but much more research is needed to understand its potential and risks.