Could SAI Delay the Recovery of the Ozone Layer?

Models suggest that SAI could cool the planet in a similar way to large volcanic eruptions that release sulphur dioxide (SO2) into the atmosphere. The SO2 reacts with other molecules in the atmosphere to form tiny sulphate particles that reflect incoming sunlight and persist in the stratosphere for one to two years.

However, implementing SAI would also affect stratospheric chemistry, with potential impacts on the recovery of the ozone layer.¹ The extent of these impacts is uncertain. Studies suggest the impacts would depend on factors such as the chemical composition of deployed aerosols and the geographic location of deployment.

How humans have damaged the ozone layer

Chlorofluorocarbons (CFCs)​ – a group of chemicals once commonly used in packaging, solvent cans, and refrigeration – ​have damaged the ozone layer. Though these chemicals are not toxic and persist for a long time in the lower atmosphere, they undergo rapid transformation once they enter the stratosphere ​and destroy ozone​.²

Ozone destruction primarily occurs on the surfaces of polar stratospheric clouds, which form at extremely cold temperatures (<−78°C), mostly over Antarctica during winter in the Southern Hemisphere. CFCs become concentrated on the surfaces of ​these​​ clouds​,​ where they form reactive compounds that break down ozone when sunlight reappears in polar spring.³

Strong winds circle around Antarctica in the Southern Hemisphere winter, trapping the atmosphere in a polar vortex – a large region of rotating air. These strong winds persist into spring, enhancing polar stratospheric cloud formation, and increasing ozone depletion. This ozone-depleted part of the stratosphere is called the “ozone hole​”​.

Once the winds weaken at the end of spring, air masses with depleted ozone concentrations mix into the rest of the stratosphere. This leads to ozone thinning and increased UV level​s across​ the entire world​,​ but particularly​ in​ Southern Hemisphere​ countries such as Australia and New Zealand​.

International action averted environmental catastrophe 

Scientists began monitoring stratospheric ozone over Antarctica in 1956. By the 1980s, it became clear that ozone concentrations were decreasing, and CFCs were responsible.³ 

International response to ozone depletion began in the 1970s with the United Nations Environment Programme (UNEP) calling for intensive research and monitoring. The Montreal Protocol, which called for phasing out ozone-depleting chemicals, was adopted in September 1987. 

The Montreal Protocol brought together representatives from industry, environmental groups, and governments​. It​ was described by ​f​ormer UN Secretary-General Kofi Annan as “perhaps the single most successful international agreement to date” in 2005. 

Since 1987, ozone-depleting chemicals have been phased out of production globally. However, CFCs have a very long lifetime in the atmosphere and small amounts of CFCs continue to be emitted ​– ​mostly from old equipment.⁴ The hole in the ozone layer reappears each polar spring, but it continues to shrink as CFCs become less abundant in the atmosphere. 

How the ozone hole interacts with climate change 

​​​Damage to the ozone layer​ does not cause climate change. However, climate change can exacerbate ozone depletion.⁵​

Warming in the lower atmosphere is linked with cooling in the stratosphere.⁶ This enhances polar stratospheric cloud formation and the strength of the polar vortex, which can increase ozone depletion.⁷

Although climate change can exacerbate ozone depletion, the Montreal Protocol has had its own​ unintended​ impact on climate change. Hydrofluorocarbons, adopted to replace CFCs, are powerful greenhouse gases.⁸ In 2016, an amendment to the Montreal Protocol phased out their production, which has reduced their impact on rising temperatures. 

​​Could SAI​ interfere with ozone recovery​?​ 

Studies suggest that SAI’s impacts on stratospheric chemistry could have impacts on ozone recovery. Some models have shown that ozone concentrations would increase if sulphates were to be injected at mid-latitudes in the Northern Hemisphere.⁹ 

In polar regions, however, increasing stratospheric aerosol concentrations also increases the surface area for ozone-depleting reactions. In this way, injected aerosols would behave like polar stratospheric clouds and become the sites of ozone depletion, leading to decreased ozone in spring over Antarctica​.​¹ 

Recent research suggests that SAI using ​non-sulphate​ particles to reflect sunlight could have a different impact on stratospheric ozone.¹⁰ For example, alumina particles would decrease ozone in a similar way to sulphates,¹¹ but calcite particles might avoid or limit this effect.¹⁰ However, research on non-sulphate particles for SAI is limited. 

A slow recovery of the ozone hole, made slower? 

The international community has worked together for more than 50 years to address the issue of stratospheric ozone depletion. Efforts to eliminate ozone-depleting chemicals in the atmosphere have been largely successful and the ozone hole is slowly recovering.  

Whether, and to what extent, SAI could undermine the recovery of the ozone layer is unclear, and the issue is being monitored by the Scientific Assessment Panel of the Montreal Protocol. International assessments suggest SAI using sulphates could delay ozone recovery by decades but would not lead to ozone depletion of the same magnitude as the late 20th century.¹