What is our assessment?

Injecting huge amounts of reflective aerosols into the stratosphere to counteract or mask GHG-driven warming is not a serious or plausible climate solution. Its effectiveness is highly uncertain, and its potential for harmful unintended impacts to Earth and ecological systems, as well as on human well-being, is extremely high. Based on these significant problems and risks, we conclude that deploying Stratospheric Aerosol Injection is “Not Recommended.”

What is it? 

Stratospheric aerosol injection (SAI) is a geoengineering technology that uses airplanes or balloons to inject fine particles, usually sulfates, into the stratosphere, the layer of air that begins about 6 to 20 km (20,000 to 65,000 ft) above the Earth’s surface. These aerosols would scatter some of the sunlight striking the planet, reflecting it back into space. Reducing the amount of sunlight hitting the Earth is intended to cool the planet and counteract the warming effects of increasing GHG concentrations. Because SAI does not affect the atmospheric concentration of GHGs, the direct cause of global warming, this technology is not actually a solution to climate change. Instead, it is a temporary action to mask the ongoing warming effects of GHG emissions. 

Does it work?

The injection of large amounts of reflective aerosol particles into the stratosphere does have a cooling effect on the planet. Following the 1991 eruption of Mount Pinatubo, which injected 20 Mt of sulfur dioxide into the stratosphere, average global temperatures were about 0.5°C lower for more than a year. In another example, modeling studies suggest that recent reductions in East Asian air pollution have contributed to the acceleration of global warming. Therefore, in theory, deploying SAI could achieve a similar effect. However, other than modeling simulations, SAI has never been tested in the field, and researchers agree that there are substantial uncertainties and risks. For example, the ways that GHGs and stratospheric aerosols affect global temperatures differ. GHGs warm the planet more in winter than in summer, and more in the high latitudes, especially in the Northern Hemisphere, than in the equatorial regions. Because aerosols reflect solar radiation, they have a greater impact during the summer and in the equatorial zone. Finally, the solar radiation reflective effect of SAI is temporary. Depending on the location and altitude of injection, the aerosols remain in the stratosphere for only months to a few years.  

Why are we excited?

We’re not. The only argument in favor of deploying SAI is based on the concern that we cannot reduce GHG emissions fast enough to avoid the catastrophic environmental and societal impacts of climate change. SAI proponents argue that this geoengineering approach to reduce global temperatures could be a “bridge,” buying time to cut GHG emissions and remove atmospheric CO₂ over the longer term. 

Why are we concerned?

SAI is an untested technology designed to alter planetary energy balance and atmospheric dynamics. Numerous modeling studies indicate that its efficacy to reduce global or regional temperatures as intended is highly uncertain and that it has high risks for unintended impacts on Earth, ecological, and human systems. These studies show that SAI could have substantial effects on the physics, chemistry, and circulation of the upper atmosphere, including harm to the ozone layer. It could destabilize weather and rainfall patterns, reducing the amount of sunlight striking the Earth’s surface, and changing the balance of “direct” and “diffuse” sunlight, effectively making the sky look more hazy. These effects will, in turn, have profound impacts on ecosystems, including the rates of photosynthesis in forest carbon sinks, agriculture, and human well-being. Even if it works to lower temperatures as planned, SAI will have no impact on the non-climatic effects of increasing CO₂, such as ocean acidification. SAI is also inherently a temporary intervention; it will require sustained deployment for as long as 100 years, according to one study, to avoid “termination shock” and an abrupt temperature increase if GHG concentrations are still high. SAI also poses immense geopolitical, legal, and ethical challenges, including international responsibilities for implementation, financing, compensation for negative impacts, and procedural justice questions, such as those around informed consent. And finally, beyond these scientific, environmental, political, and socio-economic concerns, SAI poses a serious “moral hazard” that could distract or delay action on real solutions to climate change.