The Sunlight Reflection Methods

Major volcanic eruptions, like Pinatubo in 1991 and Tambora in 1815, added a layer of tiny reflective sulphate particles to the stratosphere, above the tops of most clouds. These particles spread out to create a thin global layer that reflected a small fraction of incoming sunlight and cooled the earth for a few years.

Stratospheric aerosol injection (SAI) would aim to mimic this proven cooling effect by releasing tiny aerosol particles into the stratosphere. There, they would have a much longer lasting, and so much larger, cooling effect than they would in the lower atmosphere.

With hundreds of specialised aircraft lifting millions of tonnes of sulphate or other particles to the stratosphere, geoengineering with SAI could lower global temperatures by one or even several degrees Celsius. While uncertainties remain, there is little doubt that this could have a substantial cooling effect.

By lowering global temperatures, SAI might reduce many climate risks, though not all. It would affect rainfall patterns, causing greater change in some places than expected under climate change alone. It would also have some undesirable side effects such as adding a little to acid rain, making the sky a little hazier, and delaying the slow recovery of the ozone hole by some decades.

“Ship tracks” form in certain areas of the ocean when polluting ships pass underneath low clouds. The tiny pollution particles emitted by these ships act as seeds for new cloud droplets, triggering cloud formation or brightening existing clouds.

Following changes to pollution standards for ships in 2020, ship tracks have become less common and ocean clouds near busy shipping routes have become less reflective. This clean-up effort could have contributed slightly to recent global warming.

Marine cloud brightening (MCB) is an idea to produce a similar cloud-brightening effect without the air pollution impacts by spraying sea-salt particles from ships.

While this idea could work in principle, engineers have yet to develop sea-salt sprayers that would be efficient enough to be used for deployment, and the effects of the particles on clouds remains very uncertain. In fact, these aerosol–cloud interactions are one of the biggest uncertainties in climate science.

This is why a team in Australia has been conducting outdoor experiments of this idea since 2020, and several other research teams are working on plans for outdoor experiments of their own.

While SAI and MCB are receiving the most attention from researchers, there are several other SRM ideas.

Space-based SRM would involve placing material between the Earth and the Sun to deflect light before it reaches the Earth, though it would be prohibitively costly, at least for the next several decades.

Cirrus cloud thinning would aim to reduce the heat-trapping effect of high wispy cirrus clouds. Adding the right kind of particles might thin these clouds, but there are deep uncertainties about its potential and effectiveness.

Mixed-phase cloud thinning is a similar idea, though focused on a different type of cloud that forms lower in the atmosphere and contains both ice crystals and supercooled liquid. Thinning those clouds, especially in winter months at the poles, may allow some extra heat to escape and help regrow melting sea ice.

There are also several ideas for brightening surfaces, for example, developing slightly more reflective crops, building with brighter materials, or brightening and thickening sea ice. These ideas might have some potential to alleviate local impacts, but none could be scaled up to have a substantial global cooling effect.

Cutting greenhouse gas emissions is and will remain the primary strategy to combat climate change. Removing carbon already in the atmosphere and adaptation are also critical.

SRM is being explored as an additional tool to use alongside these other strategies, offering a way to reduce temperatures and limit many of the harms of climate change. However, these approaches would bring new risks and uncertainties of their own, as well as raising profound ethical and political questions.