There is evidence that SAI can offset some of the impacts of climate change, but will come with ancillary risks of its own. Crucially, it would be possible to restore the average global temperature to pre-industrial levels if sufficient quantities of aerosols were injected in a sustained fashion.⁹ However, modelling studies suggest that it is not possible to reset temperatures in every region. The same applies to precipitation, wind patterns and other aspects of climate. So, while SAI could lead to a more favourable outcome overall, there would inevitably be some winners and some losers.¹⁰

Sulphate aerosols are the most commonly proposed substance for SAI. However, sulphur’s potential to damage the ozone layer or lead to acid rain is a significant concern and this is driving research into alternative materials, such as calcites or chalk powder. A better understanding of the way all these materials interact with the upper atmosphere is much needed.

The overwhelming majority of studies of the effectiveness and consequences of SAI have been carried out through computer modelling. Most of the field tests that have been proposed have been called off after facing opposition.¹¹ But a small number have taken place, carried out by independent researchers and funded by entrepreneurs, raising concern over the possibility of unilateral action.^{12 13}

Various researchers have also proposed thinning and dispersing high-altitude cirrus clouds, which contribute to warming by trapping a disproportionately large amount of terrestrial radiation that would otherwise escape into space.¹⁶ Injecting these clouds with particles of bismuth tri-iodide allows the formation of large ice crystals within the clouds. These large ice crystals fall out more rapidly, shortening the clouds’ lifespan. Cirrus reduction can improve the transmission of long-wave terrestrial radiation into space.¹⁷

In February 2024, researchers at the US National Oceanic and Atmospheric Administration proposed injecting ice-nucleating particles into the troposphere to prevent water vapour reaching the stratosphere, where it usually absorbs outgoing radiation. This “intentional stratospheric dehydration” would help to cool the planet by increasing outbound radiation, although significant technical and practical hurdles would need to be overcome first.¹⁸

Whereas SAI has a globally uniform effect, cloud-engineering techniques produce more localised cooling, allowing them to be used in a targeted way. For instance, Australian MCB could be used to cool major coral reefs, which suffer bleaching when water temperatures become too high.¹⁹

One prominent method is to paint roofs white or other pale colours. This can have direct benefits to the local populations, including mitigating the worst effects of heatwaves. Furthermore, the cost is relatively low.²¹

In areas not covered by buildings and roads, plants with light-coloured leaves can also change the local albedo. This is cited as a potential downside of planting additional trees on grassland areas: trees are typically darker than grass, so they lower the albedo and may thus contribute some warming. There is also ongoing research into engineering paler crop plants, a task in which synthetic biology may play a role.²²

At the more extreme end of the scale, there are proposals to artificially regrow Arctic sea ice, perhaps by spraying tiny particles of silicon dioxide to encourage ice formation, or by pumping huge volumes of cold water up from the deep sea. In theory, schemes like these could restore a large area of reflective ice surface. However, the feasibility and costs are uncertain, and there may be unforeseen negative environmental consequences.²³

The simplest notion is a large occulting disc or “parasol”. This would be placed at a carefully chosen position between the Earth and the Sun, in order to produce a permanent partial solar eclipse. The ideal location would be Lagrange Point 1, where the gravitational pulls of the Sun and Earth are balanced.²⁴

Researchers have also considered a number of alternative reflectors for reducing the radiation incident on Earth.²⁵. These include Fresnel lenses, diffraction gratings and mirrors.^{26 27} The key considerations are the robustness of the design to meteoroids and other threats, and the mass of the structure — which must be carried into space by rocket, or else manufactured in space, adding to the cost.

All these technologies face considerable technical and economic barriers.²⁸ For example, an occulting disc at Lagrange Point 1 would need to have a surface area of millions of square kilometres: no structure remotely close to such a scale has ever been constructed in space. Furthermore, such projects arguably also create a dangerous single point of failure in our climate-mitigation strategies: in contrast to Earth-based forms of SRM, the scale of investment and hardware deployment required for a space-based reflector would mean putting all our eggs in one basket, with catastrophic risks if the project failed.