Solar radiation management (SRM or solar geoengineering) is a theoretical approach to reducing some of the impacts of climate change by reflecting a small amount of inbound sunlight back out into space. It is in the early stages of research, but it is already a controversial topic. It is clear that SRM has the potential to be very helpful or very damaging for those people and species most threatened by climate change, but it is very unclear what its full effects would be.
SRM would not directly reduce concentrations greenhouse gases, and therefore numerous expert reports have concluded that it could never be a complete solution to global warming and does not represent a substitute for mitigation of greenhouse gas emissions. However, they have also concluded that it might be able to reduce some climate risks to which Earth is already committed, though even for this more limited purpose whether it can be net positive to humanity and the environment is unclear.
For instance, if it could be made to work, SRM would be the only known method for quickly stopping the rise in global temperatures. It could even be used to cool the planet, should that ever be deemed necessary. As such it might be able to reduce some damages while humanity decarbonizes the global economy, or it might offer a method for dealing with some of the climate risks associated with those greenhouse gases that have already been emitted to the atmosphere. The uncertainties around the effects of SRM are large though, and there is not nearly enough evidence yet available to evaluate whether use of SRM would increase or decrease the impacts of climate change, and where.
Different SRM techniques have been proposed, but the proposals receiving the most attention from researchers would involve brightening marine clouds by spraying seawater into the lower atmosphere, or replicating the cooling effect of volcanoes by spraying reflective sulfate particles into the upper atmosphere (the stratosphere).
Stratospheric aerosol injection
When very large volcanoes erupt they blast millions of tonnes of reflective sulphate particles into the stratosphere. These particles circulate the planet on the powerful stratospheric winds, reflecting away a small amount of inbound sunlight and cooling the planet for a year or two. Stratospheric aerosol ejection would seek to replicate this effect, with aeroplanes or balloons used to inject reflective aerosol particles into the upper atmosphere.
Marine cloud brightening
Large areas of ocean are covered in marine stratus clouds. Scientists have proposed that spraying tiny droplets of seawater into these clouds could make them lighter and more reflective. Whiter, brighter clouds reflect more sunlight back out into space, and would help cool the planet.
It is not yet known what all the side effects of SRM could be, or whether they would be very damaging. Some possible side effects are known, however. For instance, stratospheric aerosols might delay the regeneration of the ozone layer. There might also be some health effects from the additional aerosols in the atmosphere, although these are not currently expected to be large.
The physical impacts of SRM are only one part of the story. Many people feel that the political dimensions could prove more difficult to manage than the physical dimensions. For instance, what would happen if a country used SRM unilaterally, and then other countries experienced extreme weather events that might have been caused by the geoengineering? How would the world achieve widespread international agreement over how much SRM cooling to do? Some countries might want the world a degree or two warmer, and some might want it a degree or two cooler. Also, there are concerns that the idea of SRM could distract politicians from the task of implementing deep cuts to greenhouse gas emissions.
Many people and organizations have called for more research. Basic calculations have indicated that SRM could be very cheap to deploy relative to the costs of climate mitigation or adaptation, and so research and discussion usually focus on risks, benefits, uncertainties, and governance.
But research itself is controversial and it raises numerous questions:
- Who decides if research proceeds, and what should be researched?
- Who pays for the research? Who benefits?
- What ensures that research is conducted in a transparent manner, and that all results are shared openly?
- How can the different research priorities of different groups be heard?
- What can be done to make sure that SRM research does not distract public and politicians from the task of cutting emissions of greenhouse gases?
These questions can shape the way that SRM develops, but there are no clear answers. A broad global conversation about SRM and the governance of related research is therefore required.