It's simple physics — if we spray aerosols into the stratosphere for long enough, eventually they will start to reflect the sun rays and reverse catastrophic global warming. That's what the proponents of solar geoengineering say.
But its many opponents say it's a dangerous gamble on a planetary level and that the potential consequences are so complicated it should not even be considered as an option.
After spending years as a relatively obscure idea, solar geoengineering is now gaining increasing prominence among lawmakers, academics and even UN climate body IPCC, with the alluring possibility of taking the planet back to pre-industrial temperatures even as humanity dithers on ramping down carbon emissions.
US President Joe Biden's administration has argued the case for a coordinated research program into solar radiation management (SRM) — a term describing geoengineering efforts to reflect the sun. Still, the United States has stressed it has no plans to launch such a program.
The EU's executive arm has called for international talks on a potential framework to address the dangers and governance issues of SRM.
How does it work?
The most prominent approach involves spraying tiny droplets of sulfur dioxide into the stratosphere — the atmospheric layer between 10 and 50 kilometers (6-30 miles) above the earth's surface. There they can survive for about one year, reflecting sunlight and cooling the planet.
The principle was demonstrated by the 1991 volcanic eruption of Pinatubo in the Philippines. Its blast was so powerful that it injected about 15 million tons of sulfur dioxide right into the stratosphere. The eruption cooled the planet by more than half a degree Celsius over the following two years.
Modeling shows that reflecting 1% of the sunlight reaching earth would be enough to cool the planet to pre-industrial levels, according to Douglas MacMartin, a senior research fellow at Cornell University in the US.
Research shows also that replicating such an injection of sulfur dioxide into the stratosphere isn't practically difficult.
MacMartin added the cost of deployment would be relatively low, with one paper estimating $18 billion (€16 billion) annually. That's much less than the damage already caused by climate-fueled extreme weather.
"Relative to the damages from climate change, it's basically free," MacMartin said.
That's not to say it would be simple. It would likely involve hundreds of aircraft stationed around the world. They would fly thousands of flights a year over the next century, constantly spraying aerosols in the atmosphere. Deploying such technology would affect the entire planet too.
It may sound reckless, but proponents point out that we already pump about 10 times more aerosols into the lower atmosphere, as a result of regular pollution. This too has a cooling effect but is associated with other health and ecological hazards.
What are the downsides?
Not all the possible effects are fully understood, but scientists already know that the aerosols would produce acid rain, and would deteriorate the ozone layer, which according to the UN is helping to prevent more warming.
It would also affect precipitation patterns, potentially exacerbating catastrophic weather systems — already made worse by climate change — in some areas. Other, more catastrophic effects may come to light with further research.
And while reflecting the sun with aerosols would cool the planet, it cannot reverse other effects of climate change, such as ocean acidification.
MacMartin argues that these risks should be balanced against those of doing nothing at all in the case that we don't cut emissions quickly enough.
"We're not in a good place. This is all about risk versus risk. And the only decision on the table today is do we do enough research so that if climate change gets really bad, we can make an informed decision."
The dangers of even looking
But hundreds of scientists believe further research into this technology should be halted altogether.
Aarti Gupta is a professor of global environmental governance and politics at Wageningen University in the Netherlands. Gupta was one of the initiators of an open letter against public funding of solar geoengineering research — a letter signed by more than 450 scientists.
The group argues that even with extensive simulations and modeling, the only way to completely understand the risks and effects of solar geoengineering is to fully deploy the technology on an unwitting planet, locking future generations into a geoengineered atmosphere.
"There are so many things we don't necessarily understand about the climate system. We would only know those sort of irreversible consequences when they actually manifest," Gupta told DW.
The group also argues that the lure of this technology will only delay the proven and urgently necessary action of reducing carbon emissions. Fossil fuel lobbyists could exploit SRM to further delay action, fears Gupta.
"It may be ineffective and may grossly exacerbate the risks and impacts. At the same time, we would take our eye off what we should be doing in terms of emission reductions and fossil fuel phase out," Gupta said.
The group also argues that solar geoengineering could never be governed in a fair and inclusive manner, and planet-altering decisions will inevitably be dominated by rich and powerful nations at the expense of poorer countries.
"The history of climate negotiations tell us that these very thorny issues — where there's somebody who will lose and somebody who will gain — are not going to be made with the small island or developing states, for example, having a veto right or having control over these technologies," Gupta said.
This skepticism is also held by the Center for International Environmental Law, which wrote that the tools to tackle the climate crisis already exist.
"Betting on future technofixes creates dangerous distractions and delays the implementation of real solutions," added the nonprofit based in Washington, D.C.
The Union of Concerned Scientists has called for research to be limited to modeling and observational studies for now, although it does not rule out small-scale experiments in the future.
Earlier this year, 110 physical and biological scientists penned a rival open letter, calling for a comprehensive, international assessment of how such interventions would affect climate risks regionally and globally.
The allure of a panacea
Other scientists see it as one of the only options to buy enough time to reduce emissions, extract enough carbon dioxide from the atmosphere and avoid the worst effects of climate change.
John Baker, for example, is a restoration ecologist who has seen first-hand the damage of climate change in his work restoring maritime forests in the US.
"We're at a very critical point where we need to use every tool and approach that's possible in order to help prevent the loss of quality of life and the degradation of our biosphere."
"I don't want a geoengineered atmosphere. But I also recognize that we're at the beginning of the sixth mass extinction. And we're losing species and habitat right now," he said.
While the debate over the future of solar geoengineering continues, scientists agree that the most pressing concern, in all scenarios, is to reduce carbon emissions as much and as fast as possible.
Edited by: Tamsin Walker