In 2021, a team of researchers from Harvard University’s Department of Earth and Planetary Sciences arrived in Kiruna, a small town in northern Sweden.
Their mission: to carry out a field trial of something they had only modeled on computers – until then.
The Stratospheric Controlled Perturbation Experiment, or SCoPEx, would entail lofting small amounts of calcium carbonate into the stratosphere with a balloon to create tiny light-scattering particles, and then measure the results.
This would have been one of the world’s first outdoor experiments in solar geoengineering – a series of methods to cool the Earth by increasing the amount of sunlight reflected back into space.
But faced with strong opposition from environmentalists and Indigenous Sámi communities, the project was canceled before it even got off the ground.
Solar geoengineering continues to sharply divide scientists, too.
On one hand, more than 500 climate scientists have signed a statement calling for an International Non-Use Agreement on Solar Geoengineering. They argue that the technology “cannot be fairly governed globally and poses unacceptable risk if implemented as a future climate policy option.”
On the other, 110 scientists, including former NASA climatologist James Hansen, have penned an open letter calling for further research to be carried out to better understand these risks.
The Union of Concerned Scientists (UCS) opposes the deployment of solar geoengineering because of the risks it poses, but it supports “continued modeling research, observational studies, and strong, inclusive public participation in decision-making.”
While the Alliance for Just Deliberation on Solar Geoengineering (DSG) acknowledges these risks, it also calls for inclusive discussions and decision making on the issue to avoid worsening climate injustices.
Solar engineering, also known as solar radiation modification (SRM), is one of two main forms of geoengineering, alongside carbon capture and storage. Rather than removing carbon from the atmosphere, it works by reflecting sunlight back into space.
The most well-studied and widely proposed method of solar geoengineering is stratospheric aerosol injection (SAI).
SAI involves using aircraft or balloons to release small amounts of a chemical such as sulfate dioxide, calcium or even diamond dust into the stratosphere, where it mixes with vapor to form aerosols that reflect sunlight back into space.
This mirrors what happens after a volcanic eruption. When Mount Pinatubo in the Philippines erupted in 1991, for example, the resulting blast of sulfate dioxide into the stratosphere is estimated to have lowered global temperatures by about 0.6 degrees Celsius over the following 15 months.
Many researchers consider SAI the most reliable of all solar geoengineering methods.
“We know that it would cool, and we understand some of the other effects,” says Douglas MacMartin, an associate professor in mechanical and aerospace engineering at Cornell University.
“There just can’t be any really big unknown unknowns, because we would have seen them after large volcanic eruptions.”
One downside is that the sulfate eventually comes down as acid rain, he acknowledges. “That is in the list of side effects, and part of the trade-off.”
However, MacMartin adds, not only are the amounts involved small compared to current pollution levels, but about two-thirds of that rain “comes down over the oceans, where it’s utterly irrelevant.”
Another solar geoengineering method is marine cloud brightening (MCB). This would deploy a fleet of ships to inject particles of sea salt into low-lying marine clouds to form smaller water droplets.
This makes the clouds more reflective, thus reducing the amount of heat absorbed by the ocean below.
Other methods that are being studied are more of a long shot.
One is cirrus cloud thinning (CCT). Cirrus clouds are found at high altitudes and tend to prevent heat from escaping from the atmosphere, keeping it trapped and thus heating up the planet.
CCT involves introducing aerosols like sulfuric or nitric acid into the upper atmosphere to thin these cirrus clouds out. The idea is that this could replace them with clouds with larger ice crystals and shorter lifespans, allowing more heat to escape from the Earth.
Then there’s the idea of building giant mirrors in space to physically reflect light away from us, which MacMartin dismisses as “hideously expensive.”
On that note, global funding for solar geoengineering remains relatively small but has risen significantly in the last 15 years, reaching USD 191.7 million last year. Just under half of that money comes from philanthropists, with governments providing most of the remainder.
One of the latest recipients of funding to study solar geoengineering is the Advanced Research and Invention Agency, ARIA, which recently received GBP 56.8 million (USD 76 million) from the British government.
ARIA’s fresh funding will go to an array of projects, including five involving small-scale outdoor experiments in both MCB and SAI.
Unfortunately, the deployment of these solar geoengineering methods could have many potential unintended side effects, such as disturbances to regional rainfall patterns and temperatures.
These potential consequences are not yet well understood, and they could have particularly devastating effects for communities across the Global South.
Another major concern is governance, says Mike Hulme, a professor in the Department of Geography at the University of Cambridge and author of Can Science Fix Climate Change?
“There’s no globally agreed way in which these technologies, from a research point of view, are being governed – never mind how their deployment should be governed,” he says.
“So, we’re going down a path where we haven’t got the protocols of research governance, let alone deployment governance.”
Then there’s the fear of termination shock. If solar geoengineering is deployed and begins to cool the planet, any abrupt stop for whatever reason could result in a sudden, potentially massive rise in temperatures afterwards.
In other words, a millennial commitment would be required.
“There’s nothing in human history to give comfort to the fact that the world would collectively be able to fairly and democratically manage such a feat,” says Mary Church, geoengineering campaign manager for the Center for International Environmental Law (CIEL).
Some experts fear that even carrying out research on solar geoengineering risks legitimizing it – not to mention distracting policymakers and businesses from cutting emissions by providing a ‘get out of jail free card.’
“That normalization within the scientific community will spill over into the public realm,” says Hulme. “Once you embark on a process of technological innovation and development, it’s actually very difficult to get off the slope.”
By addressing some of the symptoms of climate disruption, critics say, even limited deployment could create a moral hazard by reducing incentives to take the difficult measures needed to decarbonize the global economy.
“It just takes the pressure off mitigation for vested interests – for the big polluters who are still making a fortune from harming the planet and pushing the climate beyond the brink,” Church argues.
MacMartin, echoing many other experts, makes it clear that any research on solar geoengineering must be accompanied by major reductions to greenhouse gas emissions.
“Are the benefits and risks of doing so better or not than the benefits and risks of not doing so, including both the physical climate effects and the societal concerns (which are absolutely legitimate concerns)?” he poses.
For MacMartin, the answer is a resounding yes – and the fact that such a technology exists shouldn’t serve as an excuse to keep emitting.
“I’ve never heard somebody say, ‘great, I’ve got seat belts and airbags. I’m going to run this red light,’” he analogizes.
Still, there are several international conventions and regulatory regimes that create roadblocks to any further research.
These rules, Church explains, are “ultimately restrictive not just for solar geoengineering but across all geoengineering approaches.”
One example is the United Nations Convention on Biological Diversity (CBD). Its members first agreed to place a moratorium on solar geoengineering in 2010 – a position it has reaffirmed multiple times, most recently at last year’s COP16.
And at last year’s UN Environment Assembly, African countries successfully resisted efforts led by Switzerland to set up a UN expert panel to “examine risks and opportunities” related to solar geoengineering.
There’s also the 1996 London Protocol, which controls marine pollution by prohibiting the disposal of waste at sea. Parties to the Protocol must implement a precautionary approach to environmental protection.
“It currently prohibits ocean fertilization but is also looking at regulating further techniques, including two solar geoengineering techniques that take place from the marine environment,” says Church.
“You have the obligation not to cause transboundary harm.”
Yet, she adds, “the basic proposition of solar geoengineering is, it’s untestable other than through deployment. So, obviously, that runs counter to the precautionary principle.”
For Shuchi Talati, co-founder of DSG and a member of the now-defunct SCoPex advisory board, solar geoengineering is a double-edged sword.
“It could be a way to limit human suffering,” she told the New York Times last year. “At the same time, I think it can also exacerbate suffering if used in a bad way.”
So, will continued research and modeling eventually bring the two opposing sides together?
As world leaders fail to respond anywhere near adequately to the climate crisis, and greenhouse gas emissions continue to climb, there is little sign that such a convergence will happen anytime soon.
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