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Should we dim the sun? Why we need to talk about solar geoengineering


Five months after the Titanic struck an iceberg and sank in the North Atlantic Ocean in April 1912, Carroll Livingston Riker had an idea to prevent the tragedy from ever recurring.

The Brooklyn-based engineer and inventor concocted a project so grand that he believed it could tilt the axis of the Earth. 

Riker’s proposition was to redirect the Gulf Stream, a warm ocean current that works its way along the eastern coast of the US, by building a jetty off Newfoundland. Riker believed it could help warm water move into the Arctic, melting the ice and allowing ships to safely pass through. Moreover, with less ice at one end of the Earth, he reasoned that the planet “would swing around a bit” on its axis, warming up some of our world’s coldest climates. 

The engineer was no stranger to ingenious solutions — he’d built the first refrigerating warehouse in the world — but this was a proposal to engineer the Earth’s system. Today, we might call it geoengineering. 

Riker’s proposal never got off the ground, but in the past 110 years, humans have unintentionally brought his ideas to bear. By burning fossil fuels and releasing carbon dioxide into the atmosphere, we’ve essentially been running our own geoengineering project. It’s proving particularly disastrous for the ice in the Arctic, and, strangely enough, it has has affected the tilt of Earth’s axis.

In recent decades, geoengineering has become a hot topic, but not the kind Riker was proposing. The most discussed, and most controversial, is solar geoengineering, an idea that would see light from the sun dimmed by injecting reflective particles into the atmosphere. Less sunlight reaching the surface of the Earth means less heat trapped by carbon dioxide.

As world leaders, activists and academics meet in Glasgow, Scotland, for COP26, the UN’s premiere climate change conference, CNET Science is examining some of the technological advances being developed to help tackle the climate crisis. While technology might help us adapt or mitigate the effects of climate change, alone it’s not a solution to the problem. Drastic reductions in carbon emissions are required for the world to limit global warming to 1.5 degrees Celsius by the end of the century — the chief goal of the 2015 Paris Agreement — and there is no substitute.

Solar geoengineering doesn’t address the underlying cause of human-driven climate change: carbon emissions. However, some scientists argue that it could be a cheap, important tool in our climate change toolkit, if only we were able to research it more thoroughly. 

“It’s not a replacement for cutting emissions,” says Gernot Wagner, a climate economist at New York University who has spent several decades researching geoengineering. “We should be having this conversation right about now, and we should be doing the research.”

Others, including environmental organizations like Friends of the Earth, have opposed solar geoengineering, writing that it “will take us in the wrong direction” and is an “attempt by those most responsible for climate disruption to continue polluting.”

It’s a fraught topic. And yet, with 1.1 degree of warming already locked in and scientists concerned that we won’t be able to keep warming below 1.5, solar geoengineering research might finally have its time in the sun.

Managing the sun

In a nutshell, solar geoengineering refers to deliberately increasing the amount of sunlight reflected back into space. You might also sometimes see it referred to as solar radiation management, or SRM. 

The most-discussed approach, at least in recent years, involves releasing reflective particles into the stratosphere, the second layer of Earth’s atmosphere, which extends to the edge of space. These particles, or aerosols, linger in the air and would be made of sulfate or calcium carbonate. It’s known as stratospheric aerosol injection, or SAI, and scientists have been examining the risks and benefits of aerosol injection for decades

Nature is the best geoengineer, and volcanic eruptions are their own form of solar radiation management. During an eruption, plumes of smoke filled with sulfates can dim the sun. The Pinatubo eruption, the second largest in the 20th century, dropped planetary temperatures by half a degree in 1991.

Sulfates come with considerable risks. They’ve been shown to damage ozone and potentially heat the lower tropical stratosphere. It’s unclear how injecting these compounds into the atmosphere might affect rainfall patterns and whether they would disturb some of the Earth’s natural processes. In one sense, we might be changing the climate even more than we know.

One high-profile SAI project is Harvard’s Stratospheric Controlled Perturbation Experiment, or Scopex, which proposes to use a different compound: calcium carbonate. 

In computer models, calcium carbonate, commonly found as limestone and used by snails to make shells, has been shown to have a similar effect to sulfates — without the ozone-damaging side effects. In fact, some models suggest it might even reverse ozone depletion.

Another option is marine cloud brightening, which involves spraying sea salt into the air. In theory, this interacts with clouds to increase their reflectivity. This type of SRM has been trialed over Australia’s Great Barrier Reef and is intended to provide additional shade to the expansive coral system. 

The eruption of Mount Pinatubo, in the Philippines, on June 15, 1991, was the second largest volcanic eruption of the 20th century.


Arlan Naeg/Getty

Moratoriums and moral hazards

Paul Crutzen, a Dutch chemist and Nobel laureate, broached the idea of stratospheric aerosol injection in an essay 15 years ago, igniting a debate over SRM that continues to this day.

Even though solar geoengineering had been floated for decades already (some trace it back to US President Lyndon B. Johnson in the 1960s), scientists barely spoke about it. They certainly weren’t conducting the research. Crutzen’s essay, Wagner says, lifted a “self-imposed moratorium” on discussing or researching the technology. 

The silence around solar geoengineering was predominantly born of a fear that even speaking about it would create a moral hazard: If we had a technological solution like this, it could detract from efforts to cut carbon emissions, relieving pressure on the fossil fuel industry and politicians. 

But the problem is that we don’t know exactly how these aerosols work and we don’t understand the pros and cons of actually doing the research. Solar geoengineering could have unintended consequences in our atmosphere. Scientists working in the field to kick off the experiments share the same concerns as environmentalists in this regard. Some opponents take it a step further, suggesting it’s a slippery slope from scientific investigations to deployment.

To date, work has been mostly confined to computer models, which can take the science only so far. Field experiments have been few and far between. In 2021, Harvard’s Scopex team planned to change that — but met with extreme resistance.

Sunrise, sunset

The Scopex team planned to conduct the first stratospheric aerosol injection in the field earlier this year. The team’s experiment involved sending a balloon into the stratosphere above the small town of Kiruna, Sweden. Attached to the balloon would be a platform, stacked with scientific gizmos that would gather information about the flight. 

Crucially, it wouldn’t release any aerosols on its flight. 

Windmills silhouetted against a cloudy sky

Kiruna, in Sweden’s far north, was going to be the site of the first Scopex test. The city, situated in the Arctic, is warming three times faster than the rest of the world.


Jonathan Nackstrand

In February, a conglomerate of Swedish environmentalists and the Saami council, which represents indigenous peoples’ organizations in the country, penned an open letter to the experiment’s advisory board, arguing that stratospheric aerosol injection “entails risks of catastrophic consequences.” The groups also noted the lack of public engagement by the Scopex team in Sweden and argued that there were no acceptable reasons for Scopex to continue — not just in Sweden, but anywhere

A month later, the Scopex advisory board, consisting of environmental scientists, lawyers and experts in risk management, suspended the trial. It decided that until “robust and inclusive” engagement with the public occurred, the balloon would remain grounded. The earliest date for a test flight is now expected to be in 2022 after additional social engagement.

Although that was a setback, attitudes have been slowly shifting since the experiment’s announcement in 2015. “The conversation is changing — and has changed — because of the conversation around this experiment,” notes Wagner. 

Shortly before the project was suspended, the US National Academies of Science urged the US government to invest in solar geoengineering research to the tune of $100 million. In May, one of the world’s top science journals, Nature, published an editorial titled “Give research into solar geoengineering a chance.” 

Where do we go from here?

Scientists and researchers, like Wagner, aren’t advocating for solar geoengineering projects to be deployed on a planetary scale. Organizations like the Union of Concerned Scientists oppose large-scale aerosol injection tests, and even David Keith, who helps lead Harvard’s solar geoengineering team, says he loses sleep about the purported moral hazard of such research

The urgency isn’t in deployment but in basic research.

There are many open questions that span science, ethics and governance. What are the best solar geoengineering techniques? Who would deploy and oversee them? What countries would be most affected? How do you control a release effectively? Could it be weaponized? Where does solar geoengineering fit in with our decarbonization plans? Is it something the wider public understands and agrees with? What would it cost? And what happens to the atmosphere when an aerosol injection project ends? 

sunset, china, over mountains

Public engagement and governance are key aspects on solar geoengineering’s horizon.


Getty/Philippe Lejeanvre

There’s consensus that large-scale interventions should remain off-limits, that a more robust governance system should be in place and that scientists must educate the public about the risks and benefits of any proposed experiments. There’s also growing momentum from some environmental organizations, like the Environmental Defense Fund, that small-scale experiments are the way forward. Even so, progress has been slow. 

An unfortunate reality remains: We are not decarbonizing quickly enough to keep global warming below 1.5 degrees Celsius by the end of the century. Authors of the latest IPCC report worry we might even top 3 degrees Celsius, a temperature rise that comes with a catastrophic outlook for the planet. 

Solar geoengineering won’t provide a climate fix. It shouldn’t be framed in any way that detracts from the world’s need to cut carbon emissions — in fact, it should make decarbonizing even more urgent and potentially be tied to it. Having to resort to planetary-scale engineering projects is, in a word, nuts. 

But it could turn out to be even more nuts if scientists aren’t able to undertake safe, basic research to understand exactly how such a project would ultimately affect the planet.



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