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Inside the high-stakes battle over dimming the sun’s light to cool the planet

In recent days, letters from dozens of climate scientists argue for increased research into altering the sun’s rays to save the planet. Others say doing so could doom us all.

In one version of a not-very-distant future, specially engineered planes about the size of Boeing 737s dim the sun's rays to help cool the planet by dispersing loads of molten sulfur in great mists across the sky.images from adobe; globe staff photo illustration

In one version of a not-very-distant future, eight specially engineered planes about the size of Boeing 737s fly again and again into the stratosphere and disperse loads of molten sulfur in great mists across the sky. The planes repeat the trip thousands of times over the course of a year, creating a reflective blanket around the earth meant to divert enough of the sun’s heat to stave off the worst effects of global warming.

The frequency of flights in this future would gradually increase, continually replenishing and fortifying the blanket to keep rising temperatures at bay. By year 15, a fleet of 95 planes would ascend from bases around the world 60,000 times annually.


It may sound like sci-fi, but it’s not.

Scientists around the globe — led by a pioneering team at Harvard — have been researching options like this in labs for years, taking inspiration from volcano eruptions, which temporarily cool the atmosphere when they emit sulfur dioxide. And now, as the devastating tolls of climate-driven extreme weather mount, and as hope dims that the world can move quickly enough to avoid catastrophic tipping points, the work is being thrust to the fore.

Last week, a group of 90 prominent climate scientists issued an open letter urging more and faster study of so-called solar radiation management, also known as solar geoengineering, citing “devastating impacts” already threatened by the warming climate and “grave threats to public health.” Another group of scientists, across a broad range of disciplines including political science and economics signed a similar letter. The United Nations Environment Program, meanwhile, weighed in, acknowledging the possible necessity for such measures in the future and calling for rules to govern any effort to proceed.

A wave of critics has responded, pointing to profound moral, political, and legal questions cascading from an effort they say could have vast, unknown consequences — and which represents an unprecedented human intervention in natural forces.


“There’s this slippery slope and suddenly humankind is walking down the road with closed eyes to a new planet, which will be entirely geoengineered,” said Frank Biermann, a political scientist researching global sustainability governance at Utrecht University in the Netherlands. He helped lead an effort to urge international governments and the United Nations to adopt a ”Solar Geoengineering Non-use Agreement,” which was supported by some 400 other scholars and scientists.

The authors of each of last week’s letters urging more research say they do not advocate deploying the technology but rather vigorously studying it in order to be prepared in the event it is needed.

They also say they do not intend for the technology to be seen as a substitute for aggressively slashing greenhouse emissions worldwide. But they also note that it is increasingly unlikely that warming will be kept below the 1.5 degree Celsius threshold set by the Paris Agreement. The letter signed by climate scientists said the planet’s natural systems are already “approaching thresholds for catastrophic changes,” potentially necessitating drastic measures to rapidly cool the planet at some point in the next three decades.

Solar geoengineering could emerge as the planet’s best option should that need arise, said Sarah Doherty, an atmospheric scientist at the University of Washington, Seattle, who organized the letter signed by climate scientists.

“The cat is out of the bag in terms of these options existing, so they’re likely to be put on the table for consideration in the future,” she said. “The question is simply: ‘Do you want to do that with good information, or do you want to hold off research on it and make those decisions with poor information?’ ”


Some of solar geoengineering’s most vehement critics say the possible ramifications of such a mass-scale attempt are so large and potentially dangerous that such a step should not even be considered. They cite a raft of possible problems, including that it could dampen world urgency for cutting emissions.

They also say there is probably no way to know in advance the effect on world weather patterns and whether it could bring flooding or have devastating impacts on agriculture and economies, perhaps even sparking mass migrations.

“You change where and how frequently it would rain, and because the effect would be a global one, this means it could affect weather patterns at a very large regional scale, including a global scale,” said Lili Fuhr, deputy director of the climate and energy program at the Center for International Environmental Law, headquartered in Washington, D.C.

Another worry that critics cite is the difficulty of maintaining the protective blanket once it is put in place. Researchers say that once solar geoengineering is employed, the world could be dependent on it for generations to compensate for the warming effects of carbon that could remain in the atmosphere for several centuries. Any prolonged interruption of the replenishing flights — for instance, by economic collapse, war, or merely international disagreement about continuing — could cause a rapid spike in world temperatures and a phenomenon scientists are calling “termination shock,” said Raymond Pierrehumbert, an Oxford physicist and a vocal critic of solar geoengineering.


As troubling to some as the climate implications are, solar geoengineering also poses never-before raised ethical and legal questions; weather disasters that insurance policies and other legal documents once referred to as acts of God would now have human involvement, and potentially human liability.

Moreover, solar geoengineering could “increase power imbalances between nations, spark conflicts, and raise ethical, moral, legal, equity and justice issues,” wrote authors of last week’s report by the United Nations Environment Program.

Without mechanisms in place for countries to fairly navigate such conflicts, poorer countries are likely to be at the mercy of wealthier nations’ decisions — about whether to use solar geoengineering in the first place and how to manage it if it is deployed, said Biermann, the Utrecht political scientist.

“What do you do if something goes wrong? Suppose a certain part of the planet will experience a different climatic system that they don’t want. Where do you complain?”

Biermann said he has concluded that the current world order is unlikely to correct that imbalance and that geoengineering simply “cannot be governed in a fair and effective manner.”

But not all scholars agree. Jesse Reynolds, an expert on international environmental policy and author of “The Governance of Solar Geoengineering,” is a proponent of increasing research into solar geoengineering. He said that just like more research in the lab and in small-scale field experiments are needed now, so too is a long and careful international dialogue about what governance of solar geoengineering would look like. That might include the creation of a fund that could compensate nations in need, he said.


But just because it’s governable, Reynolds said, doesn’t mean it’s not fraught. “I do see it as posing environmental risks, especially if it’s used in a careless or reckless way. I can really see the ways in which it can raise serious challenges of international cooperation. And the big question is, if this was going to be explored, how can it be done in a way that keeps the focus on emissions cuts?” he said.

David Keith, a Harvard physicist who led the development of the university’s Solar Geoengineering Research Program, said more research is needed to better understand the risks. “I think it’s very important to do some field research targeting specific questions,” Keith said. “This is important, and we need to reduce uncertainty.”

At least in theory, researchers say they have broadly determined that solar geoengineering can work. But much of the research thus far has been conducted using computers and mathematical models, leaving much still to be understood.

The National Oceanic and Atmospheric Administration, directed by Congress in 2020 to begin a multi-year solar geoengineering research project, is aiming its work at the most basic levels of knowledge, “establishing a baseline understanding” of heat entering and leaving the atmosphere. The administration announced last week that it was sending a converted Cold War bomber laden with instruments to investigate atmospheric chemistry in a little-studied region over the Arctic.

Complicating some research is the intense controversy surrounding solar geoengineering. An experiment in Sweden in 2021 that would have tested a high-altitude balloon for potential use in future experiments was cancelled after an outcry from Swedish environmental groups and the Indigenous Saami Council, which called the project “a real moral hazard.”

The country of Mexico recently banned the deployment of any solar geoengineering after a startup called Make Sunsets announced it had launched balloons filled with sulfur dioxide from Baja California as part of a carbon credit scheme in which it sells “cooling credits” to offset emissions.

The real complication, in the view of some critics, is that ultimately there is no way to test solar geoengineering’s safety and impact short of doing it.

“To test its climate impact, you would need to actually deploy it at a scale and for a period that any test would equal deployment,” said Fuhr, of the Center for International Environmental Law. “So it’s basically an untestable technology.”

Sabrina Shankman can be reached at Follow her @shankman.