As Earth’s climate has steadily warmed over the past several decades, some scientists have looked into the possibility of using industrial-scale geoengineering to cool the planet down – at least temporarily until greenhouse gas emissions are eliminated.

Some still think a technique called solar radiation management (SRM) might be a prudent Plan B “fix” if lack of progress cutting back emissions leads to unbearable results. In one scenario, supersonic jets would spray hundreds of thousands of tons of fine aerosols into the Earth’s stratosphere each year to keep the sun’s rays from reaching the troposphere, or the part of the atmosphere closer to the ground where the buildup of greenhouse gases is amplifying that energy and warming the planet.

Many remain skeptical of SRM, and as more scientists have studied the implications and possible unintended consequences, it seems any improvement from this technique could well be short-lived. Scientists note that aerosols in the stratosphere will not stop carbon from building up in the lower atmosphere or in the oceans, so warming would be that much worse if, for any reason, the spraying treatment was halted, a phenomenon called “termination shock.” Furthermore, ocean acidification, harmful to  coral reefs and marine life, would continue unabated.  Also, ultraviolet light is a wild card: It is strong in the upper atmosphere and could cause unforeseen chemical reactions and physical transformations on any chemical released there.

Researching ‘the worst possible way’ to confront warming

Solar geoengineering is “the worst possible way to address climate change that we need to take seriously,” said Peter Frumhoff, chief climate scientist at the Union of Concerned Scientists. In March 2021, Frumhoff was part of a high-level committee of the National Academy of Sciences that called on the U.S. to ramp up scientific research on shielding Earth from the sun’s rays to confer “the full range of options” to future generations. The appeal to action has added fuel to discussions, and controversy, about whether sun-shading technology should be implemented, or even researched further.

Christopher Field, the chairman of the National Academy’s solar geoengineering committee, supports further study of SRM. But he says if concerns such as  over ocean acidification and termination shock can’t be assuaged, full scale deployment might be inadvisable. “If you can’t tell whether or not a bomb works until you set it off, that’s a really good reason to not build the bomb,” he says.

Interest in solar radiation management grew in the years after June 15, 1991, when Mount Pinatubo erupted in the Philippines. The volcano’s deadly blast was the world’s largest volcanic eruption in decades, immediately killing 350 people and spewing 17 million tons of sulfur dioxide into a mushroom cloud that vaulted up 25 miles into the stratosphere. 

The Mount Pinatubo summit caldera on August 1, 1991, about six weeks after the June eruption.

In the year after Pinatubo erupted, Earth’s climate cooled as high-altitude winds blew the mist around the world, shrouding the planet in a thin volcanic haze.  Global temperatures fell by 1°F and did not return to normal for about 12 months. The instantaneous planetary cooling became an important data point for scientists who had quietly discussed the idea of geoengineering for years: It allowed them to fine-tune how climate models handle particulate matter, such as fine particles or aerosols. A “Pinatubo eruption” is still a term used to denote the amount of material that would be needed to cool the planet this way.  Scientists were encouraged that global warming could be countered by intentionally injecting shading aerosols or dust particles into the stratosphere.

Worldwide use of oil, coal, and natural gas today release 60 percent more carbon dioxide than the annual total when Pinatubo blew up. Earth has warmed about 1°F since before the eruption, boosting the allure of responding with geoengineering – and amplifying the opposition.

In 1974, a scientist’s warning of inevitable necessity  

A pioneering Russian climate researcher, Mikhail Budyko, is often credited with first proposing the injection of chemicals into the upper atmosphere to block the sun. “[I]n the near future climate modification will become necessary in order to maintain current climatic conditions,” he wrote in 1974.

Fine particulate matter delivered into the stratosphere (beginning six to 12 miles above the ground, depending on latitude) is distributed widely on planet-encircling winds. There’s hardly any vertical movement of air in the stratosphere, so the material would remain aloft for months. That contrasts with material injected into the lower atmosphere, which falls or is washed to the ground in days. Budyko calculated that 40,000 tons of sulfur burned in high-flying airplanes and sprayed into the stratosphere each year could do the job.

U.S. scientists began speculating along these lines around the same time. In a 1974 paper in Science magazine, climate scientists William Welch Kellogg and Stephen Schneider, both then at the National Center for Atmospheric Research, laid out ways for cooling the planet, including by creating a “stratospheric smog,” by spraying aerosols from supersonic airplanes.

Budyko had predicted that such climate tinkering was a foreseeable, unproblematic necessity. But Kellogg and Schneider questioned such interventions.  Anticipating objections now common in the current debate, they warned that “to tamper with the system that determines the livelihood and lifestyle of people around the world would be the height of irresponsibility,” and they urged that studies on how to guard against unanticipated impacts would be needed.

But little geoengineering research of any kind was published in the following three decades. Most climate scientists in effect considered it taboo: They feared, not implausibly, that even broaching the topic could distract from the most sensible responses to climate change: decarbonizing economies by replacing fossil fuel-based energy with alternatives such as wind and solar power.

Nobel Laureate pointed to ‘grossly unsuccessful’ efforts to curb warming

Then, in 2006, the atmospheric chemist Paul Crutzen wrote an editorial in the influential journal Climate Change calling for a concerted modeling and atmospheric research campaign to determine the feasibility and pitfalls of injecting sun-blocking materials into the stratosphere. Like many papers before and since, his focused primarily on sulfurous aerosols. Natural and manufactured sulfur compounds, including the sulfuric-acid-related class called sulfates, are ubiquitous throughout the atmosphere and long recognized to influence the amount of heat absorbed at the surface. 

Crutzen, a 1995 Nobel Prize laureate for his research on the ozone hole, held that “grossly unsuccessful” progress in curbing global warming necessitated the development of an “escape route,” an assertion that is sometimes referred to as ‘Plan B.’ The argument, coming from this eminent scientist, resonated in the climate research community. The number of scientific papers published on solar geoengineering, some also calling for solar radiation management or SRM, shot up from a handful to several dozen each year. “The sense of a taboo has largely disappeared,” Crutzen and a colleague exulted in a 2017 paper.

In the past 30 years, the National Academy of Science has issued three reports considering prospects, challenges and potential hazards of SRM. The second, in 2015, recommended more computer simulations of global and regional impacts of injecting cooling compounds into the atmosphere. It also advocated active field experiments releasing small amounts of material into the atmosphere. Such experiments would be imperative before attempting to modify the climate.

In his 2006 editorial, Crutzen had said the UV light effect would be tolerable if sulfites were used. Alternative materials that might be employed for solar shading could cause different problems: Soot particles could shield the planet not by reflection but by absorbing sunlight before it penetrates to the surface. But they would also heat the stratosphere, possibly changing the movement of high-altitude air currents. 

Some false starts went … not so far

Several outdoor experiments investigating these issues have been proposed and funded since the mid 2000s, but only a single small one, conducted by a Russian scientist in 2009, three hundred miles southeast of Moscow, appears to have taken place. Yuri Izreal, a scientific advisor to Vladimir Putin, sprayed sulphate aerosols into the lower atmosphere several hundred feet above the ground. The experiment attracted scant scientific interest.  A 2011 experiment off the California coast testing the behavior of clouds spiked with particulate matter is considered relevant for SRM, though its stated purpose was to understand how clouds influence climate.

Two proposed SRM experiments have battled challenges from environmental groups and some prominent climate-research insiders. In 2011, Bristol University in England announced an experiment comparing the optical and chemical properties of a dozen candidate alternatives to sulphates for cooling the planet, including diamond powder and also calcium carbonate (the principal ingredient of chalk). Mixed with water, the materials were to have been sprayed from a nozzle dangling from a helium balloon tethered half-a-mile above the ground. But, bowing to stiff opposition from scores of environmental groups around the world, the proposed project was cancelled within a year. Critics pointed to lack of government oversight and alleged researchers’ conflicts of interest: Some of the investigators had applied for patents for the dispersal technology possibly benefiting them financially if Plan B became necessary.

A long-planned experiment proposed by a team at Harvard could also be squelched. In 2012 David Keith, a physics and public policy professor at Harvard, announced that he planned his own balloon-mounted geoengineering trial above New Mexico. Like the Bristol group, he also proposed to spray sulphates, but much higher – 12 miles above the ground, in the lower stratosphere. But the political climate, like that of the planet, was too hot. This plan, too, was soon withdrawn. 

Undaunted, Keith and his team resuscitated the Harvard experiment a few years later. This time the plan, now dubbed the Stratospheric Controlled Perturbation Experiment, would spray calcium carbonate from a high-flying balloon. Initially they planned to fly the new experiment over the U.S. southwest. In 2020, they shifted the venue to the launch center of the Swedish Space Corporation in Sweden near the Arctic Circle. They said that logistics problems caused by the COVID-19 pandemic had contributed to the decision. But environmental groups, such as the Swedish Society for Nature Conservation, Greenpeace Sweden, and Friends of the Earth Sweden, opposed the experiment. Leaders of the indigenous Sami people, who herd reindeer in the region around the launch site, also vehemently opposed the experiment. The Swedish agency withdrew its invitation to the researchers in March 2021.

National Academy report takes big step, concerns persist

In its latest SRM Report, the National Academy issued its strongest call yet for more research, including all-important atmospheric experiments. The academy, in that report, fretted that the world has not moved any closer to developing a deployable system – or even deciding whether it is wise to do so – in the six years since its 2015 study. The “state of understanding remains limited,” the report said, despite a specific recommendation in the earlier study, that the US “improve understanding of the range of climate and other environmental effects” and “unintended impacts.” Reflecting a sense of urgency and frustration at the lack of progress, the authors took an unprecedented step: They proposed a specific research budget of $100 million to $200 million for SRM studies over five years. 

“That’s a massive amount of money,” says Raymond Pierrehumbert, a planetary physics professor at the University of Oxford in England. Allocating it “is not something that it would make sense to do.” A lead author of one of the International Panel on Climate Change reports and a member of the committee of the 2015 report, Pierrehumbert has become one of the most outspoken scientists urging caution about conducting open air sun-blocking research.

He argues that if the viability of a sun blocking system is ever proven, its relatively low cost will make deployment irresistible.  But the bargain-basement expense will be misleading, he says, because solar shading carries extraordinary risks. For instance, while shading chemicals can cool the planet, they do nothing, as mentioned earlier, to slow the accumulation of carbon dioxide in the ocean, which is making seawater progressively more acidic and inhospitable to ocean plants and animals.

He makes other points too.  For instance, once underway, if shading chemicals sprayed in the sky are halted for any reason, the planet will very rapidly heat up, as predictably as a sunbather would get a sunburn if their shade umbrella is removed. And as long as carbon dioxide levels keep rising, the needed tonnage of shading chemicals will increase every year. Like the peril of living below a frail dam holding back a reservoir in flood season, the hazard of “termination shock” will increase progressively.

Pierrehumbert says that at some point it’s unavoidable that something would come up – a pandemic, a depression, a war – that would halt the temperature-masking spray, “which is almost inevitably going to end in massive death.”

What if 95% of planet benefits while 5% is made worse-off?

Alan Robock, a climate scientist at Rutgers University in New Jersey, says that in many cases, there’s no justifiable distinction between a test and deployment of sun blocking technology. He explains that in comparison to the incremental progression of climate change, natural variation is large. “How do you know that what was caused by your geoengineering wouldn’t have happened anyway?” he asks rhetorically. The only way to know, he says, is to run multi-decade-long experiments with sufficient shading to cause an effect larger than the climate’s normal fluctuations. Such an experiment would be no different from an operating system.

Using climate models, Robock has shown that spraying shading chemicals would not have a uniform cooling effect all over the planet. Some places would cool more or less than others. Moreover, shading the planet with chemicals would also reduce precipitation, though this effect, too, would be uneven, possibly causing drought in some places and floods in others. He wonders how to proceed if you could find a way to make 95% of the planet better off but 5% worse off? He asks: How are you going to compensate those made worse off?

Christopher Field, the chairman of the National Academy’s solar geoengineering committee, says he, too, worries greatly about ocean acidification and termination shock. But, he says, if implemented in moderation – only as a supplement to fossil fuel cutbacks – the drawbacks of solar geoengineering might be tolerable. 

Will the attractively low cost of sun blocking undermine arguments for such cutbacks? Field says, “That’s an important question we don’t have the answer to.” He says that it’s also not yet known whether experiments determining the effectiveness of solar geoengineering would indeed require full-scale deployment in all but name. “That’s a researchable question,” he says. If further investigations bear out the contention, he adds, some experiments might be deemed too dangerous to conduct.

Robock, whose first paper on geoengineering was “20 reasons why geoengineering may be a bad idea,” says that he supports studying the idea more to evaluate how risky it might be.  “I think probably we’ll find it so risky, that we’ll never dare to do it.”

Cartoon by Tom Toro.

Editor’s note: This article was lightly edited on June 8 and 9.

Daniel Grossman

Daniel Grossman, Ph.D., is an award-winning freelance print journalist and radio and web producer with more than 20 years of experience. He earned his B.S. in physics and his Ph.D. in political science,...