As recently as 10 years ago most scientists I interviewed and heard speak at conferences did not support geoengineering as a way to counteract climate change. Whether the idea was to release large amounts of sulfur dioxide into the stratosphere to “block” the sun’s heating or to spread iron across the ocean to supercharge algae that breathe in carbon dioxide, researchers resisted on principle: don’t mess with natural systems because unintended consequences could ruin Earth. They also worried that trying the techniques even at a small scale could be a slippery slope to wider deployment, and that countries would use the promise of geoengineering as an excuse to keep burning carbon-emitting fossil fuels.
But today more and more climate scientists openly support experimenting with these and other proposed strategies, in part because entrepreneurs and organizations are going ahead with the methods anyway—often based on little data or field trials. Scientists want to run controlled experiments to see if the methods are productive, to test consequences and perhaps to show objectively that the approaches can cause serious problems.
“We do need to try the techniques to figure them out,” says Rob Jackson, a professor at Stanford University, chair of the international research partnership Global Carbon Project and author of a book on climate solutions called Into the Clear Blue Sky (Scribner, 2024). “But doing research does make them more likely to happen. That is the knotty part of all this.”
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A tacit race may be starting among scientists and entrepreneurs. More funding is being offered to researchers, and investments are growing in companies that would pursue geoengineering. In 2023 a start-up called Make Sunsets launched balloons containing sulfur dioxide into the stratosphere, selling “cooling credits” to companies and individuals. In early September, 23 academics at the not-for-profit consortium Exploring Ocean Iron Solutions unveiled a plan to assess how much CO2 iron fertilization could sequester in the deep sea, and they hope to start trials across the northeastern Pacific Ocean in 2026. Big corporations, including oil companies, are already building large industrial facilities to pull CO2 from the air, and the U.S. government is offering them billions of dollars.
There is confusion, too. Some scientists say the term “geoengineering” should refer only to techniques that alter Earth systems. For example, sulfur dioxide in the stratosphere creates tiny droplets that reflect incoming sunlight back to space, an approach called solar radiation management (SRM). But this method could also affect weather patterns or weaken the ozone layer that protects us from ultraviolet radiation. Brightening clouds by spraying them with ocean mist from below can reduce sunlight but could interfere with rain patterns. Spreading iron across the ocean helps phytoplankton to grow and consume CO2, yet these organisms would also consume other nutrients, which could possibly starvesea life. Spreading certain kinds of pulverized rock across the sea surface can make the water more alkaline, allowing it to absorb more CO2 from the air, but it could affect ocean chemistry, too.
In contrast, using machines to pull CO2 from the air, a technique known as direct air capture (DAC), doesn’t directly interfere with natural systems and so shouldn’t be called geoengineering, some argue. Social concerns could still be high, however; for example, millions of these machines would be needed to reduce warming by a meaningful extent, and that would require huge amounts of energy. If they were powered by wind and solar, as they ideally would be to avoid more greenhouse gas emissions, the installations could compete with agriculture for land.
For years Jackson refused to endorse DAC, but he now supports research. He has come around because warming is rising relentlessly, along with fossil fuel use, creating dangerous floods, droughts and heat waves that are killing people worldwide. “We are out of time and options,” he says, “an unfortunate outcome of our inaction on climate.”
He is not quite ready to endorse SRM, he says, “because I don’t trust our ability to do it equitably around the world.” Blocking the sun above certain countries could alter rainfall patterns elsewhere, which could be particularly problematic if it occurred in poorer regions. Research shows that the huge 1991 Mount Pinatubo volcanic eruption in the Philippines, which lofted about 20 million metric tons of sulfur dioxide into the stratosphere, ended up exacerbating drought in parts of the world as well as reducing ozone levels by a small percentage. If one country launches sulfur into the stratosphere, Jackson asks, who is going to pay for drought that happens in another country?
When Ken Caldeira, a longtime climate researcher at the Carnegie Institution for Science’s Department of Global Ecology at Stanford, started to look into geoengineering in 2000, he says, older researchers warned him that delving into the taboo topic would put his career at risk. Small studies did ensue, but public opinion against geoengineering mounted and most work stopped.
A big change came in 2018, when the Intergovernmental Panel on Climate Change released its Global Warming of 1.5°C report. It stated that without carbon removal or other techniques, the world stood little chance of holding warming to no more than 1.5 degrees Celsius (2.7 degrees Fahrenheit) above preindustrial levels, the goal agreed on in the 2015 Paris climate agreement. “That was an inflection point,” Jackson says. “It pointed out the difficulty, almost the futility, of reaching the 1.5-degree-C target, or even the two-degree-C target, without carbon removal.” Even if carbon removal is not considered geoengineering, the report bolstered some scientists’ willingness to experiment.
David Keith, formerly an applied physics professor and public policy professor at Harvard University, is among them, and he prefers the term “climate engineering” to refer to techniques such as SRM. For more than two decades, Keith has been looking into SRM. In 2021 he and others were about to carry out the world’s first field trial in Sweden, but protests by Indigenous peoples and other groups persuaded the Swedish government to cancel it. Last year the University of Chicago hired Keith to oversee a new program called the Climate Systems Engineering initiative, perhaps the first program to hire people specifically to do SRM research. Keith refutes the slippery slope argument, too. “There is no ethical argument for not pursuing research,” he says.
Nevertheless, many scientists and environmental groups remain skeptical—and they have been voicing their wariness to Scientific American. Lili Fuhr, an analyst at the Center for International Environmental Law, interviewed for an article on DAC in SciAm’s September issue, said, “the reliance on these future speculative techno fixes delays real climate action right now.” Deep-sea expert Lisa Levin of the Scripps Institution of Oceanography, interviewed for a September 12 article about iron fertilization, said the technique is likely to “affect something that we don’t really understand yet.”
But to Caldeira, who is credited with coining the term “solar radiation management” in 2006, that is the reason to do “outdoor research”—not just computer modeling studies. “The key is identifying what could go wrong, and demonstrating how,” he says. “What experiment could you do to narrow the uncertainty about whether a technique is bad or okay?” If the outcomes raise threats to the environment or people, he says, “we should know that now.”
Although Caldeira is not pursuing it, one intriguing idea would be to equip a fast-mobilizing aircraft team that could fly a specially outfitted plane above a sudden volcanic eruption to test all sorts of stratospheric parameters. But it would be difficult to spend money on a special aircraft that, most of the time, would sit idle in a hangar.
Caldeira thinks support for SRM will continue to broaden, especially if drought and famine caused by climate change—which have already begun—happen year after year and disproportionately affect poorer countries. “SRM is the only way to start cooling the Earth within a few years,” he says. “There would be mounting pressure on political leaders [in poorer affected countries] to deploy it. Or at least, they could use the threat of SRM to get more aid from wealthy countries.”
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