As the globe sizzles, interest is growing in schemes to cool it through solar geoengineering—but at what risk?
What is solar geoengineering?
It’s the concept of lowering the temperatures here on Earth by shading the planet. Scientists are studying several potential ways to reduce solar radiation, including launching huge sun shields into orbit and brightening clouds with salt water to make them more reflective. But the process drawing the most attention involves seeding the upper atmosphere with reflective particles such as sulfur dioxide, which would bounce sunlight back into space. The technology, known as stratospheric aerosol injection, is inspired by the planet-cooling effects of volcanic eruptions. When Mount Pinatubo in the Philippines blew in 1991, it spewed nearly 20 million tons of sulfur dioxide into the atmosphere, lowering global temperatures by about 1 degree Fahrenheit for 15 months. Some estimates suggest a similar cooling effect could be achieved by using high-altitude aircraft to spray a million tons of sulfur dioxide annually. That would cost tens of billions of dollars a year, far cheaper than the trillions of dollars needed to quickly decarbonize the economy. It’s a radical plan, but one proponents say is necessary with planet-warming emissions and global temperatures climbing ever higher. “The idea of dialing down the temperature of the whole planet is horrifying,” said David Archer, a climate scientist at the University of Chicago. But it’s not as horrifying as “dialing it up with carbon dioxide.”
Are any projects in the works?
Most of the focus right now is on research: which aerosolized particles might be the most effective, for example, and how deployment might be governed. The University of Chicago and Harvard University have programs focused on various types of geoengineering; the European Union and U.S. government are exploring ways to limit solar radiation; and tech billionaires including Bill Gates and Jeff Bezos have pumped money into research. While most of these initiatives are proceeding cautiously, Make Sunsets—a U.S. start-up that sells “cooling credits”—has over the past two years launched 82 weather balloons filled with sulfur dioxide into the upper atmosphere. Many supporters of solar geoengineering say small, unregulated outfits like Make Sunsets are hurting the field. But some scientists say the entire concept of solar geoengineering is ethically questionable and potentially highly dangerous.
Why are they opposed?
Critics say it’s a distraction from the core task of moving beyond fossil fuels, and that there could be unintended and devastating consequences to meddling with the climate. Large-scale stratospheric aerosol injection might weaken the summer monsoon, resulting in reduced rainfall in regions that are home to billions of people, notes Alan Robock, a climate scientist at Rutgers University. Other areas could see increased rainfall and an accompanying spike in infectious diseases such as malaria. A less-intense sun could shrink crop yields. Then there are worries that if the spraying is stopped—the aerosols fall to Earth and so need to be regularly replenished—bottled-up warming would be released in a sudden burst. Raymond Pierrehumbert, an atmospheric physicist at the University of Oxford, said the planet could experience a “potentially massive temperature rise” over a decade, “hitting the Earth’s climate with something that it probably hasn’t seen since the dinosaur-killing impactor.” Even if that worst-case scenario doesn’t occur, critics worry that solar geoengineering could fuel geopolitical tensions, or even war.
How could that happen?
If one country tries to alter the climate to its benefit, other nations might suffer knock-on droughts or floods—and could respond with military force to prevent further climatic harm. The White House and the Defense Department have both identified unilateral solar geoengineering as a geopolitical threat, and scientists and political analysts agree that international consensus will be needed on a host of difficult questions before any potential use: How much cooling is desirable? And how would disputes over negative impacts be adjudicated? But if nations can’t even agree on curbing carbon emissions, how are we “going to govern a technology that could purposefully alter planetary living conditions?” asked Elizabeth Chalecki, a political scientist at University of Nebraska Omaha. Even small experiments with solar geoengineering have drawn controversy. Mexico banned the technology last year after Make Sunsets launched balloons without permission in Baja California. And a 2021 attempt by Harvard researchers to release a balloon carrying reflective calcium carbonate in arctic Sweden was nixed after objection from the indigenous Sami people, who cited “risks of catastrophic consequences.”
What do proponents say?
They don’t deny geoengineering could have some harmful consequences. But they say that those risks are being overblown—and that we have an obligation to press ahead with further study because so little progress is being made in the wider battle against warming. Global carbon dioxide emissions hit a record 36 billion metric tons in 2023, which was the warmest year on record. Meanwhile, numerous climate tipping points are coming into view, including the thawing of permafrost and the drying out of the Amazon rainforest. Geoengineering, supporters argue, is a short-term fix that will give humanity the time it needs to kick its carbon habit. For Rutgers’ Robock, the question at this point is not whether hacking the atmosphere is risky. It’s “which is riskier: doing it or not doing it?”
‘Fertilizing’ the ocean
To fight climate change through geoengineering, some scientists are looking not to the skies but the seas. One concept called ocean fertilization would mimic a natural process that occurs when iron-rich dust is blown off the land by wind or carried to the sea by rivers. That dust acts as a fertilizer for tiny phytoplankton that absorb carbon through photosynthesis. Adding extra iron to the ocean, the notion goes, will lead to the proliferation of plankton, which sink when they die, carrying the carbon they’ve absorbed to the ocean floor. Proponents say it’d be a relatively cheap and easy way to pull carbon from the atmosphere. Critics say it’s unproven and could have unintended consequences, such as algae blooms. Research has been stalled since 2012, when an entrepreneur dumped 100 tons of iron sulphate off Canada’s west coast in a rogue experiment, sparking outrage and spawning a plankton bloom. But some proponents hope to soon conduct large-scale experiments to assess ocean fertilization’s promises and risks. Ken Buesseler, a marine biologist at the Woods Hole Oceanographic Institution, said his previous caution has eroded amid rising wildfires and heat deaths. “Doing nothing,” he said, “is not really an option.”
