What Happens If We Start Solar Geo-Engineering—and Then Suddenly Stop?

It might be worse than letting climate change play out.

Volcanoes, nuclear war, and solar geo-engineering.

With a research portfolio that includes all three, Alan Robock is used to thinking about sudden, catastrophic change. A professor of environmental science at Rutgers University, Robock studies the movement of small particles and liquids through Earth’s atmosphere. In that role, he researches some very large and particle-producing events: the sudden extrusion of many tons of sulfur from an erupting volcano, or the global convection of smoke and ash from the firestorms that would follow a nuclear war.

Such a combination of interests—calamities both natural and anthropogenic—make him well-credentialed to study solar geo-engineering, which scientists widely consider to be the most promising technique to slow the increase in global temperature.

Supporters of solar geo-engineering say it’s logical: Since humans have already made the Earth better at trapping heat by releasing greenhouse gases like carbon dioxide, then maybe humans can futz with another variable—how much heat enters the planet in the first place—and release a different gas that will repel sunlight back into space. The gas most likely to get the job done is sulfur dioxide, a transparent substance that reflects light and that is naturally released by volcanic eruptions. Sulfur dioxide helped cool Earth after the 1991 eruption of Mount Pinatubo in the Philippines, for instance. Aspiring solar geo-engineers imagine a fleet of planes, spraying the skies with multiple Pinatubos’ worth of sulfur every year, in order to preserve the planet’s cooler climate for as long as possible.

But Robock is skeptical. In a paper released this week in Nature Ecology and Evolution, he and his colleagues make one argument for why solar geo-engineering could be worse than climate change itself. If humanity were to start solar geo-engineering—and then, decades later, suddenly stop—animals, plants, and ecosystems might suffer more than they would under climate change as usual.

In other words, solar geo-engineering has the potential to harm Earth’s biodiversity more than letting global warming run its course.

For ecosystems, everything depends on how quickly climate change occurs. If climate change slowly warms the globe over the course of decades, then animals and plants will have those decades to adjust. This will still be destructive—and, from a geological standpoint, it will count as speedy climate change. The Earth’s natural climate changes—such as the onset of an ice age—take thousands of years to set in.

But if solar geo-engineers can hold off that temperature rise for decades, before suddenly losing the political or technological capability to do so, then sulfur dioxide would quickly fall out of the atmosphere and global temperatures would skyrocket. Decades of global-warming-induced temperature increase would set in over the course of years. Animals and plants would have even less time to respond to the suddenly scorching planet.

“As the planet’s been warming for the last 100 years, and as it will in the future, some species can adapt to the gradual change. But if some have to suddenly move to adapt to a new climate regime, they might not be able to move fast enough,” Robock told me.

What could prompt humanity to give up solar geo-engineering? There are lots of possible causes: a global war, a debilitating recession, a crisis of international confidence. There are already some trouble spots we can foresee now, Robock said. Because sulfur dioxide will cool the land more than the sea, solar geo-engineering may weaken the monsoons that feed much of Asia—and that could upset some very powerful nations.

“You could get a big drought in India, or a big drought in China, and those countries would say, ‘You damn geo-engineers, you’re causing this,’” said Robock. “And we might say, ‘It could’ve happened anyway.’ But they’d say, ‘We know, but our citizens think you’re causing this, so you’d better stop.’”

“If people are considering using geo-engineering to avert global warming, we want them to be aware this is a potential scenario,” he added.

The researchers used computer programs that simulate Earth’s biodiversity to mimic how post-termination warming temperatures would play out across different biomes. Some of the ecosystems most harmed by climate change—like the tundra, the tropical oceans, and the Amazon basin—would also be the most ill-served by a sudden termination of solar geo-engineering. In other scenarios, shutting down solar geo-engineering leads to an area’s climate splitting in two—its temperature rises, while its rainfall might be diverted south—causing a total “fragmentation” of habitat.

Shutting off solar geo-engineering would also damage the most at-risk species. In the tropics, for instance, amphibians, corals, and fish would be the most harmed, said Christopher Trisos, an author of the paper and an ecologist at the National Socio-Environmental Synthesis Center. But even in today’s climate, amphibians are already believed to be undergoing a mass extinction. Many coral reefs are also already suffering mass-bleaching events too frequently to recover.

Gernot Wagner, the codirector of the solar geo-engineering research program at Harvard University, said that the paper “highlights an important, woefully understudied aspect of solar geoengineering: its potential impact on biodiversity.” (He was not involved in the research.) In the book Climate Shock, he and the economist Martin Weitzman argue that solar geo-engineering is so cheap that it’s almost inevitable: “Someone will surely do it based on their own self-interest, broader consequences be damned.” For that reason, he and other scientists have called for as much research on geo-engineering as possible.

Cleo Chou is an ecologist at the Princeton Environmental Institute who also was not connected to the paper. She said it was important to note that the study only examined a relatively moderate climate-change scenario, in which the Earth’s temperature rises a little more than 2.4 degrees Celsius, or about 4.3 degrees Fahrenheit, by 2100. Compared to more extreme climate-change scenarios, she said, solar geo-engineering may look better and better.

Solar geo-engineering is only one type of climate intervention. Michael Wolovick, a glaciologist at Princeton University, has proposed a scheme to isolate massive polar glaciers from warming ocean water in order to avert rapid sea-level rise. Other researchers focus on “negative-emissions technologies,” a costly and still-theoretical technique to scrub the atmosphere of carbon-dioxide pollution directly.

But solar geo-engineering remains the most popular—in part because, pending some technological breakthrough, it seems the most efficient. While carbon-removal gear would have to touch every molecule of affected carbon dioxide, a single particle of sulfur dioxide (in a solar geo-engineering scheme) would repel trillions of photons. Study of the idea has accelerated over the past few years, with off-the-record conferences and new programs in China and the United States.

It’s a far cry from 2006, when the climate scientist and Nobel laureate Paul Crutzen first called for a resumption of research into solar geo-engineering. NASA held a follow-up meeting to discuss the idea at their Mountain View, California, laboratory. Robock remembers it well. “The room was too hot, and nobody could figure out how to control it,” he said. “I thought it was pretty ironic.”