A growing body of evidence highlights the fact that the climate is changing, and human activity is the primary cause. The recent National Climate Assessment, compiled by more than 300 scientists and based on decades worth of research, found that the U.S. has already warmed nearly 2 degrees Fahrenheit (1.1 degrees Celsius) since 1900, snowpack has dwindled away, and sixteen of the warmest seventeen years on record have occurred since the year 2000.

The world is warming, and we are responsible. Now, some scientists are starting to wonder if the same global forces that humans unwittingly harnessed to drive climate change could be used to mitigate the extraordinary cost that even a couple degrees of warming would cause. They support the idea that we will have to intentionally make large-scale changes to the planet in addition to drastically cutting our global carbon footprint. Those large-scale changes—chemically capturing carbon from the air, spurring the growth of carbon-eating plankton, or creating a reflective haze in the upper atmosphere to reflect sunlight—are collectively known as geoengineering.

While many scientists believe that geoengineering the Earth may someday be necessary to preserve life as we know it, the public, so far, isn't buying it. As a result, the preliminary research to figure out if geoengineering projects would even work is proceeding with extreme caution.

“I think if research on solar geoengineering's going to move forward, it's important that it's done responsibly and that it's done at a pace that … doesn't get too far ahead of itself,” says Lizzie Burns, the project manager of Harvard’s solar geoengineering research program. “I think it's important to co-develop governance with research, and if that takes a little longer, I'm fine with that.”

Even if we somehow stopped emitting carbon dioxide completely tomorrow, many scientists say it wouldn't be enough. Our oceans are currently absorbing some of the excess heat of the planet, causing a lag in air-temperature rise. The warming we see today was set in motion by decisions made decades ago. Some scientists say we're already committed to a 1 degree Fahrenheit (.5 Celsius) increase by 2100, no matter what we do. Given the scale of the issue, geoengineering advocates say it's important to look to new technologies.

The idea isn't just coming from the fringes. The Intergovernmental Panel on Climate Change, a U.N. body that has been sounding the increasingly panicked alarm about climate change since 1990, said in October 2018 that it was looking unlikely (close to "impossible") that warming could be kept under 2 degrees Celsius without developing infrastructure to remove carbon from the atmosphere—something we can't currently do at scale.

So scientists are looking at other ways to cool the planet. Researchers have proposed brightening clouds, making sea spray more reflective, or even launching a giant mirror into space to reflect extra sunlight. The most promising and affordable of these methods is stratospheric aerosol injection, which involves spewing tiny particles into the upper atmosphere. Those particles would reflect sunlight away from the Earth, effectively dimming the sun and, in theory, cooling the planet.

Many studies using computer models have shown that this method would, in theory, work. The latest such study, published today in Nature Climate Change, used a sophisticated model that simulates extreme rainfall and hurricanes, and found that reflecting sunlight with aerosols could uniformly cool the globe with minimal additional effects.

Though a stratospheric aerosol injection experiment hasn’t yet been conducted, scientists do have an idea of what happens when tiny particles are spewed into the upper atmosphere thanks to volcanic eruptions. When Mt. Pinatubo in the Philippines erupted in 1991, the roughly 20 million tons of sulfur dioxide it tossed 20 miles up cooled global temperatures by 0.6 degrees Celsius for 15 months.

Beyond the Pinatubo eruption, we have few data points that reveal how sulfur in the stratosphere would affect the planet. The Pinatubo eruption data was "incredibly valuable" for validating models, but "an eruption's not the same as a continual emission of sulfur dioxide,” says Douglas MacMartin, a professor of mechanical and aerospace engineering at Cornell University. “If Pinatubo had erupted in a different season, it might have had different impacts. It went off at the same time as El Nino and some of the impacts are difficult to disentangle. We don't even know exactly how much material was put into the stratosphere."

We don't know a lot, it turns out. According to an opinion piece MacMartin co-authored in the Proceedings of the National Academy of Sciences in January, not only do we not know if stratospheric aerosol injection would work, we don't have a good sense of what could go wrong. In theory, injecting aerosols into the stratosphere could cool the planet at a cost of disrupting seasonal weather patterns, leading to widespread flooding or drought. We could harm our food supply, either by reducing the amount of sunlight that reaches crops or by reducing the amount of rainfall, or both. The particles could eat away at the ozone layer, reintroducing a problem that was addressed in the early 1990s by banning the production of chemicals known as chlorofluorocarbons (CFCs).

"We don't know enough about it to make informed decisions," MacMartin says. Along with his coauthor Ben Kravitz, an atmospheric scientist at Indiana University, MacMartin argues that scientists need to stop conducting “curiosity-driven” research—what happens if you do X?—and move to a “mission-driven” program of research that aims to nail down exactly what scientists need to know to “inform future societal decisions.”

A Harvard study aims to fill in some of these gaps. SCoPeX, or the Stratospheric Controlled Perturbation Experiment, is designed to study exactly how aerosols behave in the stratosphere. The first iteration of the experiment would launch a balloon 12 miles high where it would release tiny particles of calcium carbonate—harmless chalk—as it putters horizontally along at “walking speed” for about a half mile, Burns says. The balloon would then make a U-turn and putter back through the plume of chalk dust to detect the particles and measure how they change over time.

Before SCoPeX can conduct the calcium carbonate experiment, however, the Harvard team needs to test the balloon equipment. They plan to launch the balloon in a trial run using water as the payload. But before SCoPeX can even launch its engineering test flight, it needs to get approval from an advisory committee that would monitor the project and pull the plug if necessary—and the committee has yet to be selected.

“If it were not labeled geoengineering,” nobody would care, says MacMartin, who is not affiliated with SCoPeX. Neither of these experiments could even remotely be considered “geoengineering”—their payloads are far too small to have any effect on the Earth’s climate. “But the media says Harvard’s planning to blot out the sun."

Burns argues that the slow, methodical approach is necessary. “Our goal is not to tell people how to think, but to do things in a way that makes people feel confident [in what we're doing].” The costs of not gaining public confidence were seen in a British geoengineering experiment called Stratospheric Particle Injection for Climate Engineering (SPICE), which was shelved in 2012 over both conflict-of-interest concerns as well as protests from environmental groups.

Even those who support geoengineering research say that caution is necessary. “The research doesn’t have to be scary, [but] it’s scary in terms of what it implies," MacMartin says. "That we might actually try to control the entire climate is a pretty terrifying idea in some respects.”

Some scientists argue that we shouldn’t even peek down this road. Climate scientist Claire Parkinson says that “attempting to counter the damage we've done by pouring stuff into the atmosphere and oceans by pouring more stuff in … is too fraught with potential unintended consequences.”

But when it comes to geoengineering, the research falls into a catch-22 of being too risky to rush, and some scientists say, too important to delay.

When asked if scientists should conduct preliminary experiments to lower the uncertainties and risks of geoengineering, Parkinson was silent for a long time. She finally said, “If I were voting on it right now, I would vote no.” In Parkinson’s view, we need to focus on technologies that remove carbon from the atmosphere or simply use less of it in the first place.

Geoengineering research could also divert funds from known carbon-reduction strategies like solar and wind energy. Even the knowledge that we could cool the planet with aerosols, some argue, would remove the incentive to decarbonize. But most geoengineering proponents agree that even with something like large-scale stratospheric aerosol injection, we'd still need to reduce carbon in the atmosphere. If we don’t, we'll have to keep pumping more and more aerosols up there—literally forever. And dimming the sun may help fight climate change, but it doesn't alter any of carbon dioxide's other nasty effects, like ocean acidification, which is killing coral, shellfish and plankton around the globe.

Solar geoengineering is "a potential supplement, but it carries its own risk," Burns says. "It's like a painkiller. If you need stomach surgery and you took pain medication, it doesn't mean you no longer need stomach surgery." But for now, solar geoengineering is less like Aspirin and more like an untested, unregulated supplement you picked up on the street. It could relieve our climate pain—or make it much worse. We just don't know.

In the meantime, experiments like SCoPeX continue to move, slowly but inexorably, toward a likely launch in the next year or two. All the while, the Earth continues to warm.

"I think we're likely to get to a place where the consequences of not doing geoengineering are so bad,” MacMartin says, “that people are going to say some amount of geoengineering is better than not."