Solar geoengineering aims to manipulate the climate by bouncing sunlight back into space. In theory, it could ease global warming. But as interest in the idea grows, so do concerns about potential consequences.

A startup called Stardust Solutions recently raised a $60 million funding round, the largest known to date for a geoengineering startup. My colleague James Temple has a new story out about the company, and how its emergence is making some researchers nervous.

So far, the field has been limited to debates, proposed academic research, and—sure—a few fringe actors to keep an eye on. Now things are getting more serious. What does it mean for geoengineering, and for the climate?

Researchers have considered the possibility of addressing planetary warming this way for decades. We already know that volcanic eruptions, which spew sulfur dioxide into the atmosphere, can reduce temperatures. The thought is that we could mimic that natural process by spraying particles up there ourselves.

The prospect is a controversial one, to put it lightly. Many have concerns about unintended consequences and uneven benefits. Even public research led by top institutions has faced barriers—one famous Harvard research program was officially canceled last year after years of debate.

One of the difficulties of geoengineering is that in theory a single entity, like a startup company, could make decisions that have a widespread effect on the planet. And in the last few years, we’ve seen more interest in geoengineering from the private sector.

Three years ago, James broke the story that Make Sunsets, a California-based company, was already releasing particles into the atmosphere in an effort to tweak the climate.

The company’s CEO Luke Iseman went to Baja California in Mexico, stuck some sulfur dioxide into a weather balloon, and sent it skyward. The amount of material was tiny, and it’s not clear that it even made it into the right part of the atmosphere to reflect any sunlight.

But fears that this group or others could go rogue and do their own geoengineering led to widespread backlash. Mexico announced plans to restrict geoengineering experiments in the country a few weeks after that news broke.

You can still buy cooling credits from Make Sunsets, and the company was just granted a patent for its system. But the startup is seen as something of a fringe actor.

Enter Stardust Solutions. The company has been working under the radar for a few years, but it has started talking about its work more publicly this year. In October, it announced a significant funding round, led by some top names in climate investing. “Stardust is serious, and now it’s raised serious money from serious people,” as James puts it in his new story.

That’s making some experts nervous. Even those who believe we should be researching geoengineering are concerned about what it means for private companies to do so.

“Adding business interests, profit motives, and rich investors into this situation just creates more cause for concern, complicating the ability of responsible scientists and engineers to carry out the work needed to advance our understanding,” write David Keith and Daniele Visioni, two leading figures in geoengineering research, in a recent opinion piece for MIT Technology Review.

Stardust insists that it won’t move forward with any geoengineering until and unless it’s commissioned to do so by governments and there are rules and bodies in place to govern use of the technology.

But there’s no telling how financial pressure might change that, down the road. And we’re already seeing some of the challenges faced by a private company in this space: the need to keep trade secrets.

Stardust is currently not sharing information about the particles it intends to release into the sky, though it says it plans to do so once it secures a patent, which could happen as soon as next year. The company argues that its proprietary particles will be safe, cheap to manufacture, and easier to track than the already abundant sulfur dioxide. But at this point, there’s no way for external experts to evaluate those claims.

As Keith and Visioni put it: “Research won’t be useful unless it’s trusted, and trust depends on transparency.”

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