“What I want to do is create as much cooling as quickly as I responsibly can, over the rest of my life, frankly,” Iseman says, adding later that they will deploy as much sulfur in 2023 as “we can get customers to pay us” for.

The company says it has raised $750,000 in funding from Boost VC and Pioneer Fund, among others, and that its early investors have also been purchasing cooling credits. The venture firms didn’t respond to inquiries from MIT Technology Review before press time.  'A terrible idea'

Talati was highly critical of the company’s scientific claims, stressing that no one can credibly sell credits that purport to represent such a specific per gram outcome, given vast uncertainty at this stage of research.

“What they’re claiming to actually accomplish with such a credit is the entirety of what’s uncertain right now about geoengineering,” she says.

Kelly Wanser, executive director of SilverLining, a nonprofit that supports research efforts on climate risks and potential interventions like geoengineering, agreed.

“From a business perspective, reflective cooling effects and risks cannot currently be quantified in any meaningful way, making the offering a speculative form of ‘junk credit’ that is unlikely to have value to climate credit markets,” she wrote in an email.

Talati adds that it’s hypocritical for Make Sunsets to assert they’re acting on humanitarian grounds, while moving ahead without meaningfully engaging with the public, including with those who could be affected by their actions.

“They’re violating the rights of communities to dictate their own future,” she says.

David Keith, one of the world’s leading experts on solar geoengineering, says that the amount of material in question—less than 10 grams of sulfur per flight—doesn’t represent any real environmental danger; a commercial flight can emit about 100 grams per minute, he points out. Keith and his colleagues at Harvard University have worked for years to move forward on a small-scale stratospheric experiment known as SCoPEx, which has been repeatedly delayed.

But he says he’s troubled by any effort to privatize core geoengineering technologies, including patenting them or selling credits for the releases, because “commercial development cannot produce the level of transparency and trust the world needs to make sensible decisions about deployment,” as he wrote in an earlier blog post.