It’s no longer science fiction to imagine altering the Earth’s atmosphere to try to cool the planet. In fact, several major “geoengineering” experiments are already underway.

The scientists pursuing them believe that there’s already too much carbon in the atmosphere — and that to avoid catastrophic climate change, we’ll need to resort to climate-cooling technologies.

Particularly since President Donald Trump pulled the U.S. out of the Paris Climate Agreement, the United Nations estimates that we’re way off track to keep the Earth’s temperature from rising 1.5 degrees Celsius over pre-industrial levels, the goal set by the landmark 2016 deal signed by 195 nations.

Still, the current prospects are both risky — some involve altering the atmosphere or oceans — and really, really expensive. And the research has critics. Many believe that a focus on engineering the planet distracts from more level-headed decarbonization policies. If we can fix it later, after all, why reduce emissions now?

“If you want to be confident to get to 1.5 degrees [Celsius], you need to have solar geoengineering,” said David Keith, a Harvard physicist, told Reuters.

Here are three major geoengineering projects already in the works:

Carbon-sucking fans in Switzerland

A company in Switzerland called Climeworks employs huge fans that suck carbon out of the air and then puts the carbon to use growing vegetables in a nearby garden. They estimate that they’ll suck about 900 tons of carbon out of the air per year, at a cost of about $600 per ton.

Critics say that’s too expensive — even more expensive than the ultra-expensive carbon-capture technologies that would remove carbon from the exhaust from fossil fuel plants.

Still, Climeworks thinks it’s possible to scale. In order to hit the company’s goal of removing 1 percent of carbon from the atmosphere using this tech, they’d need about 250,000 of these plants operating around the globe.

Solar geoengineering

Solar geoengineering involves releasing aerosols into the atmosphere that would reflect sunlight back out into space, preventing some portion of sunlight from entering our atmosphere. A team of Harvard scientists has been studying this for years, and they’re planning their first open-air experiment in 2018. They have funding from a handful of private donors, including Bill Gates.

The experiment will involve sending a hot-air balloon some 20 kilometers up above the Arizona desert and releasing a substance, likely calcium carbonate (a very common chemical compound, the primary ingredient in both Tums and eggshells), and measuring its effects on the atmosphere.

It will be the first outdoor experiment in solar geoengineering.

Even one of the scientists leading the experiment told the Guardian that solar geoengineering was a “terrifying prospect.” Altering the atmosphere to keep the planet cool may have consequences we can’t foresee, and even natural alterations to the planet’s atmosphere can have awful repercussions. In 1815, a volcanic eruption let to what’s known as the “year without summer,” — think snow in June, crop shortages, and disease outbreaks. Scientists worry about

Feeding iron to phytoplankton

Phytoplankton, the microscopic organisms that are nearly ubiquitous in our oceans, like to eat iron. They also suck carbon dioxide out of the atmosphere. Scientists are testing — and have tested already — what dumping iron into the oceans does to phytoplankton populations, and, by extension, to their capacity to reduce the amount of carbon in the atmosphere.

There have already been some 13 major open tests of ocean fertilization, as dumping iron into the oceans is referred to, since 1990, according to Nature. Another controversial experiment might take place off the coast of Chile — purportedly with the goal of reviving Chilean fisheries, as a more robust phytoplankton bloom can travel up the food chain and lead to a larger population of fish.