Let's pretend that the US didn't recently pull out of the Paris Climate Agreement. Let's also pretend that all the other countries that scolded it for withdrawing also met their Paris pledges on deadline. Heck, let's pretend that that everyone in the whole world did their very best to cut emissions, starting today. Even if all that make-believing came true, the world would still get very hot.

Fact is, if you add up all the emissions cuts every country promised in their Paris pledges, it still wouldn't keep the planet's temperature from rising beyond the agreement's goals—to keep global temperatures from rising more than 2˚ C higher than they were before the Industrial Revolution, and as close to 1.5˚ C as possible. If Earthlings want to avoid a heat-soaked, tide-swamped, and war-clouded future, they need to do more. This raises the specter of geoengineering: things like seeding the stratosphere with sulfur, or using ice crystals to dissolve heat-trapping clouds. But geoengineering is a dirty word many climate scientists and climate policy experts avoid, because humans meddling with nature doesn't have the best track record. Which is why they say world leaders need to come up with some rules about geoengineering ASAP, before desperation over the coming climate catastrophe forces humanity to do something it might well regret.

Geoengineering strategies generally fall into two categories: removing carbon dioxide and reducing heat. The former problem has vexed researchers for years. Sure, they can do it on small scales—carbon scrubbers are essential life support aboard closed systems like the International Space Station and submarines. But installing systems large enough make a dent in all those parts per million is functionally impossible. It would be expensive, energy-intensive, and also nobody really knows how to do it. Doing the same with reforestation would require covering nearly half of all world’s landmass with trees. Not likely to happen. And despite the hype, carbon capture and storage—sucking the stuff up before it leaves the smokestack and pumping it underground—is still in its infancy.

Heat reduction is currently more practical. You can do it many ways, and all of them involve either blocking the sun's heat from coming into Earth’s atmosphere, or allowing more of Earth's heat to radiate into space. For blocking heat, sulfur injections are probably the most likely to work. "It scatters and reflects solar radiation back into space," says Ulrike Niemeier, a climate scientist at the Max Planck Institute for Meteorology, and co-author of a new paper in Science discussing that geoengineering technology and its risks. The concept comes from volcanic eruptions. Large ones send gobs of sulfur dioxide into the stratosphere, triggering temporary global cooling events. After Mount Pinatubo erupted in 1991, scientists measured 17 million tons1 of additional sulfur dioxide in the atmosphere. The Northern Hemisphere cooled by about 0.5˚ to 0.6° C in the aftermath.

Compared to carbon removal, sulfur injection isn't so hard. The basic technology already exists: High-flying jets capable of carrying tanks of sulfur into the stratosphere. The difficulty arises when you consider the scale at which you'd have to deploy those jets in order to get meaningful cooling. Niemeier and her co-author estimate that 1˚ C of cooling would require 6,700 flights a day. Over the course of a year, that would cost around $20 billion.

OK, how about that other tack, letting Earth shed more heat into space? Similarly, this strategy would involve high-flying jets. Their targets would be cirrus clouds, those wispy strokes of white common on pleasant days. Cirrus clouds form high in the atmosphere, and are made from particles of ice. "The cirrus clouds that form at high altitudes absorb some of the radiation that would otherwise be emitted to space. In that sense they act similar to greenhouse gases," says Ulrike Lohmann. That’s a different Ulrike than the author of the previous paper; Ulrike was just a popular name for girls in north Germany for a while, and this one is a climate scientist at ETH Zurich's Institute of Atmospheric and Climate Science and co-author of a separate Science paper describing how eliminating these high altitude cirrus clouds could cool the planet.