1. Whiten the clouds.
Big idea: Spray salt water into clouds at sea to make them fuller and whiter so they reflect sunshine away from earth.
Why it’ll never happen: Requires a massive, worldwide cloud-seeding operation to cool things just a little.
2. Polish the crops.
Big idea: Breed plants like corn and barley to be shinier by changing the shape, size, and waxiness of their leaves.
Why it’ll never happen: Could take decades to develop and introduce the new crop varieties, and the impact would be minor.
3. Drop acid—or diamonds.
Big idea: Deliver tiny reflective particles—sulfates, diamonds, or aluminum oxide—into the stratosphere via jets, balloon-tethered hoses, or artillery shells.
Why it’ll never happen: Sulfates could deplete the ozone layer, while the effects of diamonds and aluminum oxide are poorly understood. And, really: Spraying drain cleaner into the air?
4. Bubble the oceans.
Big idea: Hitch up aerators to ships so they create tiny bubbles near the ocean surface, making the water frothier and whiter to reflect sunlight away.
Why it’ll never happen: It’s safe, but the effects are too negligible to justify the expense. Also, ships’ captains aren’t likely to embrace the idea.
5. Throw some shade with balloons.
Big idea: Float aluminized hydrogen balloons into the stratosphere to form a solar shade.
Why it’ll never happen: It’s costly, and retrieving spent ones would be like cleaning up after the world’s biggest birthday party.
6. Make our own sun spots.
Big idea: Park trillions of self-steering disks in orbit around the sun at L1—a stable gravitational point 1 million miles from earth—to create a space umbrella.
Why it’ll never happen: It would cost trillions of dollars, even if the disks could be shot up cheaply using electromagnetic acceleration and ion propulsion.