[Below is the original script. But a few changes may have been made during the recording of this audio podcast.]
A manmade volcano erupting continuously to shade the Earth until greenhouse gas levels in the atmosphere are brought down. Thousands of mirrors boosted into geosynchronous orbit to block sunlight. Crewless ships wander the oceans spewing saltwater into the air to increase cloud cover.
The U.K.'s Royal Society has spent the last year identifying which schemes are just plain crazy and which just might be needed if the world doesn't start reducing greenhouse gas emissions right away.
Covering the world's deserts in reflective material is the most dangerous, expensive and ineffective geoengineering scheme, not least because it would likely change weather patterns. Fertilizing the ocean with iron to promote plankton blooms also gets high marks for danger because of the potential for unintended ecosystem impacts.
On the safer side? Artificial trees to suck CO2 out of the air are safe… but they don't come cheap. Painting roofs white is also safe… but not likely to make much impact. In the sweet spot of safe, immediately available as well as relatively effective and inexpensive sits—capturing excess CO2 where it largely comes from: power plants.
And the simplest and cheapest way to slow global warming on a planetary scale, it turns out, is simply to stop cutting down the forests helping control today's climate. It won't solve the whole problem but a few trees go a long way.
Of course, the craziest geoengineering scheme to date is the one we're already doing: changing the climate with an excess of greenhouse gas emissions. If we don't stop that experiment, we might have to start an even crazier one.