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We need to build an Yggdrasil
(fedia.io)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. The best ones are thoughts that many people can relate to and they find something funny or interesting in regular stuff.
Algae is the easiest and most likely to work.
We did experiments with iron seeding, the technology isn't too far, it's basically how the planet used to regulate before us.