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Optimistic perhaps, but maybe not impossible ;)

I've done some further googling and found some studies on the topic, though on-land: https://cbmjournal.biomedcentral.com/articles/10.1186/1750-0... and some more from a presentation https://www.feem.it/m/events_pages/20167111127315Zeng_presen...

Though the question still remains how securely the CO2 would be stored under the ocean. Maybe with a large enough layer of rock on top it would suffice.. I suppose it's high time we started field trials on all these approaches.

My own back-of-napkin calc says that we'd need to cut about .1 to .2 % of tropical rainforests per year and remove them from the carbon cycle, not sure if that's right.

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Oops, my calculation was way off (missed a kilo-prefix somewhere), we'd need to cut and store about the entire rainforest per year. So, this is obviously not possible as an all-around solution but might help with some Gigatonnes..