Sure, but it’s even more efficient to leave those coal mines unmined.

There is already a ton of open pit coal mines. Like here https://www.google.com/maps/@51.6155147,14.3242528,60602m/da...

I'm not advocating for new mines.

No, because as soon as a tree is dead it will begin to rot and decompose (there's fungi everywhere in the air that is just waiting for a piece of juicy fresh tree to digest), thereby releasing CO2, methane and other decomposition gases.

The only way to sequester CO2 using trees is keeping the trees alive (or blasting them with chemicals to prevent rotting).

If you bury the trees reasonably deep, that CO2 takes a very long time to get to the surface (like tens of thousands of years) so it’s good enough for our purposes.

You can also weigh it down and sink it to the bottom of oceans that don't have wood eating organisms. The Baltic and the Black Sea are the ones I know.

But of course, the solution you mention is the simplest: Treat the wood with one of the several known ways to make it not break down, and leave it in big piles somewhere.

Thus my friend's idea to CRISPR plants so that they deposit their carbon in some indigestible polymer that will not rot for the foreseeable future.

I'm sure there are no possible repercussions to doing this.

I mean, it worked for the Carboniferous, until some cheeky fungus figured out how to digest lignin. But we had a nice 60 million year run, and got lots of useful hydrocarbons out of it, so hey!

Truly bioengineering at a galactic scale. In a hundred million years, geological processes will have turned your polymers into some cool new exotic fuel source.

Plants send glucose down their roots to feed mycorrhizal fungi which build up soil organic matter.

The idea is to cover the trees with something (soil, and large amount of it)?

If they rot the CO2 will still be below the surface?

> or blasting them with chemicals to prevent rotting

That's kind of a given. People generally don't like it when their homes and furniture decompose.

After a tree dies, how long does it take before it returns all its carbon to the atmosphere?

You could even cut them down and turn them into houses that people could live in.

We don't need that many houses.