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Comment by TeMPOraL | Hacker News Reader

I had a stupid thought yesterday that I need to run numbers on, but I wonder if following would be possible:

Allocate some large area of ground for planting fast-growing trees. Dig a large and deep hole in the middle; something like 20 meters in diameter x 500 meters in depth. Let the trees grow; when they're mature, cut them down, throw into the hole, and throw a bunch of dirt (from the mound you made by digging the hole) behind them. Plant new trees, rinse and repeat until the hole is full, then dig another one somewhere near. Would that even make sense as a carbon sequestration facility?

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No, because the buried wood would decompose. Decomposing wood releases its stored carbon as carbon dioxide, which will slowly evaporate through the covering dirt.

One scheme that actually would work is to convert the wood to coal, then burying it. That could be done without adding any energy by using old-fashioned methods for charcoal burning.

https://en.m.wikipedia.org/wiki/Charcoal_burner

Because charcoal is chemically inert, the buried charcoal should stay put for up to a few hundred years,rather than a couple of decades for the decomposing wood.

Converting wood to charcoal is a process that requires a lot of energy, so you'd need to keep that in mind when you make your carbon calculations.

> No, because the buried wood would decompose.

Only if there is water. Bury it deep enough, or protect it from water (i.e. like a landfill).

Oxygen is much more important than water for decomposition.

How slowly though? Intuitively, I would think that trees piled wide and 20m high and covered over would still be mostly there in my lifetime. Is this wrong? (genuine question)

500m high is another level. If it took, say, 1000 years for them to decompose fully and release their CO2 in to the atmosphere, that's still potentially useful even if its technically net zero.

We really only need to get past the next 100 years.

If it's buried under enough dirt, there won't be enough oxygen for it to decompose.

This is possible, and there is an entire agricultural method to it called Hügelkultur

https://www.permaculture.co.uk/articles/many-benefits-hugelk...

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2266747/

Placing them in arid caves/mines would lock the carbon up much longer.

So, we could skip the hole drilling part, and set up shop near abandoned mines? That turns the idea from expensive to relatively trivial to do.

If/where suitable mines exist, railways may still exist and might be feasible from a carbon standpoint with the newer diesel locomotives or using an electric locomotive.

That said, you aren't going to make a considerably dent in the CO2 emitted in a given time period doing this. Used in combination with dozens of other things though...

I'd imagine many such plants deployed around the world. I wouldn't expect it to be anywhere close to a complete solution, but it's the easiest way to start doing carbon sequestration I can think of.

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