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We know that trees are the most efficient "tool" we have to get CO$_2$ out of the air, but the problem is that most of their fixated CO$_2$ will naturally reenter the carbon cycle through decomposition. Would it be possible to remove this carbon by sinking the trees deep underwater (and possibly putting some rocks / sand on them)? How effective would this be, if at all? (I was unable to find any studies on this, if there are please point my to them.)

Update: I found a study regarding wood burial on land, for those interested. This might or might not be easier than under water, are there are any field trials either way? https://cbmjournal.biomedcentral.com/articles/10.1186/1750-0680-3-1

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    $\begingroup$ Effective? Not at all. Transportation alone would require enormous amounts of energy. $\endgroup$ – Jan Doggen Aug 6 '19 at 13:03
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    $\begingroup$ Well, if wood didn't float... $\endgroup$ – jeffronicus Aug 6 '19 at 14:47
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    $\begingroup$ en.wikipedia.org/wiki/Deep-sea_wood $\endgroup$ – Keith McClary Aug 7 '19 at 3:45
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    $\begingroup$ Where did you take this "most efficient tool" info? What about storing Co2 by restoring wetlands? Peat is a great Co2 sink too. $\endgroup$ – J. Chomel Aug 7 '19 at 7:44
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    $\begingroup$ Just some back-of-napkin-calculation. As you need no further energy inputs into trees, that is an adavantage and scalable. aps.org/policy/reports/assessments/upload/dac2011.pdf pg 13 gives est. 600$ / ton of Co2 captured, though this may be somewhat outdated. To capture the same amount from a forest you would need between 250-500 m2 tropical forest for a year (data found in here creaf.uab.cat/Global-Ecology/Pdfs_UEG/2019%20GloChaBiol4.pdf). The main cost would probably not be land but forest work (land maybe 0,25$/m2?). Peat is great, yes, but we need diverse approaches $\endgroup$ – Quote Aug 8 '19 at 11:38
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So, there's really two questions here.

1) if we were to take wood and store it in a way that prevents decomposition, would this help reduce atmospheric CO2? and

2) Would sinking it into the bottom of an ocean trench achieve this?

So, first things first, yes removing wood from the contemporary carbon cycle does reduce atmospheric CO2. We already do this (at least temporarily) by using timber for making things - depending on the type of product this has a half-life of anything from a couple of years (paper or biomass fuel) to about 50 (for sawlogs). You can achieve a similar effect by planting new forests - so carbon is locked up in living biomass.

We could come up with a more permanent solution, and this would have to be economic (and not require more CO2 emissions to accomplish than we saved in the first place). Current research is looking at sequestration of carbon as charcoal (as this is more biologically stable than wood which is a source of food for a lot of species). This can either be spread on the land as biochar, or I suppose buried terrestrially - I'm thinking of abandoned coal mines.

I'm not aware of deep sea storage as a widely considered option, and I suspect that this is mainly due to the techno-economic cost, and lack of research - the debate is still ongoing for many other more established carbon sequestration technologies. I don't believe there has been much investigation into the stability of wood in deep water (though it lasts for thousands of years in anoxic peat bog conditions - so I see no obvious reason why it wouldn't work).

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