Trying to bury a significant portion of the global plant mass gain every year in a way that it does not emit greenhouse gases while decomposing seems like a major undertaking to me.

A quick calculation - one ppm reduction means extracting three trillion metric tons of carbon dioxide which contains about 800 billion tons of pure carbon which takes up about 400 cubic kilometers. That is a huge hole to dig even if we would just have to deal with the carbon. And that is for one ppm.

> three trillion metric tons of carbon dioxide

Billion not trillion (10^9). All the rest of your numbers are off by a similar factor.

> That is a huge hole to dig

Actually, using the correct numbers (.4 cubic km), if you made a 50 foot deep hole it would be about 3 miles by 3 miles. Not that big - a couple of city blocks.

A typical landfill is larger than that, and we have tons of those.

Nope, trillions. The mass of the atmosphere is about 5.15 × 10¹⁸ kg [1], 400 ppm of that are 2.06 x 10¹⁵ kg or 2.06 x 10¹² t which are about two trillion (short scale) or two billion (long scale) metric tons. I used the short scale [2]. And because the 400 pm are by volume and not by mass you end up with about three trillion metric tons if you take that into account, too. Of course only unless I messed things up.

[1] https://en.wikipedia.org/wiki/Atmosphere_of_Earth

[2] https://en.wikipedia.org/wiki/Long_and_short_scales

Your figure is for 400ppm. But we are talking about 1ppm.

Of course, sometimes you are just blind for the error.

Don't we have plenty of old coal mines? They seem like a natural place to sequester carbon in, given that that's where a lot of it came from in the first place...