[ars]

> 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.

[danbruc]

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

[ars]

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

[danbruc]

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