[dredmorbius]

Better yet: CO2 in seawater.

We know that the oceans are a sink for atmospheric carbon dioxide (oceanic acidification is a direct consequence of this), and that it's energetically less costly (~30%--60% of atmospheric capture) and probably technically simpler to recover CO2 from seawater than the atmosphere.

The concept's been studied for decades at M.I.T. and the US Naval Research Laboratory, the latter of which released a flurry of research articles in the 2010s on the concept. I'd written of those a ways back at Reddit:

<https://web.archive.org/web/20230604174145/https://old.reddi...>

[speleding]

If you take the CO2 out of sea water to create synthetic fuel, wouldn't that contribute to the green house effect? (Unless the CO2 is captured when that synthetic fuel is burned, but in that case why not use non-synthetic fuel, for a more economical solution with the same carbon footprint)

[dredmorbius]

Generally: no.

Three are three key points:

1. Both atmospheric and oceanic CO2 are biospheric reserves, that is, already present within the biologically-active portion of the carbon cycle, as opposed to fossil carbon, which had been sequestered over hundreds of millions of years and represents a vast store which humans have been re-introducing to the biosphere at rates millions of times greater than which it was originally sequestered. That is, it's the reintroduction of this vast bolus of carbon to the atmosphere and oceans which is problematic.

2. The atmosphere and oceans are in rough equilibrium, with as I'd said above the oceans acting as a sink for atmospheric carbon. Removal of carbon from the oceans means that further excess atmospheric carbon can enter into solution. A key current concern is how much carbon can be absorbed into the oceans, and any net removal should increase the rate at which atmospheric carbon is dissolved. Given my first point this is something of a net flush, but it means that the net effect still remains carbon neutral.

3. Ocean acidification by way of CO2 absorption is already problematic, so any incidental reduction is advantageous of itself, though I suspect net effects would be small. What impacts localised reduction might be (as in the immediate neighbourhood of Iceland) I don't know.

There are futher considerations, notably that extracted carbon might itself be sequestered or stored (we know that petroleum analogues are stable over exstremely long terms --- tens to hundreds of millions of years), and that there are current applications for which there are few reasonable alternatives to petroleum analogues, notably in marine, air, and rocket transportation.

[yetihehe]

There is already too much CO2 in seawater. Taking it from air will also reduce levels in water. If we stop adding new carbon from underground, in several thousand years levels should return to current normal, but this may be too long for our civilization.