> 1. stop overfishing by switching to plant based diets
Switching to fish raised in fish farms in the open ocean / fresh water fish raised in farms (which are often local) should already be a large improvement, even if it isn't optimal, especially right now when those captive fish are fed fish caught in open waters.
Also hope you like mussels and clams, because raising those in farms has little ecological impact and might actually reduce net CO2 by sequestering it in their shells.
Ocean electrolytic deacidification is interesting. CO2 dissolved in the ocean makes it more acidic, which disrupts marine biology and slows down further CO2 absorption. You can extract acid from ocean water using electrolysis, raising the pH back to natural levels. This requires energy input, but could be more cost-effective than direct CO2 capture. It may be able to pay for itself by selling the acid for industrial use.
I think the best bet is to figure out how to get desalination plants to also scrub CO2 and lithium from seawater. With government subsidies, it might actually turn a profit, and we'll need to do all three of those things anyway.
(Scrubbing CO2 from the atmosphere would also work, but would take longer to address ocean acidification.)
We would need an immense number of such factories. Just lookup how much is already in the atmosphere, and how much are those technologies able to capture.
This is a feel-good technology, something to placate the population that something is being done.
1. stop overfishing by switching to plant based diets
2. stop fishing ... even fish we don't fish, but decimate (as bycatch) will rebound
3. stop fishing ... there won't be as much plastic to entangle fish, mammals and turtles (49% of great pacific garbage patch is fishing nets)
4. stop animal ag ... minimize agricultural runoff from farms and fields to stop creating dead zones in the ocean
5. stop animal ag ... use technology to reforest pastures and store all excessive carbon in them
6. reform agriculture ... use technology to improve ag methods and improve our soils, storing even more carbon in them
7. stop fossil fuels
8. give nature time to repair itself
9. enjoy healthy ecosystems, saved for future generations
Or explain the scorched earth to your children.