Plants not only capture CO2 from the air (pump out entropy) but they also reverse the chemistry to bind C to something like a carbohydrate molecule, whereas in carbon capture they usually compress the CO2 to 1500 psi or so and inject it underground. If you do want to do something useful with the CO2 you might have an e-Fuel factory that turns CO2 into CO and, together with H2, builds up gasoline and diesel molecules via Fischer-Tropsch chemistry which directly competes with (say) making ethanol from biomass. You don’t need so much energy to separate CO2 from N2 and compress it.
In the ocean, where most photosynthesis occurs, it's definitely not water.
I believe it's iron. I think there have been a few small-scale experiments involving "fertilizing" the ocean by spraying iron compounds on the surface.
So GP is mistaken: if we threw environmental caution to the wind, and given sufficient funding, we absolutely could produce tangible reductions in AGW using today's technology.
Or, possibly better. The deep oceans don't have a lot of marine life because there's not much plankton, because all the nutrients have sunk to the bottom. Regions where there are natural upwellings that bring bottom nutrients to the surface tend to have massive plankton populations, leading to massive fish populations (Grand Banks, the west coast of South America, others).
That's why it would be important to do experiments to see what happens when it begins to get scaled up.
"We don't know what would happen" is not a reason to avoid experimentation (indeed, "we don't know what would happen" is pretty much the reason why you do experiments).
And so on.
The physical presence of solar cells and wind turbines is a different matter, but it's largely a local effect.
The water limit is important when you are considering agriculture and biomass projects that so far we are doing on land.
Note there are biomass energy schemes where you capture CO2 from fermentation or combustion known as
https://en.wikipedia.org/wiki/Bioenergy_with_carbon_capture_...
and these do compete directly with direct air capture.
It's not really true. The critical limit is the inefficiency of photosynthesis, it's around 3-5%. The RuBisCo enzyme just sucks, compared to many other enzymes. So you can't improve the productivity of plants by adding more water.
But if you do have a lot of fresh water, you can cultivate lands that are right now empty.
Nature gave us eyes. We invented telescopes and microscopes.
Natural selection “only” does the bare minimum that beats the competition. It didn’t design plants to be maximally efficient at removing CO2. Plants do so while also having to deal with respiration, water conservation, heat management, reproduction, nutrient intake, disease prevention, etc.
I’m sure we can do better when we don’t concern ourselves with all that other stuff.
Instead of boats it seems more efficient to replace hands/legs with fins/tails.
Tongue in cheek, but only half - natural selection works on a different scale and with different goals, and in that it seems pretty good.
In this case: if we all agreed to drastically reduce our consumption of animal-based products, we'd free up huge amounts of land which we could use now to plant rapid growing, carbon-capturing plants. Then harvest them and either process them as biochar, where roughly 50% of the carbon remains captured and use the generated heat for energy/heat generation - or maybe even bury the harvested biomass in a way which ensures no methane or carbon will be emitted.
The plants would do all the hard work, we "just" need to reap its result.
Plants a significantly extremely inefficient at capturing carbon compared to industrial technology like carbon capture using Calcium Hydroxide.
> we'd free up huge amounts of land which we could use now to plant rapid growing, carbon-capturing plants
You severely underestimate the problem and overestimate the ability of trees to fix it. Planting trees doesn't scale to the levels required to make any difference even if we used all land on Earth. It would take around ~2 trillion trees planted per year just to offset the 40-50 gigatons of CO2 being pumped out per year. That is around ~2 billion acres of land - is more than the entire land area of the contiguous US - every year, just to break even.
The idea that we can invent our way into a scenario where we can have our cake and eat it too is just a lie that kicks the can down the road. How likely does it really seem that we will find a solution in which we can continue to use more and more natural resources but magically make all the negative externalities suddenly disappear? And is that likelihood more realistic or reasonable than just stopping creating the damage as much as possible?
Barring turning everything off tomorrow at astronomical human cost, there is some optimal amount of emissions and if used effectively Carbon Capture/Storage (primarily point source) could shift the Pareto front in the right direction.