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Comment by PaulHoule | Hacker News Reader

Not necessary. The critical thing that limits CO2 capture by plants through photosynthesis is water, how they manage that gets you to the difference between C3 and C4 plants. A system based on PV might not need much water at all.

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.

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> The critical thing that limits CO2 capture by plants through photosynthesis is water

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.

> The critical thing that limits CO2 capture by plants through photosynthesis is water

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.

One concern is that if it’s not one thing it’s another: a particular spot in the ocean might be iron limited but if you supply iron you might burn up all the calcium (or something else.) so you are just bringing the carbon capture earlier in time.

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.

Iron is an interesting one. We use a huge amount of iron to produce steel, but that production process releases a massive amount of carbon into the atmosphere. The scale is such that steel is much more a byproduct of releasing that carbon rather than the other way around.

Yep. Geoengineering like seeding clouds or atmospheric aerosols (albedo modification) and fertilizing the ocean could absolutely have an impact on global temperature and CO2 levels respectively, but also would almost certainly cause environmental impacts at least on the scale of climate change, and possibly worse.

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.

> but also would almost certainly cause environmental impacts at least on the scale of climate change, and possibly worse.

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).

I definitely agree, but in the meantime we cannot depend on these strategies when making long-term plans to combat climate change because we don't know whether their side effects will prove to be tolerable.

How do you know that the "long-term plans to combat climate change" won't prove harmful in their own right? They would produce massive economic effects at a minimum. Then there are the unknown effects of (e.g.) removing a lot of energy from the desert and transporting it to other regions. I'd be very surprised if that didn't have some effect. Same thing with extracting energy from the winds on any large scale. Wind energy turned into electricity is energy that's not available to (e.g.) move rain clouds from one region to another.

And so on.

In general the incoming power from the sun is so many orders of magnitude greater than the amount of power we usefully extract that it's hard to imagine it having any material impact on the biosphere.

The physical presence of solar cells and wind turbines is a different matter, but it's largely a local effect.

if you make fuel from CO2 in the air, isn't that pointless? It'd wind up right back in the air once burnt

then you don't need to get more from underground, just cycle what is already there

Sounds like wood gas.

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