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Comment by zerealshadowban | Hacker News Reader

We should remember that CO2 is an essential fertilizer for plants and phytoplankton, all of which thrive when there is much more CO2 in the atmosphere. Their photosynthesis processes evolved in and are ideal for an environment with CO2 concentrations at least 5-10x the current levels.

At or below 150 ppm of CO2 in the air, all land-based plants would go extinct.

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Geologist here. This is misleading.

Yes, CO2 is what plants eat, but it doesn't tend to be the limiting nutrient very often on Earth today. For the ocean (where 50-85% of photosynthetic oxygen production, and equivalently carbon burial, happens [1]) there are actually maps of this [2], and you may note that CO2 does not appear. On land, it's usually water, nitrogen, or phosphorus [e.g., 3].

Accordingly, there have been many studies on CO2 fertilization, but since it's not the limiting nutrient it doesn't tend to have much of an effect [e.g. 4]. At best there are benefits for some plant species in areas where water is the limiting nutrient, since higher CO2 means plants can keep their stomata closed more [e.g. 5], but even this is a bit of a mixed bag since dry places tend to get hotter and drier under global warming.

[1]https://en.wikipedia.org/wiki/Phytoplankton#Oxygen_productio... [2] https://media.springernature.com/m685/nature-static/assets/v... [3] https://nph.onlinelibrary.wiley.com/doi/full/10.1111/j.1469-... [4] http://www.pnas.org/content/early/2016/12/07/1610156113.shor... [5] https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.124...

Biologist here. I have a simple question in response to your disinformation: What CO2 level is used in commercial greenhouses?

All scientific, controlled experiments with plants demonstrate conclusively that they thrive (faster growth, larger leaves, increased biomass, larger fruits, less need for water, etc.) at higher CO2 levels than what we have today and than we had before the Industrial Revolution. I encourage people who are intellectually curious and honest to look up the scientific papers on the topic and read them for themselves (you don't have to be a biologist or a geologist to understand the experiments, the parameters, or the outcomes).

GP said that CO2 was not the _limiting factor_. I took this to mean that something else is often the bottleneck, such as access to nutrient matter. Presumably in a greenhouse, more CO2 can be used because other bottlenecks are reduced.

Well said. Yes, if you provide unlimited water, N, P, and light in a greenhouse then of course CO2 becomes limiting, it's the only thing left!

Well, very high but that's not the point. The poster was saying that it was not the limiting factor in the environment, not in an artificial setting.

> since higher CO2 means plants can keep their stomata closed more [e.g. 5]

And that apparent silver lining's actually got a downside, too. Plants working hard to draw up water is a Good Thing, because that water is the only source of nitrogen and other elements needed to make proteins and vitamins.

CO2 just gets turned into carbs; it makes plants grow big and tall but less nutritious. This won't be a problem for us overfed gluttons in the West, but it is going to exacerbate malnutrition issues in poorer regions of the world.

See e.g.:

https://www.nature.com/scitable/knowledge/library/effects-of...

http://advances.sciencemag.org/content/4/5/eaaq1012

Woah, I learned something today! Nice references.

It is not "misleading" when the scientific evidence collected via NASA's satellites is that CO2 fertilization has been measurably greening the continents for decades: "From a quarter to half of Earth’s vegetated lands has shown significant greening over the last 35 years largely due to rising levels of atmospheric carbon dioxide." [1]

Anyone who studies the biology and evolution of plants, as well as the actual constraints of their growth processes, will understand why this gigantic transformation has been happening. There are similar measurements of major increases in ocean plankton over the last decades.

[1] https://www.nasa.gov/feature/goddard/2016/carbon-dioxide-fer...

That's a cool study. CO2 fertilization is a real phenomenon (with some caveats, which I pointed out some of), but I stand by the characterization that it is misleading to use this to suggest that we shouldn't be concerned about (or consider reversing) anthropogenic CO2 emissions, or to suggest that plants are maladapted and struggling without "CO2 concentrations at least 5-10x the current levels". Plants, algae, and cyanobacteria have had plenty of time to evolve carbon concentrating mechanisms [1] to help offset low atmospheric CO2.

My apologies if that isn't what you were doing.

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2606764/

I had a related thought that maybe someone here can answer. We've been approaching peak phosphorus, where we're using up the easy sources in much the way we do with oil. Like oil, we'll likely just find other sources as scarcity raises the price. Also like oil, the used product ends up distributed in the environment.

My question is, if photosynthetic carbon capture is nutrient limited, what happens when we are adding both CO2 and phosphorus to the environment? Does everything grow better? And what's the next limiting nutrient?

That's a good question. Someone else may be more qualified, but here's a start. For one thing, the answer will be a bit be different in marine vs. terrestrial ecosystems.

Nitrogen limitation is one common one, which can apply either on land or in the oceans in different areas. We're adding quite a lot of that to the environment too though, up to a 200% increase in reactive nitrogen availability over pre-Haber-Bosch times [1]; accordingly about half the nitrogen in your body may come from the Haber-Bosch process [2].

In the ocean, there are already a good number of places that are Fe limited (Fe is needed to make chlorophyll, among other things [3]), so they'll stay that way unless we go with Fe-fertilization or increase dust input [4].

Si limitation is another possibility in the oceans -- particularly for phytoplankton that make silica tests like diatoms [5] (though as an interesting aside, a surprising number of plants need silica to make their phytoliths [6])

On land, there's plenty of Fe and Si (from soil), so if we're supplying P, N, and CO2 then we're mostly left with water or light (photon) limitation.

If all nutrients are superabundant (particularly in aqueous settings), then we end up with runaway growth and eutrophication, which is dangerous in its own way [7]

[1] https://en.wikipedia.org/wiki/Human_impact_on_the_nitrogen_c... [2] https://en.wikipedia.org/wiki/Haber_process#Economic_and_env... [3] https://www.tandfonline.com/doi/abs/10.1080/0190416840936323... [4] https://en.wikipedia.org/wiki/Iron_fertilization [5] https://www.sciencedirect.com/science/article/pii/S096706370... [6] https://en.wikipedia.org/wiki/Phytolith [7] https://en.wikipedia.org/wiki/Eutrophication

They might thrive, if and only if, they can move to an area where they have their needed heat, water and light exposures. If atmospheric CO2 is 5x or 10x everything will need to be migrating an awful lot of miles northwards. To soils and locations that may not be suitable for ph or mineral reasons. Plants almost certainly can't migrate fast enough for current rates of change, let alone more. So there's agriculture screwed.

At or above 1,000ppm, or 2.5x current outside levels, human performance starts to decline. People will complain of drowsiness and stuffiness. Those levels can occur in buildings already. I don't look forward to a future where opening a window for fresh air delivers stuffiness, and inside buildings rise commensurately.

Who knows how many pollinating species will go extinct, and at what levels, as they can no longer breathe enough to fuel their activities.

Perhaps only if we consider CO2 in isolation without attendant water, temperature, and mineral stresses. And perhaps not on longer time scales [0]. And perhaps not everywhere [1].

[0] http://science.sciencemag.org/content/360/6386/317

[1] https://www.nature.com/articles/nclimate2995

This is completely false, misleading information. Disappointing to see it is at the top. Different plants have varying levels of ideal co2 concentrations, and many do worse when they are higher than now. Not to mention since co2 will change temperature and weather, the net effect could be bad for a given plant even if the plant would thrive with more co2 in isolation.

My main point is that if CO2 levels fall below 150 ppm there would be a mass extinction of land-based plants, and of all the animals who depend on this vegetation. If you care about life on the continents, and human life, this is a valid concern when people explore schemes to rapidly remove massive quantities of CO2 from the atmosphere.

It is unlikely that we will accidentally lower CO2 levels too quickly.

Moreover, if we do happen to accomplish such a thing, it's easy to raise C02 levels; just burn more fossil fuels temporarily.

I wonder if we'll ever increase the biomass enough that we'll actually want to increase the total carbon in the system...

> Their photosynthesis processes evolved in and are ideal for an environment with CO2 concentrations at least 5-10x the current levels

Wait a second. For the last few million years CO2 levels have been in the 180-300 ppm range (except for recently where they have gone above that).

What makes you think that plants that initially evolved hundreds of millions of years ago in very high CO2 environments did not continue evolving as CO2 levels changed, so that the plants we now have find 180-300 ppm to be ideal?

The two photosynthesis paths that evolved in land-based plants (C3 and C4) are very specific, not adaptable. There has been no such thing as further "evolution" of these chemical/physical processes in plants.

CO2 is one variable of many. Temperature is more important because it determines what enzymes can work efficiently. It's unlikely that current plants would survive the accompanying temperature changes that would occur if CO2 significantly increases.

Maybe we should go by scientific facts? plants thrive in warmer climate and higher CO2 levels, not in colder climate and at starvation levels of CO2. I suggest you look into the operations of commercial greenhouses, then study the temperature and CO2 records matching the origin and evolution of plants going back to hundreds of millions of years ago.

Yes, I certainly think we should follow basic logic. I worked at a greenhouse. Historical data is obviously not based on current plant genetics. Do you think people use a greenhouse in midsummer? There is clearly a maximum temperature that varies for different plants. I suggest you look up the thermodynamics of CO2.

Yes, there are many commercial greenhouses that operate year-round, including the summer months. The deliberately increased level of CO2 in a commercial greenhouse has no measurable thermodynamic impact.

Sure, but we want it to be in the 200 to 300 area instead of 400 ppm. No one is proposing killing all plants. We know plants do just fine at that concentration.

Exactly. I wouldn't reduce the CO2 even if it was possible.

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