What percentage of atmospheric CO2 is best for plants?

[RedDevil]

Originally Posted by Xist

I have only slept four hours a night for the last few nights. RedDevil, does that say that CO2 benefits plants as long as they have enough other nutrients?

Hah, I can better that.
Knee inflammation meant I slept less than 7 hours total over 3 consecutive days (in fact burning the midnight oil right now) AND my dancing skills have suffered; I'm for work in Romania limping about...

C3 type plants may benefit if other nutrients suffice. C4 plants are already beyond their needs.

Sleep well

[NeilBlanchard]

There was a recent study that showed crops lose nutrition when there is higher carbon dioxide.

[RedDevil]

Originally Posted by NeilBlanchard

There was a recent study that showed crops lose nutrition when there is higher carbon dioxide.

Errr... like this one?

Originally Posted by RedDevil

Yeah. Well if it were that simple.

https://www.independent.co.uk/news/science/carbon-dioxide-emissions-environment-plants-climate-change-fossil-fuels-co2-emissions-a8342521.html

Basically there are 2 types of plants: those who move CO₂ actively towards an enzyme (RuBisCO) that absorbs it to prevent it from erroneously taking on oxygen and wasting energy, the so-called C4 plants; and plants that do not (C3 plants). ...

[jamesqf]

What this is overlooking is the temperature dependence of photosynthesis. C3 efficiency starts dropping around 25C, and shuts down entirely around 45C. C4 photosynthesis works at a bit higher temperature, peaking around 35C, but even it shuts down around 55C: https://kuensting.org/school/bb/cell...osynthesis.htm

So you put enough CO2 into the air to raise temperatures into this range (even if only in the tropics), and then goes a chunk of the biosphere. And of course it's a positive feedback: fewer plants recycling CO2 means more goes into the atmosphere, which means more temperature increase, which means even less photosynthesis...

[redpoint5]

Good stuff James, I'll have to read up on it more.

What history tells us though is that 99% of Earth history has had higher concentrations of CO2 than we currently have, and our fossil fuels came from plants that flourished during times of high (average) CO2. Were actually near the low point in CO2 concentration in a geologic sense.

This isn't to say the rise is good for humans, only to say it isn't necessarily catastrophic for life in general accept in the short term pressure to adapt rapidly to the change. In the long term, life won't even notice the change in CO2.

Since we're more concerned about the wellbeing of humanity, it's this context that warming matters.

[RedDevil]

Originally Posted by jamesqf

What this is overlooking is the temperature dependence of photosynthesis. C3 efficiency starts dropping around 25C, and shuts down entirely around 45C. C4 photosynthesis works at a bit higher temperature, peaking around 35C, but even it shuts down around 55C: https://kuensting.org/school/bb/cell...osynthesis.htm

So you put enough CO2 into the air to raise temperatures into this range (even if only in the tropics), and then goes a chunk of the biosphere. And of course it's a positive feedback: fewer plants recycling CO2 means more goes into the atmosphere, which means more temperature increase, which means even less photosynthesis...

There also is a dependency to CO₂ levels; higher CO₂ allows photosynthesis at higher temperatures in C3 plants and reduces their water usage.

So on the one hand CO₂ raises the global temperature which will increase the size of regions where temperature is the limiting factor on photosynthesis, on the other hand it allows some plants to make use of conditions that would otherwise impede them. Provided nutrients suffice.

[freebeard]

Via Xist:

He gave light to the plant with a fiber optic cable which filtered out UV light, a process called ‘Himawari Sunlighting’. He also added pressurized CO2 in a gasket around the stem and root system .... Within 2 years, the tomato plant had reached 16 feet tall with 800 tomatoes. In 16 years, it was reported at 45 feet tall with 15,000 tomatoes.

This could be important in jurisdictions where the number of tomato plants is restricted by law.

It also calls into question all those statistics about airborne CO2.

[Xist]

Some politicians must really not like tomatoes!

[RedDevil]

It must be the carbon dioxide and not the Himalaya lighting.
Tomato plants grow in a few months to the top of the greenhouse anyway, if properly cared for. So?

One dubious claim can blow away decades of meticoulus scientific observations.
If it is more in line with your preferences.

[jamesqf]

Originally Posted by redpoint5

What history tells us though is that 99% of Earth history has had higher concentrations of CO2 than we currently have...

99% of Earth's history starting when? From the beginning, about 4.5 billion years ago, you've got about 2 billion prior to the Great Oxygenation Event. The Cambrian Explosion and serious multicellular life didn't get going until ~530 million years ago, so that's about 88% of Earth's history used up before we get to anything much more complex than pond slime.

In the long term, life won't even notice the change in CO2.

Depends on what sort of life we're talking about. Deep-sea vent communities probably won't notice much. Those of us living closer to the surface might do well to read up on the Permian-Triassic Extinction, which is the closest geological model we have to the current situation: https://en.wikipedia.org/wiki/Permia...tinction_event