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According to Elon Musk, there's "an argument" stating that the current level of carbon in the atmosphere (more than 400 ppm) is better than it was some hundreds of years ago (200 ppm). He even calls these low carbon levels carbon starvation.

https://www.youtube.com/watch?v=kzlUyrccbos&t=45m4s

What is this argument based on? What harm would it do if the level fell again to 200 ppm?

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    $\begingroup$ The argument is based on the fact Elon Musk is not a climatologist. $\endgroup$ – John Aug 30 at 14:14
  • $\begingroup$ @John He didn't claim to be an expert. To me it sounds as if he had picked up that argument somewhere. $\endgroup$ – Everyday Astronaut Sep 3 at 7:04
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If plants were growing poorly when atmospheric CO2 was 200ppm, it was probably because Earth was in the middle of an ice age and covered with glaciers, not because plants were starved of carbon dioxide.

All things being equal, increasing the amount of carbon dioxide available in the atmosphere can benefit some plants, but the problem is that all things do not remain equal.

  • CO2 is only a fertilizer in environments where CO2 is the limiting input for a plant; plants in the outdoors may receive too little moisture, too much warmth, or too little sun for additional CO2 to make much difference.
  • Adding CO2 to the atmosphere also changes global and regional climates, reducing precipitation and increasing temperatures in ways that offset benefits from CO2 fertilization.

The authors of Plant growth enhancement by elevated CO2 eliminated by joint water and nitrogen limitation in Nature Geoscience found that

the presence of a CO2 fertilization effect depends on the amount of available nitrogen and water. Specifically, elevated CO2 levels led to an increase in plant biomass of more than 33% when summer rainfall, nitrogen supply, or both were at the higher levels (ambient for rainfall and elevated for soil nitrogen). But elevated CO2 concentrations did not increase plant biomass when both rainfall and nitrogen were at their lower level. We conclude that given widespread, simultaneous limitation by water and nutrients, large stimulation of biomass by rising atmospheric CO2 concentrations may not be ubiquitous.

Scientific American tackled the question in Ask the Experts: Does Rising CO2 Benefit Plants?, concluding that "Climate change’s negative effects on plants will likely outweigh any gains from elevated atmospheric carbon dioxide levels."

Researchers cited in that article pointed out that added CO2 provides diminishing returns, with Samuel Myers, principal research scientist in environmental health at Harvard University, noting that “We know unequivocally that when you grow food at elevated CO2 levels in fields, it becomes less nutritious."

The second part of your question was what would happen if CO2 levels fell to 200 ppm. This is unlikely to happen on human timescales; the current CO2 level is about 410ppm, rising 3-5 ppm per year. CO2 levels were at about 280ppm for most of human civilization, only increasing substantially when fossil fuel use became widespread in the 1800s. CO2 has a residence time in the atmosphere of several centuries.

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  • $\begingroup$ Were plants actually growing poorly during periods of low CO2? In particular the last Ice Age. Plant growth seems to have been sufficient to have supported extensive (and now mostly extinct) megafauna ecosystems. $\endgroup$ – jamesqf Aug 30 at 18:45
  • $\begingroup$ Up to 30% of the planet was covered with ice during the Pleistocene, compared with about 10% today. I'd figure plants covered by several hundred meters of ice fared poorly. $\endgroup$ – jeffronicus Aug 30 at 20:28
  • $\begingroup$ Obviously the plants under the ice weren't growing at all, but that's not relevant. What we're looking at is the growth rate of plants that weren't under the ice. While I don't know of any actual data (and can't really think of a way to get it), it is obvious that the growth rate was high enough to support a diverse biosphere which included larger species (of land animals) than exist today. $\endgroup$ – jamesqf Aug 31 at 16:42
  • $\begingroup$ "reducing precipitation and increasing temperatures"? No. Increase temperature, so increase evaporation, and so increase precipitation. $\endgroup$ – Helder Velez Sep 6 at 18:17
  • $\begingroup$ @HelderVelez Not uniformly. Wet climates are expected to get wetter; dry climates are expected to get drier. $\endgroup$ – jeffronicus Sep 6 at 19:07
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Musk didn't state his reasoning, but the only benefits of higher CO2 levels that I can see are that a) plants grow more vigorously, so therefore we can expect better crops, and where damaged forest is left alone it will regenerate more quickly, b) climate warming is an insurance against the next ice age or little ice age. There was a Little Ice Age between about 1350 and AD 1810 (estimated dates vary, as there were no accurate climate records in the Middle Ages). LIAs are believed to be associated with periods of low sunspot activity called Maunder Minimums, and some scientists say we are now on the verge of another.

Milankovitch Cycles only tell you when a proper ice age is likely, they are rather complex and don't accurately predict Ice Ages. We can be sure that there will be another ice age, but no one knows exactly when.

The down sides of higher CO2 levels are so well known that it shouldn't be necessary for me to enumerate them. The question was: are there any advantages to higher carbon levels, so I have answered it.

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    $\begingroup$ Except that plants in general do not grow more vigorously with increased CO2 levels. Their growth is almost always limited by other factors such as water, temperature, available nitrogen, phosphorus, and other nutrients... $\endgroup$ – jamesqf Aug 29 at 18:10
  • $\begingroup$ You're wrong about the CO2, but obviously if plants are short of water and nutrients they will grow less vigorously,a kid of six could see that. It's not a revelation. I don't see the connection with CO2. Some greenhouse owners deliberately enhance CO2 levels to make their produce grow more quickly. $\endgroup$ – Michael Walsby Aug 29 at 18:24
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    $\begingroup$ Some greenhouse owners may enhance CO2 believing that it enhances growth, but AFAIK there's little actual evidence that it does. Even if it does in a greenhouse (where all the plants' other needs are presumably met), there's no evidence that it does in the natural world. An easy way to test would be to examine tree rings. If the increased atmospheric CO2 over the last century actually enhanced growth, it should show up there. But apparently it doesn't: cbc.ca/news/technology/co2-trees-1.5000709 $\endgroup$ – jamesqf Aug 30 at 4:26
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    $\begingroup$ The bottom line is that in the natural world (rather than lab or greenhouse), plant growth is almost always limited by something other than the available CO2. $\endgroup$ – jamesqf Aug 30 at 4:28
  • $\begingroup$ I think the ice age point is valid. $\endgroup$ – Everyday Astronaut Aug 30 at 12:59

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