This is a complex article showing how high CO2 related to plant evolution changing into forms that supplied food and made agriculture possible. As a result there was a new supply of edible energy and a massive growth in a human population:

https://wattsupwiththat.com/2017/03/24/effect-of-atmospheric-co2-concentrations-on-earl...

"The purpose of this paper will be to show that it is very likely that it was the increase in the CO2 content of the atmosphere, to levels above 250 ppm, that created the conditions for the plants that served as food sources to humans to thrive, and that made it worthwhile for humans to spend the time and energy in cultivating crops, which then allowed for the creation of cities and much more complex social orders. Prior to reaching this turning point, it was more efficient for humans to let wild animal species forage for the more limited vegetative offerings available, convert the plant material to protein and fat, and harvest the animals. Until about 8000 years ago, the human diet was composed primarily of meat from wild animals, supplemented with nuts, berries, mushrooms, and fungi in the local area. The transition from hunter/gatherer to agriculturalist was not necessarily a one-way process. Climatic changes could have necessitated a return to past methods when necessary for survival, such as occurred during the Younger Dryas.

"However, as the following graphs will show, the viability of numerous plants drops considerably as CO2 concentrations diminish. Biomass and seed yield were only about 40% of current plant production rates during the glacial periods; levels that would make agriculture an inefficient use of time and energy in those early cultures.

"To clarify, there are three different photosynthetic pathways: C3, C4 and CAM. For the purposes of this discussion, only the C3 and C4 pathways are of concerned. CAM is utilized by cacti and similar plants that are not a large component of the human diet. The C3 process evolved first, over 400 million years ago when CO2 levels where many times current levels and are utilized by about 85% of the existing plant species today. The C4 process evolved much more recently, about 30 to 40 million years ago, when CO2 levels had dropped to levels still above todays levels, but much lower than when C3 plants evolved.

"It is believed that the C4 process was a natural adaptation to lower atmospheric CO2. With CO2 levels rising from levels of 180 ppm during the last glaciation to about 400 ppm currently, the C3 plants show a larger response, but the C4 plants also benefitted to a considerable degree, due to increased drought resistance and mycorrhizal colonization of plant roots. Examples of C3 plants are beans, rice, wheat, barley, rye, oats, soybean, peanut, cotton, sugar beets, spinach, potatoes, all woody trees and most lawn grasses. The C4 plants include corn, sugarcane, sorghum, millet, Bermuda grass and poa.

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"Many plants species evolved at much higher CO2 concentrations than we are experiencing currently. The predecessors to Gymnosperms, or evergreens, evolved about 360 million years ago when CO2 levels were about 4000 ppm, 10 times today’s levels. The Angiosperms, or flowering and deciduous trees, evolved about 160 million years when CO2 levels were about 2200 ppm, over five times current levels. So at the levels experienced during the last glacial period of 180 ppm, the plant kingdom was clearly under great stress. We are all quite aware that the survival of the more advanced members of the animal kingdom which includes humans, are clearly dependent upon the well-being of the plant kingdom. So it would appear that during recent glacial periods, much of the life on earth was in jeopardy. (my bold)

"Will rising CO2 levels enhance the growth of many plant species existing today? The answer is clearly yes. .... However, it would appear that within certain limits the increase in atmospheric CO2 has been beneficial to date, and very likely will continue to be for some time into the future.

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"During the recent glacial advance, when CO2 levels dropped to 180 ppm, mark the very lowest levels of CO2 during the past 500 million years, and probably much longer. It should be noted, that below levels of 180 ppm, things become extremely dire. Were we to return to levels much below 250 ppm we would probably lose 70 to 80 percent of the human population to starvation and the societal turmoil that would ensue as we have to forgo the benefits of agriculture and go back to being foragers.

"The IPCC warns us that at CO2 levels above 300 ppm we face dire consequences. It appears that the quandary we are facing is this: Do we allow CO2 levels to rise, face a modestly warming earth, but one with abundant plant growth, or try to lower CO2 levels which could have much more disastrous consequences for mankind? Ironically, if past geologic history is any indication, we could be approaching the end of the current interglacial and will then have to deal with the glacial narrative. "(my bold)

Comment: It is obvious a higher CO2 level is better. What we must remember is the next glacial period may be coming. Note my bolds. The whole article is fascinating. What will God do if He does?