More linkies: TL;DR: most plant species studied appear to have optimal growth characteristics with CO2 at well above current levels, in particular at from 743ppm (measured) to 2700ppm (estimate of combination of CO2 and water usage optimization). That is, plant life on this planet appears optimized for atmospheric concentration levels of around 1200ppm. Atmospheric CO2 Concentrations At 400 PPM Are Still Dangerously Low For Life On Earth http://notrickszone.com/2013/05/17/atmospheric-co2-concentrations-at-400-ppm... Over the Earth's history, atmospheric CO2 concentrations have ranged from 180 ppm to 7000 ppm, see Figure 1 below. On that scale we are in fact today barely above the Earth's record lows. Figure 1: Atmospheric CO2 concentration is just barely above the life-sustaining levels of 150 ppm. … It’s a fact that biologists have shown that once the atmospheric CO2 level falls below the 500 ppm level, plants really begin to suffer. Many of us have seen the video showing http://www.youtube.com/watch?v=P2qVNK6zFgE how plants grow faster under higher CO2 concentrations. The following charts show the growth curves of some plants as a function of CO2 concentration: Study: why CO2 levels are lower during global cold periods https://wattsupwiththat.com/2017/03/15/study-why-co2-levels-are-lower-during... As recently as 18,000 years ago, at the height of the most recent major glaciation, CO2 dipped to its lowest level in recorded history at 180 ppm, low enough to stunt plant growth. This is only 30 ppm above a level that would result in the death of plants due to CO2 starvation. CO2 myth busted: Why we need more carbon dioxide to grow food and forests https://www.naturalnews.com/039720_carbon_dioxide_myths_plant_nutrition.html … as CO2 levels are raised by 1,000 ppm photosynthesis increases proportionately resulting in more sugars and carbohydrates available for plant growth. Plant Growth and Carbon Dioxide for Maximum Yield http://www.jasons-indoor-guide-to-organic-and-hydroponics-gardening.com/plan... Plant growth requires a tremendous amount of Carbon Dioxide (also known as CO2). At the center of every plant cell is an atom of Carbon, which the plant has absorbed from the surrounding air. When all other growing conditions are kept ideal, Carbon Dioxide becomes the growth limiting factor. This means, as you increase the CO2 in your garden area, you will also increase the plant growth rates....and your yields. The ideal level of CO2 supplementation to maximize plant growth in a well maintained garden is generally 1500 ppm. In one final twist of irony, as you begin to increase plant growth and Carbon Dioxide levels you will reach a point where temperature becomes the limiting factor. In order to benefit from the highest levels of CO2 supplementation (1500-2000 ppm), you actually need to run your garden area warmer than normal (80-85 degrees). On average, plants will also require an extra 30 watts of light/sq.ft. (70-80 watts/sq.ft.). Plants will use extra water and nutrients under these conditions, so make sure they are available! This has one implication which may offer you (the indoor gardener) a very nice solution to one of your most difficult problems. If all other factors are perfect in your garden EXCEPT your temperature runs a little high, you may be able to maximize plant growth AND solve your heat problem at the same time with the addition of CO2. The optimal CO2 concentrations for the growth of three perennial grass species https://bmcplantbiol.biomedcentral.com/articles/10.1186/s12870-018-1243-3 Here, we examined the optimal atmospheric CO2 concentration effect on CO2 fertilization and further on the growth of three perennial grasses in growth chambers with the CO2 concentration at 400, 600, 800, 1000, and 1200 ppm, respectively. … the CO2 fertilization effect may sustain up to about 1000 ppm for leaf photosynthesis [46, 49] and 1800 ppm for grain yield of crops [50]. For example, Xu [23] examined the optimal atmospheric CO2 concentration of the CO2 fertilization effect on the growth of winter wheat and found that the optimal atmospheric CO2 concentration was 894 and 968 ppm for total biomass and leaf photosynthesis… … Elevated CO2 effects on plant biomass We found very strong CO2 fertilization effects on the aboveground and total biomass of the three species. The optimal CO2 levels for the aboveground biomass were 945, 915, and 1151 ppm, and for the total biomass were 915, 1178, and 1386 ppm for tall fescue, perennial ryegrass, and Kentucky bluegrass, respectively (Fig. 1). However, an optimal CO2 of 895 ppm for the belowground was found only for the tall fescue, while no obviously optimal CO2 of the belowground biomass for the other two species was detected. Beyond the optimum, further elevating the ambient CO2 concentration significantly reduced the growth of perennial grasses, indicating the adverse impacts of high CO2 concentration on the grass species. … we quantified the relationship between CO2 and WUE [water use efficiency] of perennial ryegrass through quadratic models and found that the optimal CO2 for WUE would occur at about 2700 ppm, which was much higher than those of the other two species (Fig. 2).