Responding to the CO2 Coalition's "Fact #3" on "CO2 is Plant Food"

 CO2 Coalition's "fact #3" repeats the mantra "CO2 is plant food." This of course is superficially true, but CO2 Coalition oversimplifies this point and then carefully omits important information. The effect is to mislead. Below is a picture of Dr. Craig Idso with 4 trees apparently showing that increasing CO2 produces larger trees. The citation for this apparently is another political propaganda group CO2 Science and "Idso 2013," which is cited as if it's a peer-reviewed study, but in fact it's just another political tract written by Idso at the "Center for the Study for Carbon Dioxide and Global Change" and "CO2 Science."

Let's be clear at the beginning that it is of course superficially true that "CO2 in plant food" in the sense that photosynthesis requires CO2. The chemical reaction for photosynthesis is:

6CO2 + 6H2O → C6H12O6 + 6O2

This reaction requires solar energy for plants to take water and carbon dioxide to produce the sugars they use to grow. If you increase the number of CO2 molecules in the atmosphere, then in principle, you can get more photosynthesis. But the reaction also quite clearly shows that for every 6 CO2 molecules you also need 6 H2O molecules to produce sugar and oxygen. If there isn't sufficient water, photosynthesis rates can't increase regardless of how much CO2 you add to the atmosphere. Additionally, photosynthesis rates do not automatically increase with CO2 where other environmental conditions are altered that negatively affect the climates that plants have adapted to. A few things should be obvious from this:

1. Increasing CO2 pushes climate towards warming, so where increased temperatures cause stress for plant life adapted to lower temperatures, increases in CO2 can harm plant life even if additional CO2 is available to them.
2. Warmer air holds more water, so in more humid areas this means more rainfall, and in some places, an increase risk from flooding. In more arid areas, this means in increase in vapor pressure deficit (VPD), which can dry out plants and soils, limiting photosynthesis. And high temperatures combined with arid conditions increases the risk of wildfires.
3. What is beneficial to plants may also be beneficial to disease and pests, which can limit plant growth (more on this in a future post).

These three basic facts show that just repeating mantras like "CO2 is plant food" is not really helpful. To answer questions about whether increased CO2 is good for plant life, we need to do more than repeat mantras. We have to look at actual empirical data about how plant life is responding to global warming and elevated CO2 globally and regionally.

What propaganda groups like CO2 Coalition and CO2 Science frequently do is promote the results of controlled experiments in which CO2 is increased while controlling for other variables impacting plant life, such as water, temperature, disease and pests. In greenhouse studies (like the trees in the photograph from Idso), you can increase CO2 and make sure the plants you study have optimum sunlight, temperature and water. You can physically eliminate competition from other plants and keep plants free from disease and pests. Under these controlled situations, plants do respond positively to increased CO2, and nobody suggests otherwise. However, in real life, virtually all the plant life on planet Earth grow in open air environments, not in greenhouses, and they are sensitive to changes in temperature, humidity, flooding, sunlight, pests, disease, competition from other plants, and a whole host of other factors. It turns out that the relationship between increased CO2 and plant growth is far more complex than what propaganda groups like the CO2 Coalition and CO2 Science would like you to believe. While some plants and crops may well benefit from increasing CO2, others will not. We need empirical evidence here, not just the repetition of mantras like "CO2 is plant food."

There is far more that can be said here, but since there are other so-called "facts" on the CO2 Coalition's website that are on this topic, I'll leave fuller responses to my response to those to save space here. For now, it may be helpful to show recent evidence that increased VPD from CO2-caused global warming has in fact been a limiting factor in photosynthesis. This has been found in several studies. Yuan et al 2019[1] shows that greening has decreased over the last 20 years or do to increases in VPD, which causes water stress in plants, particularly in the tropics and in boreal forests. As boreal forests warm, they also see an increase in insect outbreaks that kill trees, and the combination of more dead wood and higher VPD in boreal forests fuels more wildfires like those we've seen in Canada in recent years. Similar results showing a negative correlation between vegetative "greening" and VPD have been documented in the peer-reviewed literature.[2][3][4][5] Even here, further analysis shows that there may be exceptions to the general rule. In northern peatlands, one study[6] showed that increasing VPD alone cannot explain vegetation suppression with warming. Nevertheless, it seems clear that we should not expect the continuation of increasing CO2 levels to cause continued "greening" without disruptions to the hydrological cycle, soil temperature regimes, aridity, and numerous other factors that have significant impacts on global and regional plant life.

References:

[1] Yuan, W. et al. (2019) Increased atmospheric vapor pressure deficit reduces global vegetation growth, Science Advances, https://advances.sciencemag.org/content/5/8/eaax1396

[2] Fu, Z. et al. Atmospheric dryness reduces photosynthesis along a large range of soil water deficits. Nat. Commun. 13, 989 (2022). Article 

[3] Liu, L. et al. Soil moisture dominates dryness stress on ecosystem production globally. Nat. Commun. 11, 4892 (2020). Article 

[4] Sulman, B. N. et al. High atmospheric demand for water can limit forest carbon uptake and transpiration as severely as dry soil. Geophys. Res. Lett. 43, 9686–9695 (2016). Article

[5] Novick, K. A. et al. The increasing importance of atmospheric demand for ecosystem water and carbon fluxes. Nat. Clim. Change 6, 1023–1027 (2016). Article

[6] Chen, N., Zhang, Y., Yuan, F. et al. Warming-induced vapor pressure deficit suppression of vegetation growth diminished in northern peatlands. Nat Commun 14, 7885 (2023). https://doi.org/10.1038/s41467-023-42932-w