Is More CO2 Good for Plants?

How are plants likely to react as climate change intensifies, temperatures get hotter and levels of carbon dioxide (CO2) rise ever higher? Will plant growth increase? If so, will it strengthen or weaken the ability of plants to offset greenhouse gas emissions by increasing their rate of photosynthesis? Will excess CO2 threaten phytoplankton productivity in the ocean? These are difficult questions to which scientists, as yet, have no precise answers. We just don't know if more CO2 is good for Plants.
Rice Field Climate Change
Rising temperatures could slow down plant growth across much of the tropics. Photo: © Ali Mohammadi/imaggeo.egu.eu

Roughly 28 percent of all the carbon dioxide (CO2) emitted by the burning of fossil fuels, cement manufacture, and deforestation during the period 2002–2011, was absorbed by plants during photosynthesis. (About 26 percent went into the oceans and roughly 46 percent ended up in the atmosphere.) 1

This unique, natural form of carbon capture and storage, is a very efficient way to absorb greenhouse gas emissions, and one we need to safeguard if our climate crisis is to be brought under control.

The climate change denial movement says that CO2 is a key driver of plant growth, and that more CO2 will help plants to grow even more. Which means more food, more oxygen, but less CO2 in the atmosphere to drive the greenhouse effect.

Sounds good. Unfortunately, as we shall see, while rising CO2 may indeed provide benefits for crops, these benefits should not be weighed in isolation. Because the overall effects of climate change on plants may outweigh these benefits, and cause damaging effects to humans as well as to agriculture.

Is More CO2 Good for Plant Photosynthesis?

Yes. To begin with, a rise in CO2 typically leads to more photosynthesis, and more plant or crop growth. Tests show that a doubling of atmospheric CO2 levels – from today’s 414 parts per million (ppm) to around 800 ppm – does boost the carbohydrate productivity (photosynthesis) of crops and trees.

Wheat, corn, rice and soybeans, for instance, all benefit from this so-called ‘CO2 fertilization effect’. Wheat productivity has been shown to increase by about 11 percent and corn by about 8 percent. Studies also show that tree productivity goes up by more than 20 percent.

That said, evidence indicates that plants adjust to elevated carbon dioxide levels, so their effect on photosynthesis diminishes with time. 2 In fact, sometimes the more CO2 rises, the less and less benefit the plant receives.

More importantly, increasing the amount of CO2 in the air does not necessarily result in more photosynthesis or more growth. There are other factors that have a negative impact on productivity, and they too need to be taken into account.

Factor 1. Shortage of Nitrogen

Nitrogen is generally the major limiting factor for crop growth and agriculture productivity 3 4

Nitrogen acts as a brake on how much biomass a plant produces, and although commercial crops are well supplied with nitrogen fertilizer in the United States and other developed countries, numerous countries in Africa and Asia lack adequate supplies. Forests and other uncultivated vegetation are also affected.

According to one recent study, rising temperatures and CO2 levels, are causing a shortage of nitrogen. 5

Factor 2. Soil Moisture Deficit

Soil moisture deficit is another strong influence on photosynthesis productivity in plants. Numerous studies have shown that lack of water reduces photosynthesis and plant health. 6 7 See also: Why is Soil So Important to the Planet?

Factor 3. Extreme Heat

High temperature, whether temporary or long-term, causes physical, biochemical and molecular changes that reduce tree photosynthesis and growth. 8

Unfortunately, the frequency, duration, and severity of periods with particularly high temperatures are increasing in line with global temperature projections, which means that plants will experience more episodes of heat stress. (Note: heat stress events are defined by WMO as episodes of 5 or more continuous days with air temperatures over 5°C above daily maximum temperatures.) Heat stress is known to lead to decreased physiological performance and growth in a range of plant species. 9

Heatwaves are a critical threat to crop productivity. The productivity of grain plants, for example, relies on the success of sexual reproduction, which is very sensitive to heat stress. 10

This doesn’t mean that all heat is bad for plants. On the contrary, heat is often essential for increased crop growth and performance. However, some plants perform at their best only within certain temperature parameters. Once they are exceeded, their productivity, germination, reproduction and other physiological processes begin to decline.

Conclusion

As far as photosynthesis and plant growth is concerned, the amount of carbon dioxide in the air is just one of several relevant factors. As well as an abundance of CO2, plants also need nitrogen, moisture and optimum heat, to name but three essentials. So, as to the question ‘is more CO2 good for plants?’ the answer is ‘Yes, but it can’t ensure crop growth by itself’.

Is More CO2 Good for Plant Oxygen Production?

This question encounters similar difficulties to the one above. A plant can only produce more oxygen if it performs more photosynthesis and, as we have seen, photosynthesis is not just dependent on CO2. It also needs nitrogen, water and the right amount of heat.

But oxygen production is not the sole preserve of terrestrial plants. It is also produced by microscopic plants in the ocean. Known as phytoplankton, these tiny creatures generate at least half (maybe more) of the world’s oxygen as a by-product of their photosynthesis in the near-surface layer of the ocean. They also serve as the foundation of the marine food web, being preyed on by zooplankton, who in turn are devoured by krill, who are eaten by everyone from sardines to whales.

The point is, warmer ocean temperatures (caused by climate change) are causing a drop in both the numbers and productivity of phytoplankton. According to one study, numbers have plummeted by 40 percent over the past 60 years or so. 11

So, in answer to the question ‘is more CO2 good for plant oxygen production?’ the answer is No. More climate change-generated CO2 will only warm the oceans more and kill more phytoplankton in the process.

Is More CO2 Good for Food Production?

Once again, this question is based on a false assumption, namely that raised CO2 delivers more food, an assumption we now know is false. Even if it was true, it’s doubtful if it would result in more food.

It’s well documented, for example, that when food is grown at elevated CO2 levels, it becomes less nutritious. It loses significant amounts of zinc and iron, while grains also lose protein. 12

Other scientific studies have indicated that high temperatures may cause a severe drop in the production of certain staple crops, such as soybeans and corn, irrespective of CO2 levels. 13

These are just sample studies. The nutritional deficiency argument is well substantiated across the internet.

So, in answer to the question ‘is more CO2 good for food production?’ the answer is No. More CO2 is no guarantee of more plants, and food grown under conditions of high CO2 levels is known to be deficient in important nutrients.

Is Climate Change Good for Crops?

No. Although elevated CO2 can spur crop growth, it doesn’t guarantee anything. Besides, any benefits it offers are going to be overwhelmed by the massive damage it causes to the biosphere in general, and the local ecosystem in particular.

The effects of global warming are too numerous to catalogue here. Suffice it to say that the Australian wildfires along with their dreadful loss of biodiversity – plant life and animals – as well as the other extreme weather events that we hear about almost every day, are quite sufficient to remind us that anthropogenic CO2 is no friend of plants or the planet.

The effects of global warming on the oceans include a cascade of attacks on marine plant life. Examples include: marine heatwaves that decimate mangrove forests; ocean acidification that destabilizes photosynthetic kinetics due to increasing seawater pCO2 levels and lower pH; 14 a mix of ocean warming and acidification that destroys coral reefs, and also changes microbes on the surface of kelp plants, causing disease and possible risk to fisheries. 15

A more widespread danger to marine plant life, comes from a combination of sea level rise and storms affecting coastal plant communities, including dense mangrove forests (mangals), seagrass beds and kelp forests. These plant habitats must be preserved due to their enormous contribution to coastal protection. 16

References

  1. “How Much CO2 Can the Oceans Take Up? []
  2. “CO2 is making Earth greener – for now.” []
  3. “Early Diagnosis and Management of Nitrogen Deficiency in Plants Utilizing Raman Spectroscopy.” Chung Hao Huang, et al. Front. Plant Sci., 05 June 2020. []
  4. “Nitrogen uptake, assimilation and remobilization in plants: challenges for sustainable and productive agriculture.” Celine Masclaux-Daubresse, et al. Ann Bot. 2010 Jun; 105(7): 1141–1157. 2010 Mar 18. []
  5. “Isotopic evidence for oligotrophication of terrestrial ecosystems.” Joseph M. Craine, et al. Nature Ecology & Evolution, 2018; 2 (11): 1735. []
  6. “Responses of photosynthetic capacity to soil moisture gradient in perennial rhizome grass and perennial bunchgrass.” Zhenzhu Xu, Guangsheng Zhou. BMC Plant Biol. 2011; 11: 21. Published Jan 25, 2011. []
  7. “Leaf Photosynthesis Response to Change of Soil Moisture Content in Sugarcane.” Sugar Tech. June 2019. []
  8. “Effects of high temperature on photosynthesis and related gene expression in poplar.” Song, Y., Chen, Q., Ci, D. et al. BMC Plant Biol 14, 111 (2014). []
  9. “Timing Effects of Heat-Stress on Plant Ecophysiological Characteristics and Growth.” Dan Wang, et al. Front. Plant Sci. 02 November 2016. []
  10. “High temperature susceptibility of sexual reproduction in crop plants.” Neeta Lohani, Mohan B Singh, Prem L Bhalla. Journal of Experimental Botany, Volume 71, Issue 2, 7 January 2020, Pages 555–68. []
  11. “Global phytoplankton decline over the past century.” Boyce, D., Lewis, M. & Worm, B. Nature 466, 591–596 (2010). []
  12. “Increasing CO2 threatens human nutrition.” Myers, S., Zanobetti, A., Kloog, I. et al. Nature 510, 139–142 (2014) []
  13. “Consistent negative response of US crops to high temperatures in observations and crop models.” Schauberger, B., Archontoulis, S., Arneth, A. et al. Nat Commun 8, 13931 (2017). []
  14. “Response of Photosynthesis to Ocean Acidification.” Katherine R.M. Mackey, et al. October 2, 2015. []
  15. Future climate change is predicted to affect the microbiome and condition of habitat-forming kelp.” Zhiguang Qiu, et al. Proceedings of the Royal Society B: Biological Sciences, 2019 []
  16. “The gathering storm: optimizing management of coastal ecosystems in the face of a climate-driven threat.” Mick E Hanley, et al. Annals of Botany, 2020; 125 (2): 197 []
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