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By Vivek Saini
Claim: Farmers pump CO2 into greenhouses as plants love it so much. Higher atmospheric CO2 levels would also benefit reforestation by enhancing plant growth and creating more resilient microclimates.
Fact: Half-truth. In unmanaged forests, fields, and other ecosystems, plant growth is influenced by various other factors, as opposed to the results scientists obtain in the laboratory, which are typically not representative of what occurs in the immensely more complicated real world.
Claim post:
What does the post say
Dr Simon Goddek, in his viral tweet dated 17 July ’23, stated that high atmospheric CO2 levels would benefit reforestation by enhancing plant growth and creating more resilient microclimates. He questions the narrative that CO2 harms our planet because plants love CO2 so much that farmers pump it into greenhouses.
What we found
The claim made by the science journalist, Dr Goddek, is merely a cherry-picking of the process called CO2 fertilization. Although plants require carbon dioxide to flourish, they may not thrive in situations with high levels of carbon. Different plants react differently to increased carbon. Additionally, CO2 is not the sole component that affects growth in the plant kingdom, despite the carbon enthusiasts there. As anyone interested in gardening knows, plants require the proper ratio of water and soil nutrients to convert excess carbon dioxide into growth.
Given the way our climate is changing, this is an issue. Droughts in areas like the American West get worse due to climate change brought on by too much CO2 in the atmosphere. That decreases the water supply for the plants there while also raising the danger of disastrous wildfires. Other locations will see a rise in pests that prefer warmer winters, as well as more frequent calamities like flooding and heat stress, exposure to saltwater from rising seas, and other environmental challenges for plants. And although the concept of planting millions more trees has been floated frequently to remove some CO2 from the atmosphere, it is unclear if the planet would have enough nutrients in the soil to support such growth.
What is the CO2 fertilization effect?
Photosynthesis is a fundamental concept in biology: plants use sunlight to convert water and CO2 into chemically bound energy and oxygen. This process allows plants to synthesize the chemical building blocks required for growth while providing oxygen and food for animals and humans. While sunlight is abundant, water, atmospheric CO2, and other necessary input factors are limited, limiting the scale at which photosynthesis can occur.
As previously stated, CO2 can be a limiting factor in the photosynthesis process. Thus, if more CO2 becomes available – and the other input factors are abundant – the photosynthesis process would be scaled up, increasing the amount of atmospheric CO2 bound by plants. The result would be a slower increase in CO2 concentrations in the atmosphere than expected based solely on global CO2 emissions. Furthermore, higher CO2 levels in the atmosphere have been shown to increase water use efficiency, allowing plants to survive in drier conditions. As a result of these two processes, the biosphere absorbs more CO2 and thus increases the land carbon stock. This phenomenon is known as the CO2 fertilization effect.
CO2 boosts plant productivity, but other factors also count
Photosynthesis occurs when plants combine sunlight, atmospheric carbon dioxide, and water to create oxygen and carbohydrates for growth and energy. The carbon fertilization effect results from increased photosynthesis caused by rising CO2 levels in the atmosphere. According to a recent study, global plant photosynthesis increased by 12 per cent between 1982 and 2020, paralleling a 17 per cent increase in atmospheric CO2 levels. This increase in photosynthesis was mainly brought on by fertilizing with carbon dioxide.
Plants utilise less water for photosynthesis when there is more CO2 in the air. Stomata, which are holes in plants, allow for the absorption of CO2 and moisture release into the atmosphere. Plants can sustain a high rate of photosynthesis and partially cover their stomata when CO2 levels rise, reducing a plant’s water loss by 5 to 20%. Scientists have hypothesized that this would cause plants to retain more water on land, soil, and in streams by releasing less water into the atmosphere.
The study discovered that the nitrogen-fixation temperature response is separate from the photosynthesis temperature response, which uses nitrogen-containing enzymes. These enzymes may function less effectively at higher temperatures. Rubisco is the primary enzyme that converts carbon dioxide from the atmosphere into carbohydrates during photosynthesis. Still, when temperatures rise, it “relaxes” and loses precision in the form of the CO2 pocket it retains. As a result, the enzyme mistakenly fixes oxygen instead of carbon dioxide 1/5 of the time, decreasing photosynthetic efficiency and wasting the plant’s resources. Rubisco can become entirely inactive at even higher temperatures.
Current trends and projections on the CO2 fertilization effect
Whether photosynthesis is increased due to increased atmospheric CO2 levels or weakened due to higher temperatures – and, for that matter, the general development of the land carbon sink – is a question that depends on a plethora of interactions that science is attempting to answer using observations, field experiments, and computer models. The fertilization effect appears to be still at work (between 2000 and 2009, approximately 14 gigatonnes of CO2 equivalent were additionally absorbed per year compared to the pre-industrial area (1750), albeit at a slower rate: the rate at which photosynthesis is enhanced is declining in comparison to the rate at which CO2 is released into the atmosphere. The photosynthetic process will be slower in the future, according to predictions, as we go from a fertile to a warming-dominated time. In this scenario, plants cease to operate as a sink for CO2, reducing global warming, and instead become a source of CO2 that causes temperatures to rise more quickly.
While some agricultural yields may improve, the amount of critical nutrients in crops is impacted by rising CO2 levels. According to one study, the protein content of potato tubers, wheat, rice, and barley grains fell by 10 to 15 per cent with increased CO2. Essential minerals, including calcium, magnesium, phosphorus, iron, and zinc, are also lost by crops. According to a study done on different rice varieties in 2018, elevated CO2 levels enhanced vitamin E while lowering vitamins B1, B2, B5, and B9.
Along with increasingly frequent droughts and heat waves, climate change is predicted to harm the carbon fertilization effect. While agricultural yields typically decline during hot growing seasons, the interaction of heat and dryness may result in maize yields reducing by 20% in some US regions and by 40% in Eastern Europe and southeast Africa. In areas where crop yields are expected to rise due to warmer temperatures, such as the northern US, Canada, and Ukraine, the combination of heat and water constraints may also lower crop yields.
References:
- https://climate.mit.edu/explainers/freshwater-and-climate-change
- https://climate.mit.edu/explainers/wildfires
- https://climate.mit.edu/explainers/extreme-heat
- https://climate.mit.edu/explainers/sea-level-rise
- https://climate.mit.edu/ask-mit/why-dont-we-just-plant-lot-trees
- https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.16866
- https://doi.org/10.1016/j.tplants.2019.04.003
- https://www.researchgate.net/publication/224942969_Trends_in_the_sources_and_sinks_of_carbon_dioxide
- https://www.ipcc.ch/report/sixth-assessment-report-working-group-i/
- https://scitechdaily.com/new-research-shows-plants-are-photosynthesizing-more-in-response-to-more-co2-in-the-atmosphere
- https://onlinelibrary.wiley.com/doi/10.1111/j.1365-2486.2006.01240.x
- https://www.nature.com/articles/s41477-021-01090-x
- https://doi.org/10.1038/s41559-017-0274-8
- https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapter06_FINAL.pdf
- http://hdl.handle.net/10871/124090
- https://onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2486.2007.01511.x
- https://www.science.org/doi/10.1126/sciadv.aaq1012
- https://news.climate.columbia.edu/2021/09/20/increased-heat-drought-combinations-could-damage-crops-globally-says-study/
