One of the oldest “arguments” against mainstream climate science is the claim that because plants “eat” carbon dioxide, a rise in atmospheric CO₂ would be good for plants, and therefor good for us. As with all good misinformation, there’s a grain of truth here. All things being equal, a plant will grow better with higher CO₂ levels. The main problem with this argument is that it ignores the other factors that help plants grow. Drought, flooding, and heatwaves all harm plant growth, and this was pretty well demonstrable a decade ago:

And in case all that wasn’t bad enough, it seems that a higher concentration of CO₂ is particularly good for poison ivy, both in terms of its growth, and its toxicity. It’s just one of the many irritating things about the current situation. There’s more to it than that, though. Even if we can protect our crops from heat and drought, it seems that higher CO₂ levels can result in less nutritious food for us plant-eaters:

For years, scientists have seen enhanced photosynthesis as one of the only possible bright sides of increasing levels of atmospheric carbon dioxide (CO₂) — since plants use carbon dioxide for photosynthesis, it is anticipated that higher levels of the gas will lead to more productive plants. In a review publishing in Trends in Plant Science on November 3, scientists from Institute for Plant Science of Montpellier in France explain why this effect may be less than expected because elevated levels of CO₂ make it difficult for plants to obtain minerals necessary to grow and provide nutritious food.

“There are many reports in the literature showing that the CO₂ levels expected at the end of the twenty-first century will lead to a lower concentration of nitrogen in most plants, mainly affecting the protein content in plant products,” says first author Alain Gojon, research director of France’s National Research Institute for Agriculture, Food and the Environment. “It is very important to understand why growing plants at elevated CO₂ has such a negative effect on the protein content of most staple crops and the future of food.”

Plants use photosynthesis to incorporate CO₂ into sugars that they derive their energy from. However, photosynthesis does not provide plants with the key minerals they need to grow. For most plants, these minerals, such as nitrogen, phosphorus, and iron, are picked up from the soil through their root systems. Nitrogen is particularly important as it is a key building block for the amino acids that plants use to make proteins.

A nitrogen deficiency not only means that a plant will have difficulty building its tissues, but also that it will provide less nutrition to humans. “What is clear is that the nutrient composition of the main crops used worldwide, such as rice and wheat, is negatively impacted by the elevation of CO₂. This will have a strong impact on food quality and global food security,” says corresponding author Antoine Martin, researcher of the French National Centre for Scientific Research.

“Two main nutrients that are essential for human nutrition may be affected by this phenomenon,” adds Gojon. “The first one is proteins built from nitrogen. In developing countries this can be a big issue, because many diets in these countries aren’t rich in proteins and plants grown at elevated CO₂ can have twenty to thirty percent less protein. The second one is iron. Iron deficiency already affects an estimated 2 billion people worldwide.”

Beyond global food systems, lowered mineral status of plants at increased atmospheric CO₂ levels may lead to a negative feedback loop for mitigating climate change. “The terrestrial carbon sink associated with enhanced photosynthesis may be limited if most of the vegetation is deficient in nitrogen and other minerals, which may prevent any additional increase of CO₂ capture from the atmosphere” says Gojon.

“We would like to really understand the mechanisms that are responsible for the negative effects of elevated CO₂ on the mineral composition of plants,” says Martin. “For example, we are currently exploring the natural genetic variation behind these negative effects, that could be used afterwards to improve crops nutritional value under future CO₂ atmosphere.”

This seems like something that could be mitigated with different farming practices, but it serves once again to demonstrate that carbon dioxide is not the only thing that can limit a plant’s wellbeing. Even if they didn’t result in ocean acidification and temperature rise, our emissions wouldn’t magically increase all other nutrients that plants need, and that has always been pretty obvious.

As ever, these arguments don’t exist to make a compelling case. They exist as weapons in a propaganda war, to be re-used for as long as possible, facts be damned. It’s good to keep doing this kind of research, but most of the climate denial we see in the world is not caused by a lack of information. Ideology and greed are doing far more harm than simple ignorance ever could.