Climate deniers can take a variety of approaches. They may deny the greenhouse effect altogether, the sources of excess CO2, or the impacts it will have. One particular refrain, “CO2 is just plant food”, crops up again and again across social media comments and denialist publications.
This catchphrase is a tactic that is based on an overly simplistic understanding of plants that a lot of people would have. From there, deniers encourage people to draw erroneous conclusions. It’s effective because the question of how plants (and other photosynthetic organisms) will respond to the increase in atmospheric carbon dioxide sounds easy enough to answer when using only that overly simplistic understanding. But from the level of molecular biology to global ecology, photosynthesis isn’t a very simple phenomenon.
Many of us learned about photosynthesis in primary school: plants breathe in carbon dioxide and absorb water, and then by using energy from sunlight they convert those into sugars before exhaling oxygen. It is therefore commonly assumed that any increase in carbon dioxide is inherently good for plants. It’s “plant food”, right?
As with everything in biology, it’s a tad more complex than what we learn as children. And that’s okay – it’s okay to have simplified models when we don’t need all of the details. But when it comes to global climate change we have to be more careful.
To begin with, there is nothing that a living organism needs that doesn’t become detrimental in sufficiently large quantities. For example, humans need water. We need it to live and that’s non-negotiable. And yet, if we drink far too much in a short span of time, we can get water poisoning and die. Too much of anything that we need to live and the buildup in our bodies will kill us.
There is an optimum amount of water intake between zero and death. And there is a saturation point: the point above which increasing intake has diminishing returns. You still have gains to be made, but once you’re above saturation you get less and less output from more and more input.
For land plants, the saturation point is around 1000 ppm of CO2, far higher than the current concentration in the atmosphere of around 420 ppm. But there are some caveats to bear in mind before we make conclusions.
First, we have to remember that photosynthesis is not one process. It’s an umbrella term for a larger number of chemical reactions, some sequential and some simultaneous, all occurring within multiple interacting components of a cell. The rate of production of sugars and oxygen (ie the rate of photosynthesis), is determined by the values of many different variables.
All of the chemicals involved are made of, and therefore depend on, more than just carbon, oxygen, and water. There are other nutrients essential to photosynthesis such as nitrogen and phosphorus. Micronutrients are necessary building blocks of important biochemical structures. CO2 increases won’t matter if plants are still limited by insufficient nutrient availability, for much of the world CO2 is not the limiting nutrient for plant growth.
On top of that, temperature affects the rates of chemical reactions, and the kinetic energy of molecules. More CO2 in the atmosphere means higher temperatures, which is not always congenial to plants, and will alter the water cycle creating another uncertainty for plant growth.
And then on top of that, plants have evolved multiple different pathways for performing photosynthesis, each with different sensitivities to CO2, temperature, and water availability.
And then on top of that, if we want to know how plants will respond only to rising atmospheric CO2, assuming everything else is ideal, we still face problems. Laboratory set-ups are often quite different from, and fail to properly replicate, real-world ecosystems. It’s one thing to take a whole leaf and zoom in on it, it’s another to successfully test an entire plant, an entire crop of plants, or a whole simulated and diverse ecosystem. The mid to late 2000s saw an experimental design called FACE (Free-Air CO2 Enrichment) which provided a more realistic setup of test plants that better replicated real world conditions. And these gave inconsistent results of photosynthesis and growth at higher CO2 than in prior experiments.
And on top of that, we haven’t even addressed what “growth” means. That’s a very complex term. Do we simply mean an increase in total mass of the plant? In which case, were ignoring the different chemicals and structures plants build during growth. When it comes to food crops, this is a very important distinction, because not every part of a plant is nutritious.
In a Scientific American article, environmental health scientist Samuel Myers explained, “We know unequivocally that when you grow food at elevated CO2 levels in fields, it becomes less nutritious,” adding that crops “lose significant amounts of iron and zinc—and grains [also] lose protein.” The scientific literature has shown this again and again.
And we still haven’t even gotten to how aquatic photosynthesis will be affected.
Ultimately, there is no one simple answer as to how plants will respond to rising CO2. It will depend on the particular species, the conditions of its environment, its sensitivity to the other effects of climate change, and even then more mass isn’t the same as healthier plant. The phrase “CO2 is plant food” is nothing more than a cop-out by deniers to shut down scientific discourse.