“Plants need CO2” was once a popular slogan for climate skeptics, meaning people who refused to believe that higher atmospheric carbon dioxide concentrations would lead to higher average global temperatures. Those climate change skeptics were denying the laws of physics, but they were right about one thing; plants rely on CO2 to survive just as humans rely on oxygen. However, too much of a good thing can be a bad thing. We need water, but drinking too much of it in one sitting can kill a person by diluting salt levels in the bloodstream. Somewhat similarly, science is discovering that CO2 saturation appears to be diluting the nutritional value of the foods we eat.

The battle for better food security is fought on two fronts. First, the world needs more food to feed a rising human population. The United Nations says the world is home to more than 8 billion people and could see a population of 9 billion by 2050. That means we could see a 12.5% increase in base food demand before accounting for rising consumption through higher incomes. Second, food security means ensuring people have access to healthy and nutritious food, not just more of it.

Climate change threatens food supplies; droughts and floods destroy crops. Now, scientists are increasingly concerned that increasing levels of greenhouse gasses threaten nutrition, as well. The conversation on what to do about it has only just begun.

Crowding out nutrition

Too much water in a person’s system dilutes sodium levels in the blood, and salt is critical to human survival. It’s not a perfect analogy, but research has found that something similar is happening with plants.

Rising CO2 in the atmosphere seems to be diluting the concentrations of nutrients—vitamins and minerals—in plant tissues, including the edible portions of some common crops. Sometimes these nutrients are essential for plant health, and sometimes they are not, but in most cases, the nutrients we are losing are essential for human health. Lithium is but one example—researchers say plants don’t necessarily need lithium to survive, but people do.

How do rising CO2 levels in the atmosphere lower nutrient levels in plants? The academic literature written on this topic so far agrees that one important way CO2 is weakening the nutritional value of crops is by simply crowding nutrients out of plants’ systems.

A more CO2-saturated atmosphere is improving the efficiency of plant photosynthesis. Normally, this could be interpreted as a good thing—more CO2 equals more crop biomass and thus more food. But there’s a price to pay—if crops have more ready access to carbon to grow their bodies, then they don’t necessarily need to rely on other inputs as much. Thus, they take in fewer nutrients that are beneficial to human health.

The result is a crop that’s as big as any grown under pre-industrial atmospheric conditions, perhaps even bigger. But gram for gram, that crop is less nutritious, especially for what plant scientists call C3 crops, common staples like rice, wheat, potatoes, barley, and more. Scientists fear the world’s food supply is already losing its nutritional value due to more efficient photosynthesis and higher intakes of carbon in plant tissues.

“Experiments exposing wheat, rice, and other C3 plants to concentrations of CO2 expected later this century show declines of about 10% in protein, 5%–10% in iron and zinc (and potentially other micronutrients), and up to an average of 30% in individual B vitamins,” as Ebi et al. wrote in a recent publication. More troubling, they point to signs that these nutritional declines are already happening in the field, and not only under laboratory conditions. “There is limited evidence suggesting that declines in nutrition may have already occurred in response to recent CO2 increases.”

The threat is real. Iron deficiency and zinc deficiency are already massive problems in developing countries. These nutrients are important for brain function, as is vitamin B. Pregnant women and children especially need these nutrients to ensure healthy families and a healthy future population.

Getting ahead of the threat

C3 plants are plants that metabolize carbon through photosynthesis in a particular way. The University of Illinois RIPE project (Realizing Increased Photosynthetic Efficiency) says the C3 process is the most common way plants absorb carbon. “The majority of plant species on Earth uses C3 photosynthesis, in which the first carbon compound produced contains three carbon atoms,” the RIPE project researchers explained.

More research is needed, but studies on the effect of rising CO2 concentrations on plant nutrition suggest that C3 crops are the most susceptible to this carbon “crowding out” effect.

Many agricultural scientists argue that one way for smallholder farmers to get ahead of climate change threats is to introduce more crop diversity to their fields. They argue that, by planting a wider variety of crops,  farmers can hedge their bets against droughts, pests, and diseases. Similarly, some scientists argue that governments should promote the cultivation of C4 crops to manage the risk that CO2 poses to nutrition in C3 crops.

The problem is, there aren’t that many C4 crops to choose from. “There are currently only five economically important C4 food crops maize, sorghum, sugar cane, onion, and pearl millet,” as the research team Jobe et al. explained in a recent study.