Increased CO2 acts as a fertilizer for most plants, boosting photosynthesis and accelerating growth. At today’s atmospheric concentration of roughly 424 ppm (up from about 280 ppm before industrialization), plants are already growing faster and using water more efficiently than they did two centuries ago. But the full picture is more complicated than “more CO2 equals more growth.” The benefits vary by plant type, come with nutritional tradeoffs, and can be capped or reversed by other environmental factors.
Why Plants Grow Faster With More CO2
Plants pull CO2 from the air and convert it into sugar through photosynthesis. The key enzyme in this process, called Rubisco, grabs CO2 molecules and uses them to build the carbon-based compounds that fuel plant growth. When there’s more CO2 in the air, the concentration gradient between the atmosphere and the inside of the leaf increases, pushing more CO2 into the plant. Rubisco works faster because it encounters CO2 molecules more frequently, and the overall rate of carbon fixation goes up.
There’s a second, equally important benefit. Rubisco is an imperfect enzyme. It sometimes grabs oxygen instead of CO2, triggering a wasteful process called photorespiration that burns through energy without producing useful sugars. When CO2 levels rise, Rubisco is more likely to grab CO2 and less likely to grab oxygen. This means the plant wastes less energy on photorespiration and channels more into actual growth.
C3 vs. C4 Plants: Not All Species Respond Equally
About 85% of plant species, including wheat, rice, soybeans, and most trees, use what’s called the C3 photosynthetic pathway. These plants benefit the most from rising CO2 because their Rubisco is directly exposed to atmospheric CO2 concentrations, and photorespiration is a significant drain on their energy budget. More CO2 addresses both problems at once.
C4 plants, which include corn, sugarcane, and many tropical grasses, already have a built-in mechanism that concentrates CO2 around Rubisco, effectively eliminating photorespiration. Because of this internal pump, C4 plants were long assumed to be unaffected by rising CO2. That assumption turns out to be too simple. USDA research has shown that several C4 species still respond to elevated CO2, with some C4 weeds increasing biomass by up to 25%. Weeds as a group showed roughly twice the photosynthetic boost (+19%) compared to crop species (+10%). Still, the overall response of C4 plants is smaller than that of C3 plants.
Crop Yields in Real-World Conditions
The most reliable data on how crops respond to elevated CO2 comes from Free-Air CO2 Enrichment (FACE) experiments, where crops grow in open fields with CO2 pumped to roughly 550 to 580 ppm. These experiments show meaningful but moderate yield gains. Meta-analyses of FACE data found wheat grain yields increased by about 14%, rice yields by 12 to 16%, and a combined analysis of rice, wheat, and soybean showed an average yield increase of 14% when CO2 was raised from about 370 ppm to around 580 ppm.
These numbers are consistently lower than what laboratory and greenhouse experiments predict. Controlled environments tend to overestimate yield gains by 20 to 50% compared to FACE results, likely because real fields impose constraints like variable weather, pest pressure, and uneven soil nutrients that don’t exist in a growth chamber.
Plants Use Water More Efficiently
One of the most significant effects of rising CO2 is that plants partially close the tiny pores on their leaves, called stomata, through which they both absorb CO2 and lose water. When CO2 is abundant, plants don’t need to open their stomata as wide to get the carbon they need. A study of nine common plant species in Florida reconstructed a 34% reduction in maximum stomatal conductance for every 100 ppm increase in CO2 over the past 150 years.
The result is a substantial improvement in water use efficiency: plants take in more carbon while losing less water. This makes vegetation more drought-resistant and could benefit agriculture in water-limited regions. It also means plants need less investment in the internal plumbing that transports water through their leaves.
Bigger Plants, Less Nutritious Food
Here’s where the story gets less optimistic. Plants grown under elevated CO2 tend to produce larger yields with lower nutritional quality. Wheat grain protein drops by about 7.4% under high CO2, driven by reductions in several essential amino acids. The mineral content of staple grains also declines. In rice, iron fell by 5.2%, zinc by 3.3%, and copper by 10.6%. In maize, iron dropped by 5.8%, zinc by 5.2%, and copper by 9.9%.
The underlying mechanism connects back to plant chemistry. Faster growth under elevated CO2 dilutes nitrogen within plant tissues because the plant is building more carbon-based structure without proportionally increasing its nitrogen uptake. Since nitrogen is the backbone of protein and is involved in mineral absorption, the plant essentially becomes starchier and less protein-rich. For the billions of people who depend on rice and wheat as primary protein sources, this dilution effect is a genuine food security concern.
More Carbon Goes Underground
Elevated CO2 doesn’t just make shoots grow faster. The extra carbohydrates produced by ramped-up photosynthesis flow down to the root system, increasing root growth and the amount of carbon that roots release into the surrounding soil. This shift in carbon allocation can stimulate soil microbes and alter nutrient cycling belowground. However, the increase in root carbohydrate delivery doesn’t scale proportionally with the increase in leaf-level sugar production, meaning roots get more carbon but not as much as you might expect from the boost in photosynthesis.
Over longer periods, the relationship between roots and shoots adjusts. Plants growing in nutrient-poor soil tend to redirect more growth toward roots in an attempt to scavenge nitrogen and phosphorus, which become increasingly limiting as the plant tries to keep pace with its CO2-fueled growth.
Nitrogen and Phosphorus Set the Ceiling
The CO2 fertilization effect has a hard limit: soil nutrients. A plant can only convert extra CO2 into growth if it also has enough nitrogen and phosphorus to build proteins, DNA, and cell membranes. In many ecosystems, these nutrients are already scarce, and adding more CO2 without more nutrients is like pressing the gas pedal with the parking brake on.
Modeling research shows that plants partially compensate by reallocating resources toward fine-root growth and ramping up biological processes that pull more nitrogen and phosphorus from the soil, including nitrogen fixation by soil bacteria and the release of enzymes that free up bound phosphorus. But these adaptations only go so far. Long-term experiments consistently show that the initial growth surge from elevated CO2 fades over years as nutrient limitations take hold.
Rising Temperatures Can Cancel the Benefits
CO2 doesn’t rise in isolation. It comes packaged with warming, and heat stress works against the very yield gains that CO2 provides. A meta-regression covering wheat, rice, soybean, and maize found that warming reduces cereal yields by 7.6% per degree Celsius for wheat and 9.5% per degree for rice and maize. When researchers modeled mid-century and late-century climate projections (higher CO2 paired with higher temperatures), the CO2 fertilization benefit on yields was greatly reduced or entirely negated for maize, rice, and wheat. Soybean was the exception, but only because experimental warming treatments hadn’t exceeded the crop’s optimal temperature range.
Changes in Plant Defense Chemistry
Elevated CO2 also alters the production of defensive compounds in plants. Research on oil palm seedlings found that tripling CO2 from 400 to 1,200 ppm increased flavonoid production in leaves by 132% and phenolic compounds by up to 91%. These compounds play roles in protecting plants from UV damage, pathogens, and herbivores. The boost appears to be driven by the same nitrogen dilution that reduces protein content: as nitrogen becomes scarcer within the plant, the amino acid phenylalanine gets redirected away from protein synthesis and toward building these carbon-rich defensive molecules. The practical implications vary by species, but for crops valued for their antioxidant content, elevated CO2 could increase the concentration of certain health-promoting compounds even as it decreases protein and minerals.