How Does Carbon Dioxide Levels Affect Photosynthesis?

Rising carbon dioxide directly fuels photosynthesis in most land plants, acting like a richer supply of raw material for the chemical reactions that convert light energy into sugar. The boost is real but far from unlimited. Over time, plants adjust to higher CO2 in ways that blunt the initial gains, and shortages of water, nitrogen, or phosphorus can cap the benefit well before the chemistry runs out of room. The relationship between CO2 and photosynthesis is one of the most studied questions in plant biology, and the answer keeps getting more layered as researchers track what happens not just in lab chambers but across decades of field experiments and satellite observations.

Why More CO2 Speeds Up Photosynthesis

The central enzyme in photosynthesis, called Rubisco, grabs CO2 molecules and attaches them to a sugar molecule to kick off the chain of reactions that builds plant tissue. Rubisco is famously slow and imprecise. It works below its optimal CO2 concentration under current atmospheric conditions and regularly grabs oxygen molecules by mistake, wasting energy in a process called photorespiration. When the air around a leaf contains more CO2, Rubisco picks up CO2 more often and oxygen less often, so photosynthesis speeds up and photorespiration slows down.¹ At low CO2, when the pool of dissolved carbon inside the leaf runs thin, Rubisco leans even harder on oxygen, essentially spinning its wheels.² Raising CO2 relieves that bottleneck directly.

Not All Plants Respond the Same Way

Plants fall into a few broad photosynthetic categories, and those categories predict how much extra CO2 helps.

C3 Plants Get the Biggest Boost

Most of the world’s crops and forest trees are C3 plants, meaning CO2 goes straight to Rubisco without any preliminary concentration step. Because Rubisco in C3 species is perpetually undersaturated with CO2, these plants show a strong photosynthetic response when CO2 rises. When researchers doubled CO2 in experiments with C3 species like wheat and kale, photosynthesis increased markedly, while C4 species grown alongside them showed no comparable jump.³

C4 Plants Respond Less, but the Picture Is Complicated

C4 plants, including maize, sugarcane, and many tropical grasses, have a built-in carbon-concentrating pump that already feeds Rubisco a rich supply of CO2. Textbooks long treated them as nearly unresponsive to rising atmospheric CO2. That turns out to be an oversimplification. A meta-analysis of wild C3 and C4 grass species found that C4 grasses still increased total biomass by about a third under elevated CO2, with higher carbon assimilation rates of roughly 25%, compared with about 33% for C3 grasses. The authors concluded that the assumption of low C4 responsiveness needed to be re-evaluated.⁴ Even more striking, a 20-year grassland experiment found that the expected pattern reversed partway through: during the first 12 years, C3 plots benefited most from elevated CO2, but during the following 8 years, C4 plots overtook them.⁵ Part of the C4 advantage likely comes through water savings rather than direct photosynthetic stimulation.

CAM Plants Have Their Own Surprises

Succulents and other plants that use crassulacean acid metabolism (CAM) take in CO2 mainly at night through a different enzyme, PEPCase, which was assumed to be saturated at current CO2 levels. Yet multiple CAM species show increased nighttime CO2 uptake under elevated CO2, with one cactus species nearly doubling its nocturnal uptake rate when CO2 was doubled.⁶ The increases come from both the nighttime PEPCase pathway and the daytime Rubisco pathway, meaning both halves of the CAM cycle respond.⁷

Stomata, Water, and the Efficiency Tradeoff

When leaves sense higher CO2 in the air, they partially close their stomata, the tiny pores that let CO2 in and water vapor out. This conserves water while still allowing plenty of CO2 to reach Rubisco. A reconstruction of nine Florida plant species over the past 150 years found that maximum stomatal conductance dropped by about a third for every 100 ppm increase in atmospheric CO2.⁸ Forest-level measurements tell a similar story: water-use efficiency has been climbing, with trees apparently keeping the CO2 concentration inside their leaves roughly stable even as atmospheric levels rise. The observed increase in forest water-use efficiency turned out to be larger than most models predicted, and it tracked alongside rising ecosystem photosynthesis and net carbon uptake.⁹

This water-saving effect has practical consequences. Under drought, elevated CO2 can partially cushion plants against water stress by reducing stomatal opening and thereby slowing water loss. A global meta-analysis found that elevated CO2 did not conserve soil water per se, but it improved leaf water status under drought, keeping leaves more hydrated because less water was escaping through the stomata.¹⁰

Why the Initial Boost Fades Over Time

Plants exposed to elevated CO2 for weeks or months often show a gradual decline in photosynthetic capacity, a phenomenon researchers call acclimation or downregulation. The leading explanation involves sugar buildup. When photosynthesis outpaces the plant’s ability to use or export its sugars, carbohydrates accumulate in the leaves. That excess sugar appears to trigger molecular signals that reduce the production of Rubisco itself, meaning the plant dials back the very engine that was running faster.¹¹ Limitations on the plant’s ability to regenerate the molecule Rubisco works on (RuBP) also contribute to the slowdown.¹² So while elevated CO2 can push photosynthesis higher in the short term, many plants adjust internally and partially erase the gain.

Nutrients Put a Hard Ceiling on CO2 Benefits

Extra CO2 gives plants more carbon, but plants also need nitrogen, phosphorus, potassium, and other nutrients to grow. If those are in short supply, the additional carbon cannot be converted into new tissue. A global analysis found that the strength of CO2 fertilization is primarily driven by nitrogen in roughly 65% of the world’s vegetation and by phosphorus in about 25%, with the balance modulated by the type of fungal partnerships the plant roots maintain.¹³ In tropical forests, where phosphorus is often scarce, the CO2 fertilization effect on vegetation carbon was roughly 50% to 70% lower in models and experiments that accounted for nutrient limitation compared to those that did not.¹⁴ This means many Earth System models may be overestimating how much extra carbon tropical forests will absorb in the coming decades.

Elevated CO2 as a Buffer Against Heat and Drought Stress

One of the more practically important findings is that elevated CO2 can partially offset the damage caused by heat and drought. In tomato plants subjected to high temperatures, elevated CO2 improved photosynthetic rates and helped the photosynthetic machinery recover from heat damage.¹⁵ In maize, a C4 crop where direct photosynthetic stimulation from CO2 is modest, combining elevated temperature with elevated CO2 helped plants recover photosynthetic rates and improved water-use efficiency compared to elevated temperature alone.¹⁶

Drought experiments show a consistent pattern. In soybean under water stress, elevated CO2 increased the thickness of water-conserving cell layers, stimulated production of protective surface waxes, and maintained higher photosynthetic rates and Rubisco activity than ambient CO2.¹⁷ In Datura stramonium, photosynthesis under drought at ambient CO2 collapsed to near zero within eight days, while plants at elevated CO2 maintained measurable photosynthesis throughout the same period.¹⁸ These buffering effects suggest that in a future with both higher CO2 and more frequent droughts, some of the drought damage may be cushioned, though not eliminated.

What Happens to Crop Yields in Real Fields

Laboratory and greenhouse studies tend to paint an optimistic picture, but field experiments tell a more cautious story. Free-air CO2 enrichment (FACE) experiments, which raise CO2 levels in open fields rather than in enclosed chambers, consistently show smaller yield gains than controlled-environment studies. An early comparison found that elevated CO2 enhanced crop yields about 50% less in FACE trials than in chamber studies.¹⁹ A later review of 30 years of FACE data covering 186 studies of 18 C3 crops found that raising CO2 by about 200 ppm boosted yields by roughly 18% under non-stress conditions. Legumes and root crops did better, cereals did worse. And when nitrogen was deficient, the average gain shrank to about 10%. Warming by a couple of degrees Celsius cut the benefit by a similar margin.²⁰

Part of the gap between chamber and field results comes from the CO2 delivery itself. In FACE experiments, wind constantly disrupts the gas plume, creating CO2 fluctuations more than ten times larger than what plants experience in nature. Research shows that photosynthetic gains under oscillating elevated CO2 were only about 65% as large as gains under a constant elevated concentration with the same average level.²¹ A wheat FACE trial in Australia recorded yield increases of about 26% when daytime CO2 was raised from 365 to 550 ppm, on the higher end of FACE results and closer to chamber estimates.²² Still, even optimistic field results land well below the dramatic doublings sometimes implied in popular discussion.

More Growth, Less Nutrition

A troubling side effect of CO2 fertilization is that it can dilute the nutritional quality of food crops. C3 grains and legumes grown at mid-century CO2 levels showed lower concentrations of zinc and iron, and C3 crops other than legumes also had lower protein concentrations. C4 crops appeared less affected.²³ Rice specifically showed declines not only in protein, iron, and zinc but also in several B vitamins, while vitamin E went up.²⁴ Because billions of people depend on rice and wheat for the bulk of their nutrition, even modest percentage declines in micronutrient density could translate into large increases in deficiency-related disease, especially in low-income countries where diets are less diverse.

Global Greening and Its Limits

Satellite data over the past few decades show that much of the planet has gotten greener, and CO2 fertilization is one of the main drivers. On the Tibetan Plateau, nearly a quarter of the landscape showed a significant positive relationship between greenness and CO2 concentration, making the CO2 fertilization effect more widespread there than the effects of temperature or precipitation changes.²⁵ In tropical and subtropical drylands, the greening trend appears consistent with improved water-use efficiency under rising CO2: over time, a given amount of soil moisture has sustained more leaf area.²⁶

But this trend is weakening. An analysis spanning 1982 to 2015 showed that global CO2 fertilization effects on vegetation photosynthesis declined across most terrestrial regions over that period, correlating with changing nutrient availability and soil water. Current carbon cycle models captured a declining trend too, but a substantially weaker one than the satellite observations showed.²⁷ The implication is sobering: the land carbon sink that has been absorbing a chunk of humanity’s CO2 emissions may be losing strength faster than models predict.

What Happens Underwater

Rising CO2 does not just affect land plants. When atmospheric CO2 dissolves in the ocean, it increases the amount of dissolved carbon available to marine photosynthesizers, but it also lowers pH (ocean acidification). The net effect on photosynthesis varies wildly by species. Many marine algae and cyanobacteria have carbon-concentrating mechanisms that already saturate their photosynthetic enzymes with CO2, so adding more to the water does not help much. A broad review across marine photosynthetic organisms found that responses to ocean acidification were relatively small for most species and highly variable, possibly because any photosynthetic benefit is offset by the energy costs of coping with lower pH.²⁸

In nutrient-poor tropical and subtropical oceans, the picture may actually tilt negative. Experiments in the North Pacific and the South China Sea found consistent reductions in community primary production under acidified conditions, driven largely by small eukaryotic phytoplankton that are already nitrogen-limited. Prokaryotic phytoplankton showed little change or even positive responses, but they could not compensate for the decline in eukaryotic productivity.²⁹ In nutrient-rich coastal waters, the story can reverse: a mesocosm experiment in eutrophic southern Chinese waters found that higher CO2 led to significantly greater phytoplankton biomass and organic carbon production.³⁰ As on land, nutrient availability is what separates a CO2 benefit from a CO2 non-event.

How Elevated CO2 Changes Weed Competition and Plant Disease

The differential response between C3 and C4 plants has implications for agriculture beyond simple yield numbers. Many of the world’s worst crop weeds are C3 species, and they stand to benefit more from rising CO2 than C4 crops like maize and sorghum. Research has shown that the C3 weed lambsquarters responded to higher CO2 with increased photosynthesis and reduced stomatal conductance, while the C4 weed pigweed showed no such changes.³¹ In fields where C3 weeds compete with C4 crops, rising CO2 could tip the competitive balance toward the weeds.

Plant disease is another complication. Elevated CO2 tends to boost the salicylic acid defense pathway (effective against certain biotrophic pathogens) while suppressing the jasmonic acid pathway (important against necrotrophic fungal pathogens). The practical upshot: some fungal diseases may become less severe under high CO2, while others get worse.³² A review of the broader literature concluded that the interaction between CO2 and plant immunity is specific to each plant-pathogen combination and cannot be captured by a single prediction.³³

A Hidden Effect on Plant Breathing

Photosynthesis gets most of the attention, but elevated CO2 also affects the opposite process: mitochondrial respiration, the way plants burn sugars for energy at night and in non-green tissues. There is strong evidence that doubling CO2 directly inhibits certain respiratory enzymes, including cytochrome c oxidase and succinate dehydrogenase.³⁴ This inhibition has two layers: an immediate, reversible suppression seen in leaves, stems, roots, and even soil microbes, and a longer-term acclimation effect specific to C3 species that appears after extended growth in high CO2.³⁵ Reduced respiration means less carbon is released back to the atmosphere at night, which could slightly amplify the net carbon gain from higher photosynthesis during the day. But it also means the plant may produce less of the metabolic energy it needs for growth and maintenance, so the consequences are not straightforwardly positive.

How Rubisco Evolved Under Changing CO2

The relationship between CO2 and photosynthesis has been shaped by billions of years of atmospheric change. Rubisco evolved when atmospheric CO2 levels were far higher than today and oxygen was scarce, so its tendency to grab oxygen was not originally a problem. As CO2 declined over geological time, organisms adapted. Analysis of Rubisco genes in marine algae revealed bursts of adaptive evolution timed to periods of falling atmospheric CO2, including the emergence of carbon-concentrating mechanisms in ecologically important groups like diatoms and coccolithophores.³⁶ Across the tree of life, the wide variability in Rubisco’s speed, specificity, and CO2 affinity reflects a long evolutionary tuning process driven by whatever CO2 and oxygen levels organisms encountered.³⁷ Today’s rapid rise in atmospheric CO2 is essentially rewinding one of those evolutionary pressures, giving C3 plants a taste of the higher-CO2 world their enzyme was originally built for, while organisms with carbon-concentrating mechanisms find they already have more CO2 than they can use.