Carbon dioxide is the gas removed from the atmosphere during photosynthesis. Plants, algae, and certain bacteria pull CO2 from the air and convert it into sugars and other organic compounds, releasing oxygen as a byproduct. Globally, forests alone absorb nearly 16 billion metric tons of carbon dioxide per year, and ocean-dwelling phytoplankton fix another 30 to 50 billion metric tons of carbon annually. Despite this enormous uptake, atmospheric CO2 reached a record average of 422.8 parts per million in 2024.
How CO2 Enters the Leaf
Leaves are covered in tiny pores called stomata, each flanked by a pair of guard cells that open and close in response to environmental conditions. When stomata open, CO2 from the surrounding air diffuses into the leaf’s interior while water vapor exits through the same pores. This simple exchange is the entry point for all land-based photosynthesis.
Light intensity is the primary trigger for stomatal opening. As light increases, stomata widen, allowing more CO2 in. Under complete darkness, stomata stay mostly closed and photosynthesis effectively stops. The relationship isn’t linear forever, though. At first, brighter light causes a rapid increase in CO2 uptake, but the rate eventually plateaus as other factors become limiting.
What Happens to CO2 Inside the Plant
Once inside the leaf, CO2 reaches the chloroplasts, the cell structures where photosynthesis takes place. The process has two major stages. In the first, light energy from the sun splits water molecules, releasing oxygen and generating the energy carriers the plant needs for the next step.
In the second stage, an enzyme called Rubisco grabs CO2 molecules and attaches them to an existing sugar molecule, splitting it into smaller compounds the plant can then rebuild into glucose. More than 90% of all inorganic carbon converted into living matter on Earth passes through this single enzyme. Rubisco is, by some measures, the most abundant protein on the planet, precisely because every photosynthesizing organism depends on it.
Three Strategies Plants Use to Capture CO2
Not all plants handle CO2 the same way. The differences matter because they determine how efficiently a plant removes carbon from the atmosphere, especially in hot or dry conditions.
C3 plants (rice, wheat, potatoes) use the simplest approach: CO2 enters the leaf and goes straight to Rubisco. This works well in cool, moist climates but becomes inefficient in heat because Rubisco sometimes grabs oxygen instead of CO2, wasting energy in a process called photorespiration.
C4 plants (maize, sorghum, sugarcane) evolved a workaround. They first capture CO2 using a different, faster enzyme, then shuttle it to specialized inner cells where Rubisco operates in a high-CO2 environment. This concentration mechanism suppresses the oxygen-grabbing mistake and gives C4 plants roughly 50% higher photosynthetic efficiency than C3 plants, along with better water and nitrogen use.
CAM plants (cacti, succulents, pineapples) take a time-based approach. They open their stomata at night to collect CO2, store it as an acid, then release it internally during the day for photosynthesis while keeping stomata closed. This minimizes water loss in arid environments.
Plants Also Release CO2
Photosynthesis removes CO2, but plants also produce it. Like all living organisms, plants respire around the clock, breaking down sugars for energy and releasing carbon dioxide in the process. Between 20% and 80% of the carbon a plant fixes through photosynthesis gets released again through its own respiration. The wide range reflects differences between species, growth stages, and environmental conditions.
What matters for the atmosphere is the net balance. During active growing seasons, healthy plants fix far more carbon than they release, creating a net carbon sink. This is why atmospheric CO2 measurements show a seasonal sawtooth pattern: concentrations dip each Northern Hemisphere summer as forests and crops photosynthesize intensely, then rise again in winter when growth slows and decomposition continues.
The Ocean’s Invisible Contribution
Land plants get most of the attention, but microscopic ocean organisms called phytoplankton are responsible for roughly 40% of all carbon fixation on Earth. These single-celled organisms collectively fix 30 to 50 billion metric tons of carbon per year despite making up only 1 to 2% of total global plant biomass. Their small size, rapid reproduction, and access to dissolved CO2 in seawater make them extraordinarily productive per unit of biomass.
There’s a catch, however. Without a continuous supply of nutrients from outside the ocean (river runoff, upwelling from deep water, atmospheric dust), open-ocean phytoplankton photosynthesis does not function as a significant long-term sink for excess atmospheric CO2. The carbon they fix tends to cycle back to the atmosphere relatively quickly as organisms die and decompose.
Why More CO2 Doesn’t Mean More Removal
You might expect that rising atmospheric CO2 would turbocharge photosynthesis, with plants simply growing faster and pulling more carbon out of the air. For a while, this happened. The so-called CO2 fertilization effect did boost global plant growth in recent decades. But satellite and ground-based measurements from 1982 to 2015 show that this effect has been declining across most of the world’s land surface.
The reason is that CO2 is only one ingredient plants need. As carbon dioxide rises, other resources become the bottleneck. Soil nitrogen and phosphorus, both essential for building proteins and DNA, limit how much extra growth plants can achieve. Water availability matters even more. In arid and semi-arid regions, the fertilization effect weakens or disappears entirely when precipitation drops. Grassland experiments have shown that the CO2 growth benefit can be completely negated when spring rainfall falls too low.
This declining trend has real implications for climate projections. If plants become less effective at absorbing extra CO2 over time, the atmosphere retains more of what human activity emits, and warming accelerates faster than many current models predict. Research published in Science found that carbon cycle models substantially underestimate how much the fertilization effect has already weakened, suggesting that the natural system’s ability to buffer rising emissions is smaller than previously assumed.
How Much Carbon Photosynthesis Actually Removes
Net primary productivity, or the total amount of carbon plants fix minus what they lose to their own respiration, is the key metric for understanding how much CO2 photosynthesis truly pulls from the atmosphere. It represents the carbon that actually gets built into leaves, wood, roots, and other plant tissue rather than being immediately recycled back to the air.
This number is enormous but not unlimited. Estimates suggest humans already appropriate a significant fraction of the planet’s total net primary productivity for food, fiber, fuel, and land use. One analysis concluded that only about 10% of retrievable terrestrial productivity remained available for future human demands. Even under the most optimistic scenarios, no more than 12% of current oil and gas consumption could be replaced with plant-based bioenergy. The planet’s photosynthetic machinery is powerful, but it operates within hard physical and chemical limits that no amount of rising CO2 can override on its own.