Diffusion in the carbon cycle refers to the passive movement of carbon-containing gases, primarily CO₂, from areas of higher concentration to areas of lower concentration across natural boundaries like the ocean surface, soil layers, and leaf tissues. It is one of the most fundamental physical processes shuttling carbon between the atmosphere, oceans, land, and living organisms. Though each molecule moves randomly and slowly, diffusion operating across the enormous surface areas of oceans, soils, and forests adds up to billions of tonnes of carbon exchanged every year, making it a quiet but powerful engine of the global carbon cycle.
How Diffusion Moves CO₂ Between Air and Sea
The single largest diffusion-driven carbon exchange on Earth happens at the ocean surface. When atmospheric CO₂ concentrations are higher than the CO₂ dissolved in surface seawater, molecules diffuse from air into the water. When the ocean holds more dissolved CO₂ than the air above it, the flow reverses. This two-way traffic is driven entirely by the concentration difference across a thin boundary layer at the water’s surface, sometimes only fractions of a millimeter thick.
The speed of this exchange depends on several factors. Temperature and salinity change how readily CO₂ dissolves; warmer water holds less CO₂, so warming can slow or reverse the net absorption. Research has shown that temperature and salinity affect the interfacial concentration of dissolved CO₂ primarily through their effect on solubility, with smaller contributions from vapor pressure and the relationship between fugacity and partial pressure.1Journal of Geophysical Research: Oceans. On the calculation of air‐sea fluxes of CO2 in the presence of temperature and salinity gradients Wind also plays a major role: stronger winds mix the surface layer and thin the boundary, accelerating diffusion. At low wind speeds, the thin “cool skin” of the ocean surface can slightly retard CO₂ uptake, while the warm layer that builds on calm sunny afternoons can enhance outgassing, each by a few percent.2Journal of Geophysical Research: Oceans. Biases in the air‐sea flux of CO2 resulting from ocean surface temperature gradients
Once CO₂ diffuses into seawater, it does not simply sit as dissolved gas. It reacts with water to form carbonic acid, which then dissociates into bicarbonate and carbonate ions. This chemical buffering effectively “pulls” more CO₂ across the surface by keeping the dissolved gas concentration lower than it otherwise would be, maintaining the gradient that drives further diffusion. Adding alkalinity to seawater enhances this uptake; under typical surface ocean conditions, roughly 83% of any increase in dissolved inorganic carbon translates to additional atmospheric CO₂ absorbed.3Chemical Reviews. Ocean Carbon Dioxide Removal and Storage This chemistry is why the ocean has managed to absorb about a quarter of the CO₂ humans have emitted.
Below the Surface: Diffusion Versus Turbulence in the Ocean
Molecular diffusion alone is remarkably slow in water. If the deep ocean relied solely on individual CO₂ molecules bumping their way downward, it would take centuries for carbon to travel even modest distances. In reality, ocean currents and turbulent mixing dominate transport below the surface. Studies using deep-ocean CO₂ experiments have found that ocean currents and turbulence can increase the net rate of CO₂ release from a concentrated source by several orders of magnitude compared to molecular diffusion alone.4Journal of Geophysical Research: Oceans. Turbulent diffusion and transport from a CO2 lake in the deep ocean So while diffusion is the gatekeeper at the air-sea interface, once carbon crosses that threshold, mixing and circulation take over as the main carriers moving it into the deep ocean.
Pressure, interestingly, does not change how fast CO₂ diffuses through water very much. Extensive experimental work has shown that pressure has a very limited effect on the diffusion coefficient of CO₂ in water, at least up to around 45 megapascals and temperatures up to about 200°C, because liquid water is barely compressible under those conditions.5ACS Publications. Diffusivity of CO2 in H2O: A Review of Experimental Studies and Molecular Simulations in the Bulk and in Confinement This means the basic rate of molecular diffusion stays fairly consistent across the range of pressures found in most of the ocean, and it is other factors, particularly turbulence, temperature, and biology, that control how fast carbon moves.
Diffusion Through Soil
Beneath your feet, soils are constantly producing CO₂. Roots respire, fungi break down dead plant matter, and bacteria metabolize organic carbon. All that CO₂ accumulates in the tiny air pockets between soil grains and must diffuse upward through the soil profile before reaching the atmosphere. This makes soil a major source of CO₂ to the air, and diffusion is the dominant mechanism moving it.
The rate at which CO₂ escapes depends heavily on how wet the soil is. Water fills pore spaces and blocks gas pathways, so saturated soil traps CO₂ much more effectively than dry soil. Research on permanent grassland soils found that under moderate to high soil moisture, CO₂ concentrations in the soil tracked temperature on a daily cycle: warmer afternoons meant more microbial respiration and higher CO₂. But when soils dried out, a different pattern emerged. CO₂ peaked in the early morning and dropped through the afternoon, because wind-driven pressure fluctuations at the surface pumped air through the drier pore spaces and flushed out accumulated gas.6Elsevier. Temporal changes in soil pore space CO2 concentration and storage under permanent grassland So at night, when the surface is calm, pure molecular diffusion dominates and CO₂ builds up; during windy days, physical pressure pumping speeds up the release.
This matters for climate models because soils hold far more carbon than the atmosphere. How quickly that carbon escapes as CO₂ depends directly on these diffusion dynamics. Anything that changes soil moisture, soil structure, or temperature patterns, whether from land use changes or climate warming, shifts the balance of how much carbon stays locked underground and how much vents to the sky.
How Plants Use Diffusion to Capture Carbon
Every plant on Earth captures carbon through diffusion. When a leaf opens its stomata, the tiny pores on its surface, CO₂ from the surrounding air diffuses inward along a concentration gradient. Inside the leaf, photosynthesis consumes CO₂, keeping the internal concentration low and maintaining the gradient that draws more in. The efficiency of this process is constrained primarily by two resistances: the resistance at the stomatal opening and the resistance within the leaf’s interior tissue, called mesophyll resistance.7Journal of Integrative Agriculture. Responses of leaf stomatal and mesophyll conductance to abiotic stress factors
This is a bottleneck that plants have been navigating for hundreds of millions of years. The concentration of CO₂ in air is relatively low, around 420 parts per million today. The enzyme that fixes carbon during photosynthesis, called Rubisco, works sluggishly at those concentrations. Under standard atmospheric conditions, Rubisco operates at no more than about 30% of its maximum capacity.8Plant Physiology (Oxford Academic). How Do Algae Concentrate CO2 to Increase the Efficiency of Photosynthetic Carbon Fixation? Many aquatic organisms, including microalgae, evolved carbon-concentrating mechanisms to actively pump CO₂ inward and boost the concentration around Rubisco far beyond what diffusion alone would deliver. Land plants, for the most part, rely on diffusion as their sole supply line, which is one reason stomata are so critical. If a plant closes its stomata to conserve water during a drought, it simultaneously chokes off its CO₂ supply, and photosynthesis drops.
Diffusion in Lakes and Wetlands
Freshwater systems are surprisingly important pieces of the carbon cycle, and diffusion plays a distinctive role in them. Lakes and ponds receive organic carbon from surrounding land, and microbes in the sediments break it down, producing both CO₂ and methane. These gases can escape to the atmosphere by two routes: steady diffusion across the sediment-water interface and water surface, or bubbling (ebullition) when gas pressure in the sediment exceeds the weight of water above it.9Limnology and Oceanography. A simple approach to quantifying whole‐lake methane ebullition and sedimentary methane production, and its application to the Canadian Lake Pulse dataset
The relative importance of these two pathways varies. In shallow ponds, bubbling tends to dominate, contributing roughly 56% of total methane emissions in one study of northern ponds and lakes. But in deeper lakes, where the water column suppresses bubble formation, diffusion becomes the primary escape route, with ebullition contributing only around 18–22% of total methane emissions when averaged across the whole lake surface.10Limnology and Oceanography. Methane ebullition and diffusion from northern ponds and lakes regulated by the interaction between temperature and system productivity Temperature amplifies both pathways but affects bubbling more dramatically than diffusion, which makes sense: warmer sediments produce gas faster, and the more gas builds up, the more likely it is to escape in bursts rather than by slow seepage.
Sea Ice as a Diffusion Barrier
In polar regions, sea ice acts as a cap over the ocean, and its effect on carbon diffusion is striking. Even when the CO₂ concentration difference between the water below the ice and the atmosphere above it is large, the ice itself can block the exchange almost entirely. Observations of landfast Arctic sea ice during early spring melt found that the low CO₂ permeability of the ice matrix, combined with overlying snow cover, effectively impeded CO₂ from escaping to the atmosphere despite a strong concentration gradient driving it that way.11Journal of Geophysical Research: Oceans. Inorganic carbon system dynamics in landfast Arctic sea ice during the early‐melt period
As the ice warms and brine channels open up, permeability increases, and CO₂ can start diffusing through. This seasonal cycle means that polar carbon exchange is not a steady drip but a highly seasonal process, with most gas exchange compressed into the months when ice is thinning or absent. With Arctic sea ice declining in extent and thickness, the ocean surface exposed to direct air-sea diffusion is growing, which changes the regional carbon budget in ways scientists are still working to quantify.
Permafrost, Freeze-Thaw, and Changing Soil Diffusion
Arctic and subarctic soils contain enormous stores of organic carbon locked in permafrost. As the climate warms, permafrost thaws, and microbes gain access to that carbon, producing CO₂ and methane. But the physical structure of the soil itself also changes in ways that affect how fast those gases can diffuse out.
Freeze-thaw cycles physically deform soil pore networks. Laboratory experiments on newly thawed permafrost aggregates from Alaska found that repeated freezing and thawing decreased the spatial connectivity of pore networks, producing more dead-end pores and reducing the volume of water-filled pores connected to the broader network.12Geoderma. Soil pore network response to freeze-thaw cycles in permafrost aggregates These shifts in pore architecture could either trap gases longer by reducing connectivity or create new pathways as larger cracks develop. The net effect likely depends on local soil composition and the intensity of freezing. This kind of micro-scale soil restructuring is one of the harder things to represent in climate models, yet it directly influences how much of the vast permafrost carbon reserve reaches the atmosphere and how quickly.
Volcanic and Geological Diffusion
Diffusion is not confined to the biosphere. The Earth’s interior is a significant source of CO₂, and much of that gas reaches the surface by diffusing through soil and rock rather than erupting from a volcanic vent. Volcanoes release carbon both through dramatic eruptions and through quieter, continuous “diffuse degassing” from surrounding soils. At Piton de la Fournaise volcano, for instance, periodic deep magma recharge events induce intermittent CO₂ degassing through overlying soils, feeding a hydrothermal system that gradually vents carbon to the atmosphere.13Geochemistry, Geophysics, Geosystems. Tracking Deep Magma Migration Through Diffuse CO2 Degassing: Insights From Piton de la Fournaise Volcano (2013–2023) This diffuse soil degassing is widespread across volcanic regions and can persist between eruptions, representing a slow but steady leak of deep Earth carbon into the atmosphere.
On the storage side, the same diffusion physics matters for efforts to sequester CO₂ underground. In geological carbon storage, CO₂ is injected into porous rock formations capped by low-permeability layers called caprocks. The caprock’s job is to prevent upward diffusion and migration of the stored gas. But CO₂ dissolved in water can react with minerals in the caprock, potentially altering its permeability over time. Whether these reactions tighten or loosen the seal depends on the mineral composition: some reactions precipitate new minerals that plug pores, while others dissolve existing minerals and open pathways.14OnePetro. The Significance of Caprock Sealing Integrity for CO2 Storage Understanding diffusion through these barriers is central to ensuring stored carbon stays put for centuries.
Measuring Diffusion in the Carbon Cycle
You cannot directly watch individual CO₂ molecules crossing the ocean surface or creeping through soil pores. Scientists use a range of clever indirect methods. For air-sea exchange, natural and bomb-produced radiocarbon, dissolved oxygen, radon-222, and the trace gas sulfur hexafluoride have all been used to estimate how fast gases move across the ocean surface.15Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences. Tracers of air-sea gas exchange Each tracer has strengths and weaknesses: radiocarbon integrates exchange over years, giving a long-term average, while sulfur hexafluoride tracks exchange over hours to days, capturing short-term variability.
In the atmosphere near the surface, researchers use eddy covariance towers to measure turbulent gas fluxes. Recent work has refined the ability to separate turbulent diffusion from non-diffusive transport caused by mean airflow, such as the upward Stefan flow driven by surface evaporation.16Boundary-Layer Meteorology. Disentangling Turbulent Gas Diffusion from Non-diffusive Transport in the Boundary Layer This distinction matters because the two types of transport respond differently to weather conditions and land surface characteristics. Lumping them together can bias estimates of how much CO₂ is actually being exchanged between an ecosystem and the atmosphere.
Carbon isotopes offer yet another window. When CO₂ diffuses between air and seawater, the lighter carbon-12 isotope moves slightly faster than the heavier carbon-13, producing a measurable fractionation. Experimental work found that the isotopic signature of CO₂ crossing the air-sea interface differs depending on the direction of travel: roughly −10 per mil for air-to-sea and −8 per mil for sea-to-air at typical ocean temperatures.17Geochimica et Cosmochimica Acta. Carbon isotopic fractionation during the CO2 exchange process between air and sea water under equilibrium and kinetic conditions By measuring these isotopic ratios in ocean water and atmospheric samples, scientists can figure out not just how much CO₂ crossed the surface, but which direction it was going and how fast.
Diffusion in Carbon Capture Technology
Understanding how CO₂ diffuses through materials is not only useful for studying nature; it also drives the design of technologies to pull carbon out of the air. Direct air capture membranes rely on selective diffusion: the material lets CO₂ pass through much more readily than nitrogen or oxygen. Recent work on graphene-doped polymer membranes found that CO₂ diffuses through these structures by a combination of mechanisms, dissolving into the polymer, moving along the interface between polymer and filler particles, and traveling between the layered sheets of graphene oxide. Adding amine-functionalized graphene oxide introduced a facilitated transport mechanism that preferentially shuttles CO₂, boosting selectivity over other gases.18Carbon Capture Science & Technology. Graphene-doped membranes for direct air capture (m-DAC) of CO2
The challenge for these technologies mirrors the challenge that plants face: CO₂ is dilute in ambient air, and driving enough of it through a membrane by diffusion alone requires either enormous surface area or materials that are extraordinarily selective. In a sense, engineers building carbon capture membranes are solving the same problem that Rubisco has struggled with for billions of years, getting enough CO₂ to the reactive site when it is only a tiny fraction of the surrounding gas. The parallels between biological and engineered diffusion systems are not just poetic; lessons from how algae concentrate CO₂ have genuinely informed membrane and reactor design.
Diffusion in Animal Gas Exchange
The carbon cycle ultimately includes living organisms, and at the smallest scale, every animal that breathes is participating in diffusion-driven carbon exchange. The final step of gas transfer in insects occurs by diffusion from air-filled tubes called tracheoles directly to the mitochondria inside cells, bypassing the need for a circulatory system to carry gases. Some amphibians exchange CO₂ and oxygen directly through their skin, though this pathway is limited by skin thickness, which creates a substantial diffusion barrier.19PubMed. Phylogeny of gas exchange systems These biological examples illustrate a universal constraint: diffusion is effective only over very short distances. Scale up the distance, and you need a pump, whether that is a heart, ocean currents, or wind.
That constraint is worth keeping in mind when thinking about the carbon cycle as a whole. Diffusion is fast enough to move CO₂ across a leaf’s cell wall in milliseconds, across the ocean’s surface boundary layer in seconds, and through a few centimeters of soil in minutes. But it cannot, on its own, carry carbon from the ocean surface to the deep seafloor or from the atmosphere to a rock formation kilometers underground. Wherever diffusion operates in the carbon cycle, it is typically the first or last step in a longer chain of transport, a handoff between the atmosphere and whatever medium takes over from there.