Photosynthesis is the single largest process that removes carbon dioxide from the atmosphere, but it’s far from the only one. The ocean, rocks, soil, and even new technologies all pull CO2 out of the air through different mechanisms and on vastly different timescales. Together, land ecosystems absorbed about 2.3 billion metric tons of carbon in 2023, while the ocean absorbed another 2.9 billion metric tons, according to the Global Carbon Budget 2024.
Photosynthesis: The Primary Engine
Every green plant, tree, and blade of grass removes carbon dioxide through photosynthesis. The core chemistry happens through what scientists call the Calvin cycle: an enzyme called RuBisCO grabs CO2 molecules and attaches them to a sugar molecule inside the plant’s cells. The plant then uses energy from sunlight to convert that captured carbon into glucose and other organic compounds it needs to grow. Oxygen is released as a byproduct.
This process is staggeringly productive. Forests, grasslands, and croplands collectively function as a “terrestrial carbon sink” that pulls billions of tons of carbon from the air each year. The ten-year average from 2014 to 2023 was about 3.2 billion metric tons of carbon annually, though the number fluctuates with weather patterns. In La Niña years, when tropical regions tend to be cooler and wetter, land plants absorb significantly more. In 2022, a La Niña year, the land sink reached 3.9 billion metric tons. By 2023, it dropped 41% to 2.3 billion metric tons.
The carbon that plants capture doesn’t stay locked away forever. When leaves fall and decompose, or when trees die or burn, much of that carbon returns to the atmosphere. The net removal is the difference between what plants take in and what decomposition and disturbance release back.
How the Ocean Absorbs CO2
The ocean removes carbon dioxide through two linked systems: a chemical process and a biological one.
The chemical side, sometimes called the solubility pump, works because CO2 dissolves readily in seawater. Once dissolved, it reacts with water to form bicarbonate and carbonate ions. These charged molecules don’t escape back into the air the way dissolved gas can, effectively trapping the carbon in the ocean for centuries. The ocean’s enormous volume and surface area make this a powerful sink, absorbing roughly 2.9 billion metric tons of carbon per year.
The biological side starts with phytoplankton, microscopic algae that photosynthesize at the ocean surface just like land plants do. When these organisms die or are eaten, their carbon-rich remains sink toward the deep ocean floor. Zooplankton and fish accelerate this process: they feed on phytoplankton near the surface during the night, then migrate hundreds of meters deeper during the day, where they excrete or respire that carbon. Ocean currents and eddies also physically push organic particles and dissolved carbon into deeper water. The deeper the carbon sinks, the longer it stays sequestered, potentially for hundreds to thousands of years.
Blue Carbon Ecosystems
Coastal ecosystems like mangroves, salt marshes, and seagrass beds are unusually efficient carbon sinks. NOAA reports that mangroves and salt marshes remove carbon from the atmosphere at a rate ten times greater than tropical forests and store three to five times more carbon per acre. This is because waterlogged, oxygen-poor soils slow decomposition dramatically, locking carbon in place for millennia. The catch is that these ecosystems are disappearing fast. When a mangrove forest is destroyed, all that stored carbon can be released.
Rock Weathering Over Geological Time
On timescales of thousands to millions of years, the weathering of rocks is the planet’s ultimate carbon thermostat. When rain absorbs CO2 from the atmosphere, it forms a weak acid. That acidic rainwater dissolves silicate minerals in rocks, releasing ions that wash into rivers and eventually reach the ocean. There, those ions combine with dissolved carbon to form carbonate minerals that settle on the seafloor as sedimentary rock, locking carbon away for millions of years.
This process is extraordinarily slow by human standards, but it has regulated Earth’s climate over geological history. Periods of mountain building, which expose fresh rock to the atmosphere, have been linked to long-term cooling because more exposed rock means more CO2 gets consumed through weathering.
Soil Carbon and Farming Practices
Soil holds more carbon than the atmosphere and all plant life combined. The carbon gets there when plant roots, dead leaves, and other organic matter decompose and mix into the ground, where microorganisms process it into stable compounds that can persist for decades or longer.
Certain farming practices can increase how much carbon soil stores. Research published in Frontiers in Sustainable Food Systems found that cover cropping on arable land sequesters an average of 0.58 metric tons of carbon per hectare per year. No-till farming, which avoids plowing and leaves soil structure intact, averages about 0.48 metric tons. Combining cover crops with no-till roughly doubles the effect, reaching about 1.01 metric tons per hectare per year. On land with woody perennials like vineyards, the numbers are even higher: cover cropping alone averaged 1.31 metric tons, and pairing it with no-till reached 1.43 metric tons per hectare per year.
These numbers are modest compared to the scale of global emissions, but farmland covers so much of the planet that even small per-hectare gains add up when adopted widely.
Technological Carbon Removal
Direct air capture (DAC) is the most discussed engineered approach. These systems use chemical sorbents to pull CO2 directly from ambient air. Traditional approaches require heating the sorbent to very high temperatures to release and collect the captured CO2, which consumes a lot of energy. Newer designs are bringing those temperatures down considerably. A class of materials called “charged-sorbents” uses electrically deposited hydroxide ions in the pores of a carbon electrode to capture CO2. These sorbents can be regenerated at just 90 to 100 degrees Celsius using simple electric heating powered by renewable energy, a significant improvement over older methods.
Enhanced rock weathering takes the natural geological process described above and speeds it up. Farmers spread finely crushed basalt rock across agricultural fields, dramatically increasing the surface area exposed to air and rain. The basalt reacts with CO2 in the same way natural weathering does, but over years instead of millennia. This approach has a dual benefit: it sequesters carbon while also improving soil health. Research estimates the global potential at gigatons of CO2 removal, and basalt is abundant enough to make it highly scalable.
How These Processes Compare
The scale and speed of these processes vary enormously:
- Photosynthesis removes carbon within minutes but stores it for years to decades in living biomass, unless a forest burns or is cleared.
- Ocean absorption operates continuously and stores carbon for centuries in deep water, though it’s making the ocean more acidic as it absorbs more CO2.
- Natural rock weathering locks carbon away for millions of years but operates too slowly to address the current pace of emissions.
- Soil sequestration works on a timescale of decades and depends on sustained farming practices to maintain gains.
- Direct air capture can store carbon permanently when paired with geological storage, but current capacity is tiny compared to what’s needed.
Nature already removes enormous quantities of CO2 each year. The problem is that human emissions, around 10.1 billion metric tons of carbon from fossil fuels alone in 2023, outpace what natural sinks can absorb. The 5.9 billion metric tons that accumulated in the atmosphere that year represents the gap between what we emit and what Earth’s systems can handle.