The ocean is the biggest carbon sink on Earth, holding roughly 38,000 gigatons of carbon in its waters. But when it comes to actively pulling carbon dioxide out of the atmosphere each year, the land and ocean run neck and neck. In 2024, land-based ecosystems absorbed an estimated 3.2 gigatons of carbon while the ocean absorbed about 3.0 gigatons, according to the Global Carbon Budget. Together, these natural sinks remove slightly more than half of the carbon dioxide humans emit each year.
How the Ocean Stores Carbon
The ocean absorbs about 27% of the carbon dioxide produced by fossil fuel burning each year. It does this through two main processes: CO2 dissolves directly into surface waters (cold water absorbs more than warm water), and tiny marine organisms like phytoplankton pull carbon from the water during photosynthesis. When those organisms die, they sink, carrying carbon to the deep ocean where it can stay locked away for centuries.
This massive storage capacity comes with a catch. The ocean’s ability to keep absorbing carbon is finite, and the rate of CO2 uptake has already slowed over the last decade. Warmer ocean temperatures dissolve less CO2, meaning that as climate change heats the seas, this critical sink weakens. Rik Wanninkhof, an ocean carbon cycle expert at NOAA, has noted that the ocean’s capacity to act as a carbon sink has clear limits.
Land Sinks: Forests and Soils
On land, carbon storage is split between two reservoirs: living vegetation and soil. Soil is by far the larger of the two. In forest ecosystems alone, over two-thirds of all stored carbon sits in soils and peat deposits rather than in the trees themselves. Globally, forest vegetation and soils together hold about 1,146 gigatons of carbon, with high-latitude (boreal) forests accounting for nearly half of that total.
How forests store carbon varies by region. Tropical rainforests keep most of their carbon locked in living plant biomass, the massive trunks, roots, and canopy of the trees themselves. Boreal forests in Canada, Scandinavia, and Russia take the opposite approach, storing the bulk of their carbon underground in thick, cold soils that decompose slowly. This makes boreal soils an enormous carbon warehouse, though one that is increasingly vulnerable to warming.
The land sink fluctuates more than the ocean sink from year to year. In 2023, land ecosystems absorbed only 2.3 gigatons of carbon, well below the ten-year average of 3.2 gigatons. The 2024 rebound to 3.2 gigatons followed the shift from El Niño (which tends to suppress land uptake through heat and drought) back to neutral conditions.
Permafrost: A Sink at Risk
Arctic permafrost holds an estimated 1,400 to 1,600 gigatons of organic carbon, nearly twice the amount currently in the entire atmosphere. Most of this carbon is ancient plant and animal material frozen into the ground over thousands of years. The top meter alone contains roughly 472 gigatons, with hundreds more gigatons locked in deeper layers.
As long as it stays frozen, permafrost is a stable carbon store. But rising temperatures are thawing it. As the ground warms, microbes break down the previously frozen organic matter and release carbon dioxide and methane. Projections vary by emissions scenario: under moderate warming, about 180 gigatons of carbon could become available for decomposition by 2100. Under the highest warming scenarios, that figure rises to 300 gigatons. Abrupt thawing, where the ground collapses rather than gradually warming, could release an additional 60 to 100 gigatons by 2300.
Blue Carbon Ecosystems
Mangroves, salt marshes, and seagrass meadows are sometimes called “blue carbon” ecosystems because they punch well above their weight in carbon storage. They cover a tiny fraction of the ocean floor but sequester carbon at rates far higher per unit area than most terrestrial forests. Seagrass meadows in the Caribbean, for example, store roughly 241 metric tons of organic carbon per hectare in the top meter of soil, with short-term accumulation rates around 122 grams of carbon per square meter per year.
These ecosystems store carbon primarily in their waterlogged, oxygen-poor soils, where decomposition is extremely slow. A single seagrass bed can accumulate carbon steadily for thousands of years. The challenge is that blue carbon habitats are disappearing fast due to coastal development, pollution, and rising sea temperatures, releasing stored carbon back into the water and atmosphere when they’re destroyed.
Are Carbon Sinks Keeping Up?
Natural carbon sinks currently absorb a little over half of annual human emissions, with the remainder accumulating in the atmosphere (about 5.9 gigatons stayed in the atmosphere in 2023). That ratio has held roughly steady for decades, but there are warning signs. The ocean sink is weakening as waters warm. Land sinks swing wildly depending on drought, fire, and climate patterns. And permafrost is shifting from a carbon store to a potential carbon source.
The core problem is scale. Humans emitted over 11 gigatons of carbon from fossil fuels and land-use change in 2024. Natural sinks absorbed roughly 6.2 gigatons. The gap between what we emit and what nature can absorb is what drives the steady rise in atmospheric CO2, now well past 420 parts per million. Even the planet’s biggest carbon sinks have limits, and those limits are getting closer.