Carbon Sink — Climate Words

Carbon Sink

Definition coming soon!

RESEARCH
Research by Micheala Chan

20 February 2026

Carbon sinks are environments that absorb more carbon than they release, forming a key part of the global carbon cycle and helping offset climate risks. They cover roughly 30% of Earth’s surface and store around 45% of all land‑based carbon. Major natural sinks include oceans, forests, mangroves, and seagrass: oceans absorb about 25% of global emissions and produce half of the world’s oxygen; forests store vast carbon stocks but are increasingly vulnerable to deforestation and wildfires; mangroves and seagrass capture carbon far more efficiently than terrestrial forests. Some major sinks, such as the Amazon, are now emitting more carbon than they absorb due to human pressures.
Other natural carbon sinks include soils, certain rocks, fungi, and large herbivores. Soils hold more carbon than the atmosphere and all living organisms combined, and sustainable farming could increase this storage. Some rocks naturally absorb carbon through mineralisation, while fungi may store a significant share of global emissions, but are threatened by modern agriculture. Large herbivores can enhance carbon capture by reducing wildfire fuel and pushing carbon into soils. These sinks complement but cannot replace the need for rapid emissions cuts.
The ocean absorbs carbon dioxide through phytoplankton photosynthesis and by converting dissolved CO₂ into bicarbonate, which stays trapped in seawater. As humans emit more CO₂, the ocean takes up more, increasing acidity and reducing its ability to absorb additional carbon, especially as warming slows the mixing of surface and deep waters and lowers phytoplankton productivity. Decades of measurements show that ocean carbon uptake varies widely with winds and circulation, making human‑driven changes hard to detect. Ongoing modelling and new data are helping scientists understand how the ocean carbon sink is evolving.
Mangroves are among the most carbon‑rich ecosystems on Earth, storing more blue carbon per hectare than any other coastal habitat. Although they occupy only a tiny share of global coastlines, they hold a large portion of the carbon buried in coastal sediments, thanks to slow decomposition and deep sediment layers. Mangroves also stabilise coasts, export carbon to nearby seagrass beds, and support biodiversity; sea‑level rise, storms, aquaculture, urbanisation, and pollution threaten mangroves immensely. Conservation relies on community‑based management, ecological and hydrological restoration, and economic incentives such as Payments for Ecosystem Services, which can strengthen local livelihoods while protecting these high‑value carbon sinks.
Soils store carbon from decaying plant matter, especially where decomposition is slow, and agricultural soils could potentially sequester over a billion additional tonnes of carbon each year. Practices such as planting deep‑rooted crops and reducing tilling can increase soil carbon, but large‑scale adoption would require major changes to farming systems. Soil sequestration cannot offset current emissions on its own, so it must be paired with rapid cuts in greenhouse gases.
Artificial carbon sinks are technologies designed to capture carbon dioxide that natural sinks cannot absorb, and they are increasingly seen as essential for meeting climate goals. Direct Air Capture (DAC) pulls CO₂ from the atmosphere using chemical filters, allowing it to be stored underground, deposited in deep‑sea formations, or reused in products such as fuels, plastics, cement, and graphene. Some approaches aim to boost natural uptake, including chemically treated artificial trees or iron fertilisation to stimulate phytoplankton, though the latter is controversial due to ecological risks. These systems can be installed at industrial sites or deployed as mobile units, but their effectiveness, cost, and environmental impacts remain active areas of debate.
The Paris Agreement leaves only a narrow remaining carbon budget, and with annual emissions exceeding 40 GtCO₂, this limit could be breached within years. Most 1.5 °C pathways therefore rely on negative‑emissions technologies (NETs), since exceeding the budget would require actively removing CO₂ from the atmosphere to return to safe levels.
Direct Air Carbon Capture and Sequestration (DACCS) removes CO₂ directly from the air using chemical sorbents, allowing the captured carbon to be stored underground or reused. It is more energy‑intensive than conventional carbon capture but requires far less land and water than BECCS, though sorbents pose chemical risks, and the technology raises ethical and policy concerns. (6)
Land‑based carbon sinks are increasingly recognised as cost‑effective climate solutions, but global finance still undervalues them. Countries with major forest resources have little economic incentive to protect them, and public climate investments often yield lower returns than business‑as‑usual. Carbon markets and international finance have also struggled to mobilise sufficient funding, partly because payments depend on proven reductions in deforestation, which must occur before money flows. As a result, conservation finance remains far below what is needed to protect the world’s most important natural carbon sinks.
Indigenous Peoples and local communities manage vast carbon stocks (about 17% of all carbon stored in the world’s forests) across both above‑ and below‑ground biomass. Their lands experience lower deforestation rates, and legal recognition of land rights has been shown to sharply reduce forest loss, such as in the Peruvian Amazon. Yet only a small share of Indigenous and community territories are formally recognised, limiting protection of these major carbon reservoirs. Securing land rights is a highly cost‑effective climate strategy, often far cheaper than technological mitigation options.

1
Konyn, Carol. “Explainer: What Are Carbon Sinks?” Earth.org, May 16, 2024.
2
Broom, Douglas. “What Are the World’s Biggest Natural Carbon Sinks?” World Economic Forum, July 26, 2023.
3
Riebeek, Holli. “The Ocean’s Carbon Balance.” NASA Earth Observatory, July 1, 2008.
4
Choudhary, Bhavesh, Venerability Dhar, and Anil S. Pawase. “Blue Carbon and the Role of Mangroves in Carbon Sequestration: Its Mechanisms, Estimation, Human Impacts and Conservation Strategies for Economic Incentives.” Journal of Sea Research 199 (2024): 102504.
5
Melillo, Jerry, and Elizabeth Gribkoff. “Soil‑Based Carbon Sequestration.” MIT Climate Portal, July 25, 2025.
6
Millennium Technology Prize. “Artificial Carbon Sinks Explained.” Millennium Technology Prize, July 20, 2022.
7
Gambhir, Ajay, and Massimo Tavoni. “Direct Air Carbon Capture and Sequestration: How It Works and How It Could Contribute to Climate‑Change Mitigation.” One Earth 1, no. 4 (2019): 405–409.
8
Jenkins, Michael, Rupert Edwards, and Genevieve Bennett. “Carbon Sinks Are Our Best Climate Hedge. So Where’s the Money?” Forest Trends, March 7, 2019.
9
Reytar, Katie. “Safeguarding the Carbon Stored in Indigenous and Community Lands Is Essential to Meeting Climate Goals.” World Resources Institute, September 13, 2018.