Carbon sink

Carbon sinks are natural or artificial systems that remove carbon-containing greenhouse gases—primarily carbon dioxide—from the atmosphere. Since the mid-nineteenth century global carbon sources have exceeded global sinks, resulting in steadily rising atmospheric CO₂ concentrations. Understanding and managing carbon sinks is therefore central to climate change mitigation, as these systems play a crucial role in regulating the global carbon cycle and rebalancing anthropogenic emissions.

Definition and Conceptual Framework

In the context of climate science, a carbon sink is defined as any natural process, activity or mechanism that removes a greenhouse gas, an aerosol or a precursor of a greenhouse gas from the atmosphere. The broader term carbon pool refers to all reservoirs in which carbon resides for variable periods of time, including the atmosphere, ocean, soils, vegetation and fossil fuel deposits. Carbon sinks represent those pools that take up more carbon than they release, while carbon sources release more carbon than they absorb.
Vegetation and the ocean constitute the two largest global sinks. Soil is also a major carbon reservoir, storing more carbon than the atmosphere and all terrestrial vegetation combined. Intensive agriculture, land-use change and deforestation have caused significant depletion of several natural sinks, contributing to the acceleration of climate change.

Types of Carbon Sinks

Natural Carbon Sinks

Terrestrial vegetation removes carbon dioxide through photosynthesis, storing carbon in biomass and soils. Forests, grasslands and shrublands all function as carbon sinks during periods of active growth. Blue carbon ecosystems—such as mangroves, salt marshes and seagrass meadows—form a crucial subset of coastal sinks. These systems store large quantities of carbon in both plant biomass and sediment, with deep-sea carbon pools extending into continental-shelf waters, abyssal zones and underlying sediments.
Oceans absorb carbon dioxide through both the solubility pump and biological pump. Dissolved CO₂ reacts with seawater to form bicarbonate and carbonate ions, while marine organisms incorporate carbon into organic matter and skeletal materials. Although the ocean is an effective sink, its carbon uptake efficiency is influenced by temperature, circulation patterns and biogeochemical processes.
Soils hold substantial quantities of organic carbon derived from plant litter, root exudates and microbial activity. Soil organic matter includes relatively labile compounds and more recalcitrant biopolymers such as lignin, waxes and terpenoids, which can persist as humus. Soil carbon accumulation varies with climate: cold or semi-arid regions tend to retain more carbon, while warm, humid tropical soils experience rapid decomposition and leaching. Grassland soils, with their dense root networks, store significant carbon, especially in semi-arid zones where decomposition rates are slow. Fires, although releasing CO₂, can enhance long-term carbon storage through the formation of biochar.

Artificial Carbon Sinks

Artificial or engineered sinks include strategies that store carbon in long-lived materials or geological formations. Geologic sequestration involves injecting carbon dioxide deep underground into stable rock formations. Mass timber construction stores atmospheric carbon absorbed by harvested trees, and replacing steel or concrete with engineered wood in mid-rise buildings could substantially increase long-term carbon storage. Sustainable forest management and the reuse or preservation of old timber are essential to maintain this carbon benefit.
Other plant-based materials such as bamboo, straw and hempcrete offer additional opportunities to store carbon in buildings and manufactured goods. Although promising, large-scale artificial removal methods remain limited and have yet to operate at the industrial scale required for significant global impact.

Soil Carbon and Land Management

Soils represent a dynamic carbon pool sensitive to land-use practices. Intensive farming, ploughing and monoculture agricultural systems have depleted soil carbon globally. Conservation methods that increase soil organic matter—collectively termed carbon farming—include no-till agriculture, cover cropping, residue mulching and crop rotation. These practices enhance sequestration while improving soil fertility and resilience.
Tropical regions, despite their lush vegetation, hold relatively low soil carbon owing to rapid decomposition. In contrast, boreal forests and temperate grasslands exhibit high soil carbon densities. Historically, large areas of grasslands have been converted to cropland since the nineteenth century, releasing substantial quantities of soil carbon.

Forests, Wetlands and Marine Ecosystems

Forests are among the most effective natural sinks. Old-growth forests store large quantities of carbon in biomass and soils, while reforestation and afforestation can increase carbon uptake. Tidal marshes, mangroves and seagrasses are highly efficient coastal sinks with long-term sediment carbon storage. Deep ocean waters hold vast reservoirs of dissolved inorganic carbon, although enhancing deep-ocean sequestration artificially remains technically challenging and environmentally uncertain.

Enhancing Natural Carbon Sinks

Climate mitigation strategies increasingly emphasise the protection and enhancement of natural sinks. Measures include forest conservation, expansion of blue-carbon habitats, restoration of degraded soils and sustainable land management. Proposed ocean-based sequestration methods—such as seaweed cultivation, iron fertilisation, artificial upwelling and ocean alkalinity enhancement—have not yet achieved deployment at large scales and require careful evaluation due to ecological risks. Direct injection of CO₂ into deep-sea environments has largely been abandoned for environmental reasons.

Role in Climate Change Mitigation

Carbon sinks reduce atmospheric greenhouse gas concentrations and thereby moderate global warming. Since human activities currently emit more carbon than natural sinks can absorb, strengthening and expanding these sinks is vital for achieving long-term climate stabilisation. International agreements such as the Kyoto Protocol encourage the use of sinks for carbon offsetting, and growing public awareness has increased interest in protecting forests, soils and marine ecosystems.