carbon sink

A carbon sink is any reservoir—oceanic, terrestrial, or geological—that accumulates and stores carbon-containing compounds for an indefinite period, removing carbon dioxide (CO₂) from the atmosphere faster than it emits it. The concept is foundational to the global carbon cycle and is formally embedded in international climate law: Article 4.1(d) of the United Nations Framework Convention on Climate Change (UNFCCC, 1992) obliges parties to promote the conservation and enhancement of "sinks and reservoirs," a term the Convention defines to include biomass, forests, oceans, and other terrestrial, coastal and marine ecosystems. Article 5 of the Paris Agreement (2015) reinforces this, directing parties to conserve and enhance sinks, including forests, and to implement results-based payment frameworks such as REDD+ (Reducing Emissions from Deforestation and forest Degradation). The opposite of a sink is a carbon source, which emits more carbon than it absorbs.

The Earth's principal sinks are the oceans and terrestrial vegetation and soils. Oceans constitute the largest active sink, absorbing roughly a quarter of anthropogenic CO₂ annually through the physical "solubility pump" and the biological pump driven by phytoplankton; this absorption simultaneously drives ocean acidification as dissolved CO₂ forms carbonic acid. Forests sequester carbon through photosynthesis, locking it in biomass and forest soils—tropical rainforests and peatlands are especially carbon-dense. Blue carbon ecosystems—mangroves, salt marshes and seagrass meadows—store disproportionately large amounts per unit area. Geological sequestration and engineered approaches such as Carbon Capture, Utilisation and Storage (CCUS) and afforestation create or augment artificial sinks. A sink's effectiveness depends on permanence: deforestation, wildfires, soil disturbance and warming can convert a sink into a source, as seen when parts of the Amazon began emitting net carbon.

For India, the sink concept is operationally central to its climate commitments. India's Nationally Determined Contribution (NDC) under the Paris Agreement pledges to create an additional carbon sink of 2.5 to 3 billion tonnes of CO₂ equivalent through additional forest and tree cover by 2030—a target tracked via the biennial India State of Forest Report (ISFR) of the Forest Survey of India, and supported by the Green India Mission under the National Action Plan on Climate Change (2008) and afforestation under the CAMPA fund (Compensatory Afforestation Fund Act, 2016). As of 2026, expanding sink capacity remains a pillar of India's stated goal of net-zero emissions by 2070, announced at COP26 (Glasgow, 2021).

For the UPSC examination, carbon sinks appear in GS Paper III (environment, conservation, climate change) and frequently in Prelims through the Environment & Ecology segment. Typical question angles include: distinguishing sinks from sources; identifying blue carbon ecosystems; linking India's NDC sink target to specific missions; and connecting the ocean sink to acidification. Prelims often pairs the term with UNFCCC/Paris Agreement provisions or with REDD+, so candidates must recall the exact treaty articles and the 2.5–3 billion tonne NDC figure precisely.