In simple terms
A carbon sink is anything that takes in more carbon than it gives back, so its net effect is to pull carbon dioxide out of the air and hold it somewhere else. The ocean, forests and soils are the large natural ones. The opposite is a carbon source, which releases more than it absorbs.
Nothing is destroyed. Carbon moves between stores, and whether a store counts as a sink depends on which way the flow runs.
How it works
Carbon cycles between reservoirs. Most of Earth’s carbon sits in rocks and sediments, and the rest is in the ocean, the atmosphere and living organisms. [1] Plants move carbon from air into the biosphere by photosynthesis, while respiration, excretion and decay send it back. [1] Carbon dioxide is described as sequestered when plants absorb it in this biological cycle. [2]
The ocean is the largest active sink: it holds about 50 times more carbon than the atmosphere, trades it quickly at the surface, and can keep it for centuries in deep water. [1] Land is measured as a sink too. In the United States, land use, land-use change and forestry form a net sink that offsets about 13 percent of national greenhouse gas emissions, which came to 6,343 million metric tons of carbon dioxide equivalent in 2022, excluding the land sector. [2] Those are one country’s accounts; elsewhere a land sector can be a net source.
Why it matters
Sinks are why atmospheric carbon dioxide has not risen as fast as emissions alone would suggest, and they are not free. The ocean absorbs much of the carbon dioxide released by burning fossil fuels, and that uptake lowers seawater pH, a process called ocean acidification, which interferes with the ability of corals, crabs and snails to build shells and skeletons. [1] Even so, atmospheric carbon dioxide is higher than at any point in the last 3.6 million years. [1] A sink can also weaken or reverse, because carbon held in trees and soil returns to the air when they burn or are cleared.
Where you’ll see it
- National greenhouse gas inventories, which report emissions and sinks as separate lines [2]
- Climate pledges and carbon markets, where stored carbon is counted against emissions
- Coverage of the ocean’s role in slowing warming, and of ocean acidification [1]
- Arguments over forests, peatlands, mangroves and farm soils as land uses
Example
A growing forest is a carbon sink for as long as it adds wood each year. If fire sweeps through it, that same forest becomes a carbon source.
Often confused with
A carbon reservoir is simply a store of carbon; a sink is a reservoir currently taking in more than it releases. Carbon capture and storage means engineered removal rather than natural uptake.
Key facts
- Most of Earth's carbon is stored in rocks and sediments, with the rest in the ocean, atmosphere and living organisms.1
- The ocean holds about 50 times more carbon than the atmosphere, and deep water can store it for centuries.1
- Carbon dioxide is described as sequestered when plants absorb it as part of the biological carbon cycle.2
- In the United States, land use, land-use change and forestry form a net sink offsetting about 13 percent of national emissions, which were 6,343 million metric tons of carbon dioxide equivalent in 2022.2
- Ocean uptake of carbon dioxide lowers seawater pH, the process known as ocean acidification.1
Related concepts
Quick checkWhat makes a carbon reservoir a sink rather than a source?Show answer
It takes in more carbon than it releases over the period being measured.
Sources
- NOAA Office of Education. Carbon cycle (resource collection). Undated (accessed Accessed 7 October 2026)
- United States Environmental Protection Agency. Overview of Greenhouse Gases, citing the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 to 2022. Undated (accessed Accessed 7 October 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed October 7, 2026Researched and drafted with AI assistanceReport an issue