Curious where the carbon you read about actually goes? The ocean carbon cycle moves carbon between air, surface water, deep water and seafloor, and the ocean holds about 95% of the active surface carbon.
This guide sits within the wider field of marine science, where carbon is one of the central threads.
| Item | Value |
|---|---|
| Share of active surface carbon held by the ocean | About 95% (~38,000 Gt C) |
| Ocean uptake of atmospheric carbon dioxide in 2023 | 2.9 ± 0.7 Pg C per year (about 30% of emissions) |
| Average net ocean uptake, 2015-2024 | About 3.2 Pg C per year |
| Atmospheric carbon dioxide in 2024 | About 423 ppm |
| Main marine carbon pumps | Three |
These figures come from Wikipedia’s oceanic carbon cycle page and the Coastal Wiki’s ocean carbon sink page. Estimates get revised as new data arrive, so check the source before you rely on them.
What forms does carbon take in the ocean carbon cycle?
Seawater carbon is sorted into four pools, depending on whether it is organic or inorganic and whether it is dissolved or particulate. The dividing line is practical: dissolved carbon passes through a 0.2 μm filter, and particulate carbon does not.
- Dissolved inorganic carbon (DIC): bicarbonate, carbonate and dissolved carbon dioxide.
- Particulate inorganic carbon (PIC): mainly the calcium carbonate of shells.
- Dissolved organic carbon: carbon in molecules such as proteins, lipids and carbohydrates that pass the filter.
- Particulate organic carbon: living and dead organic matter that the filter catches.
Most of the stored carbon is dissolved inorganic carbon, which the Coastal Wiki describes as about 90% bicarbonate and about 9% carbonate. Carbon dioxide can pile up this way because, unlike oxygen, it reacts with water and forms a balance of several ionic and non-ionic species.
The scale is lopsided. Of roughly 40,000 Gt C in the active pool at the surface, about 95% sits in the ocean. Marine life holds only about 3 Gt C, against about 610 Gt C in land vegetation, yet each group moves about 50 Gt C a year.
A note on units: the Coastal Wiki explains that a petagram of carbon (Pg C) equals one gigaton (Gt C), so the two labels in this guide are interchangeable.
First step: carbon dioxide crosses the sea surface
Exchange between air and sea happens across a very thin skin layer, less than 1 mm thick, where the concentration difference drives the flow. How fast gas crosses depends on turbulence at the surface, which is commonly estimated from wind speed.
Chemistry sets a second limit. The Coastal Wiki notes that uptake is limited by carbonate chemistry: extra DIC causes a much stronger rise in carbon dioxide pressure, a sensitivity captured by the Revelle factor, typically of order 10.
Timing matters too. The mixed layer is a few tens to about one hundred meters thick, and without deeper mixing it would approach equilibrium with the air in months to about a year. Moving carbon to the deep ocean takes hundreds of years, and individual atoms may stay sequestered for hundreds to thousands of years.
Second step: three pumps carry carbon downward
Surface water cannot hold carbon for long on its own. Three main pumps bring atmospheric carbon dioxide into the ocean interior and distribute it.
- Solubility pump: carbon dioxide dissolves more readily in cold water, and deep water forms at high latitudes where seawater is cooler and denser, so carbon-rich water sinks.
- Carbonate pump: organisms such as Coccolithophore phytoplankton build calcium carbonate shells, and shell formation actually increases atmospheric carbon dioxide.
- Biological pump: plankton fix carbon by photosynthesis in the sunlit zone, and some of it sinks, is broken down by bacteria, or reaches the sediment.
The solubility pump works because two processes coincide: carbon dioxide solubility rises as water cools, and thermohaline circulation forms deep water in those cold places. For a closer look at the living side, see our guide to the biological carbon pump.
Its scale is large. WorldAtlas reports that the biological pump transfers approximately 10.2 gigatonnes of carbon a year into the depths, and that without it atmospheric carbon dioxide would be about 400 ppm higher.
Third step: the return trip, recycling and burial
What goes down can come back up. When deep water upwells in warmer, equatorial latitudes, it strongly outgasses carbon dioxide to the atmosphere because the gas is less soluble there.
The deep water is rich in carbon partly because sinking organic matter is recycled on the way down. The Coastal Wiki says intermediate and deep waters are enriched in DIC by remineralization, and that outgassing is strongest in tropical and eastern-boundary upwelling systems.
Sinking carbon has three broad fates:
- It is recycled in the sunlit zone and used again for primary production.
- Bacteria decompose it and remineralize it at depth.
- A small share escapes both routes and is buried in sediment, where it can remain for millennia.
Rivers add carbon too, carrying organic carbon to the sea through weathering and erosion. On the longest scale, WorldAtlas says the flow of carbon in and out of the lithosphere moderates Earth’s climate over more than 500,000 years.
Marine organisms also link the carbon and oxygen cycles, which is one reason ocean deoxygenation belongs in the same conversation.
How are human emissions changing the cycle?
Before the Industrial Revolution, the ocean was a net source of carbon dioxide to the atmosphere. WorldAtlas puts the atmospheric level of that era at roughly 280 ppm, and the Coastal Wiki says it reached about 423 ppm in 2024.
The ocean has since become a net sink. Wikipedia says it has absorbed around a quarter of the carbon dioxide people produce, while the Coastal Wiki estimates 2.9 ± 0.7 Pg C in 2023, about 30% of emissions, and about 3.2 Pg C per year on average over 2015-2024. The shares differ because the sources use different periods and methods.
That help comes with side effects:
- Acidification: absorbing human-made carbon dioxide has also caused acidification, which affects calcifying organisms that build shells from calcium carbonate.
- Warming: WorldAtlas says the capacity to take in carbon dioxide decreases as sea surface temperatures rise.
- Circulation: warming and slowed thermohaline circulation can reduce the amount of carbon dioxide taken up from the atmosphere.
Frequently asked questions
Is the ocean a carbon sink or a source?
Show answer
It depends on place and period. Wikipedia notes that before the Industrial Revolution the ocean was a net source, while in recent decades it has acted as a sink. Regionally, upwelling zones release carbon dioxide while cold high-latitude waters absorb it.
How much carbon does the ocean store?
Show answer
About 38,000 Gt C, which is roughly 95% of the active surface pool of about 40,000 Gt C, according to Wikipedia. Most of it is dissolved inorganic carbon.
How long does carbon stay in the deep ocean?
Show answer
The Coastal Wiki says that individual atoms may remain sequestered for hundreds to thousands of years. Carbon buried in sediment can last longer.
Why do cold waters take up more carbon dioxide?
Show answer
Solubility increases at lower temperatures, and deep-water formation moves carbon-rich surface water into the interior, as the Coastal Wiki explains. Warm upwelling regions do the opposite and release it.
What is the Revelle factor?
Show answer
It describes how strongly carbon dioxide pressure rises when DIC rises. The Coastal Wiki gives it as typically of order 10, which limits how much more the surface can absorb.
