The largest carbon sink on Earth depends on how you define the question. In absolute terms, rocks and sedimentary formations hold roughly 65 million gigatons of carbon, dwarfing every other reservoir combined. But rocks cycle carbon over millions of years, so they have no meaningful impact on the climate changes happening in your lifetime. For the carbon cycle that actually matters to us, the ocean is the largest active carbon sink, storing around 41,000 gigatons of carbon and absorbing about 31% of the carbon dioxide humans release each year.
Why Rocks Don’t Really Count
Earth’s crust, particularly limestone and other sedimentary rocks, contains about 65 million gigatons of carbon. That’s more than 1,500 times what the ocean holds and orders of magnitude beyond everything in the atmosphere, soil, and living things put together. Carbon gets locked into rock through geological processes: marine organisms die, their shells and skeletons settle on the ocean floor, and over millions of years they compress into carbonate rock. Volcanic activity and weathering eventually release some of this carbon back into the atmosphere, but the timescale is tens of millions of years. When scientists and climate researchers talk about carbon sinks, they’re almost always referring to reservoirs that operate on human timescales, from decades to a few thousand years.
The Ocean as the Dominant Active Sink
The ocean stores approximately 41,000 gigatons of carbon, with the vast majority of it (around 38,000 gigatons) sitting in the deep ocean. Surface waters hold about 900 gigatons, and ocean floor sediments contain another 1,800 gigatons. Each year, the global ocean absorbs roughly a quarter to a third of all carbon dioxide emitted by human activity.
Two main processes drive this absorption. The first, called the solubility pump, works because carbon dioxide dissolves more readily in cold water. In polar regions, surface water chills down and becomes dense and salty as sea ice forms around it. This heavy water sinks, carrying dissolved carbon with it into the deep ocean, where it can remain for centuries. Deep ocean currents then move this carbon-rich water toward the equator in a slow global circulation pattern. The second process is biological: tiny marine organisms like phytoplankton absorb carbon dioxide through photosynthesis. When they die, their carbon-containing remains sink toward the ocean floor, effectively pumping carbon from the surface to the deep sea.
Global Forests Still Pull Their Weight
Forests are the second major active carbon sink. A 2024 study published in Nature found that global forests absorbed a steady 3.5 to 3.6 billion metric tons of carbon per year from the 1990s through the 2010s. That number has held remarkably stable at the global level, but the picture underneath is shifting in important ways.
Temperate forests (the kind found across much of North America, Europe, and parts of Asia) increased their carbon uptake by about 30% over that period, largely because forest area expanded through regrowth on previously cleared land. Tropical forests regrowing after deforestation also increased their absorption by roughly 29%. On the other side of the ledger, boreal forests in northern latitudes saw their carbon uptake drop by 36%, driven by intensified wildfires and insect outbreaks. Tropical intact forests, the old-growth rainforests that have never been cleared, lost 31% of their sink capacity as deforestation chipped away at their total area. These intact forests still sequester carbon, but they’re doing less of it every decade.
Soil Stores More Carbon Than Trees
Most people picture forests when they think of land-based carbon storage, but the soil beneath those trees often holds more carbon than the trees themselves. In temperate forests, belowground carbon stocks frequently exceed what’s stored in all the living and dead plant material above the surface. The average aboveground carbon stock for U.S. forests is about 55 metric tons per hectare, while soil carbon stocks vary widely and can be substantially higher, especially in organic-rich soils.
The most dramatic example of soil carbon storage is permafrost. Soils in the northern permafrost region contain an estimated 1,460 to 1,600 billion metric tons of organic carbon, roughly twice what’s currently in the entire atmosphere. For thousands of years, cold temperatures kept this carbon frozen and locked away, making permafrost a massive long-term sink. That role is now reversing. NOAA measurements indicate that permafrost ecosystems are already releasing a net 0.3 to 0.6 billion metric tons of carbon per year to the atmosphere. Warming temperatures activate microbes in the thawing soil, which break down the ancient organic matter and release it as carbon dioxide and methane. This means a reservoir that spent millennia absorbing carbon may have already crossed the threshold into becoming a carbon source.
Coastal Ecosystems Punch Above Their Weight
Mangroves, salt marshes, and seagrass beds occupy a tiny fraction of Earth’s surface, but their carbon storage capacity is disproportionately large. These coastal ecosystems, collectively known as blue carbon habitats, remove carbon from the atmosphere at a rate 10 times greater than tropical forests on an area-for-area basis. They also store three to five times more carbon per acre than tropical forests, largely because waterlogged, oxygen-poor sediments preserve organic material instead of letting it decompose. The catch is their total area is small compared to forests or the open ocean, so their contribution to the global carbon budget is modest in absolute terms. Their outsized per-acre efficiency makes them a high priority for conservation, though, since destroying a single acre of mangrove releases far more stored carbon than losing an acre of most other ecosystems.
How the Balance Is Shifting
The ocean’s role as a carbon sink is not guaranteed to stay constant. As seawater absorbs more carbon dioxide, it becomes more acidic, which gradually reduces its capacity to absorb additional carbon. Recent research has also shown that changing precipitation patterns affect how much carbon the ocean takes up. In the South Pacific and South Atlantic, extreme rainfall events actually enhance carbon absorption by diluting surface salinity and alkalinity, shifting those waters from carbon sources to carbon sinks. This means climate models that ignore precipitation patterns may be miscalculating the ocean’s carbon budget.
On land, the picture is similarly dynamic. The stability of the global forest sink masks a geographic redistribution: temperate regions are picking up slack that boreal and tropical intact forests are losing. Permafrost thaw is introducing a wild card that could accelerate warming if large-scale carbon release continues. The relative importance of each sink is not fixed. It’s being reshaped by the same warming these sinks are working to slow down.