Forests act as carbon sinks by absorbing carbon dioxide (CO₂) from the atmosphere through photosynthesis and storing the carbon in their biomass—trunks, branches, roots, and leaves—as well as in the soil. This process removes roughly 2.6 billion tonnes of carbon dioxide each year, making forests one of the most effective natural tools for moderating climate change.
The Global Carbon Cycle and Forests
Carbon moves continuously between the atmosphere, oceans, and land. Plants are central to this cycle: they take in CO₂, use sunlight to convert it into sugars, and release oxygen. When trees die and decompose, or when forests burn, that stored carbon returns to the atmosphere. The term “carbon sink” refers to any system that absorbs more carbon than it releases. Forests are the largest terrestrial sinks, holding an estimated 45% of all carbon stored on land.
Not all forests are equal in this role. Tropical rainforests, like the Amazon and Congo Basin, store vast amounts of carbon because of their dense, fast-growing vegetation. Boreal forests, stretching across Canada and Russia, hold enormous carbon reserves in their cold, waterlogged soils—often more than in the trees themselves. Temperate forests, such as those in Europe and the eastern United States, sit between these extremes, with moderate storage rates.
How Photosynthesis Drives Carbon Uptake
The engine of a forest carbon sink is photosynthesis. In each leaf, chlorophyll captures sunlight and uses its energy to split water molecules and combine CO₂ with hydrogen to form glucose. This sugar fuels tree growth, and the carbon from CO₂ becomes part of the tree’s structure—cellulose in wood, lignin in bark, and organic compounds in roots.
A single mature tree can absorb roughly 22 kilograms of CO₂ per year, though this varies widely by species, age, and climate. Young, fast-growing trees absorb more quickly, while old-growth forests continue to store carbon but at a slower rate. Importantly, the carbon stays locked in the wood for as long as the tree lives and persists after death if the wood is used in construction or furniture rather than burned or left to rot.
Soil: The Often-Overlooked Carbon Reservoir
Beneath the visible forest lies a second major carbon store: the soil. Dead leaves, fallen branches, and root exudates feed soil organisms, and the organic matter they produce can persist for centuries. In boreal forests, permafrost and waterlogging slow decomposition, so carbon accumulates in thick peat layers. Globally, forest soils hold roughly twice as much carbon as the trees growing above them.
Disturbance releases this soil carbon. Clear-cutting exposes soil to sunlight and erosion, accelerating decomposition. Draining peatlands for agriculture or plantation forestry can turn a carbon sink into a carbon source within a decade. This is why forest protection must consider the entire ecosystem, not just the visible trunks.
Why Old-Growth and Primary Forests Matter
A common misconception is that only young, growing forests absorb carbon. In reality, old-growth forests—those that have developed over centuries without major disturbance—continue to accumulate carbon in their soils and in large, dense trees. They also hold far more carbon per hectare than young plantations. A 2008 study in the journal Nature found that old-growth forests in the Pacific Northwest were still net carbon sinks, contradicting earlier assumptions.
Primary forests, which have never been logged, are particularly valuable because their complex structure supports high biodiversity and stable carbon storage. However, they are being lost at an alarming rate—roughly 10 million hectares per year globally, according to the UN’s Food and Agriculture Organization. Protecting these intact forests is often more effective for climate than planting new ones, because newly planted trees take decades to accumulate significant carbon.
Deforestation and Degradation: Reversing the Sink
When forests are cleared or degraded, they release stored carbon. Deforestation—the permanent conversion of forest to other land uses—accounts for about 10% of global greenhouse gas emissions. The Amazon rainforest, for example, has shifted in some regions from a carbon sink to a carbon source due to fires and logging. Degradation, which includes selective logging, fragmentation, and understory fires, also reduces a forest’s capacity to absorb CO₂ without fully removing it.
The rate of loss matters. Between 2015 and 2020, the world lost an average of 10 million hectares of forest each year—an area roughly the size of Iceland. Reforestation and afforestation (planting trees on land that was not previously forested) can help, but they cannot fully compensate for the loss of intact, high-carbon ecosystems. A newly planted forest may take 50 to 100 years to match the carbon storage of a mature one.
Natural vs. Plantation Forests
Not all tree cover is equal in carbon terms. Natural forests—those that regenerate on their own—typically store more carbon than commercial plantations. Plantations, often monocultures of fast-growing species like eucalyptus or pine, absorb carbon quickly when young but are usually harvested every 10 to 30 years, releasing much of that stored carbon. They also support less biodiversity and often replace more carbon-rich natural forests.
However, plantations can be part of a climate solution if established on degraded land and managed for long rotation periods. The key is to avoid converting natural forests into plantations, which results in a net loss of carbon. Agroforestry—integrating trees into farmland—offers a middle path, storing carbon while providing food and income.
Forests and Climate Feedbacks
Forests do not just absorb carbon; they also influence climate through other mechanisms. They release water vapor through transpiration, which cools the air and can form clouds. They also emit volatile organic compounds that affect cloud formation and reflect sunlight. These effects can either amplify or dampen the carbon sink, depending on the region.
Climate change itself poses a threat. Warmer temperatures can increase tree stress, making forests more vulnerable to fires, pests, and diseases. The mountain pine beetle outbreak in British Columbia, which killed over 50% of the province’s mature lodgepole pine by 2015, turned a large forest area from a sink into a source of carbon. Similarly, extreme droughts in the Amazon have increased tree mortality and reduced carbon uptake. This creates a feedback loop: more CO₂ leads to more warming, which weakens forests’ ability to absorb CO₂.
Frequently Asked Questions
How much carbon do forests store globally?
Forests hold an estimated 662 billion tonnes of carbon, with about 45% in living biomass and 55% in soil and litter. This is roughly 1.5 times the amount of carbon in the atmosphere.
Do all forests absorb carbon equally?
No. Tropical forests absorb the most due to year-round growth, while boreal forests store more in their soils. Young forests absorb quickly, but old-growth forests hold more total carbon. The rate and duration of storage vary widely.
Can planting trees solve climate change?
Planting trees helps, but it is not a substitute for cutting emissions. A widely cited 2019 study suggested that restoring forests could sequester about 200 gigatonnes of carbon, but other analyses argue that realistic limits are lower. Protecting existing forests is more effective than planting new ones.
What happens to carbon when a tree dies?
If the tree decomposes naturally, its carbon slowly returns to the atmosphere as CO₂ or methane. If it is burned, the release is immediate. If it is made into durable wood products, the carbon stays stored for decades or longer.
Are forests carbon sinks or sources right now?
Globally, forests are a net sink, absorbing about 7.6 billion tonnes of CO₂ per year—roughly 20% of human emissions. But this net figure hides regional differences: the Amazon and boreal forests are becoming weaker sinks or even sources in some years due to fires and climate stress.
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