Where the Other Half of Our CO2 Goes

When we release carbon dioxide, only about half of it remains in the atmosphere in the near term. The rest is taken up by land ecosystems and the ocean. That sounds reassuring until you ask the accounting question: if nature is already absorbing so much, why does atmospheric CO2 keep rising?

Because the sinks are taking up only a share of what we emit, not all of it. As long as emissions exceed removals, the atmospheric total grows. The land and ocean have slowed that growth substantially. They have not canceled it.

The carbon budget has to add up

The Global Carbon Project’s annual budget tracks carbon released by fossil fuels and land-use change, the increase measured in the atmosphere, and the amounts absorbed by the ocean and land. Over recent decades, roughly half of human-caused CO2 emissions have stayed in the atmosphere. The ocean and land have each absorbed approximately a quarter, though those shares vary from year to year.

Those are flows, not permanent assignments for individual molecules. A carbon atom absorbed by a forest today can return to the air through decomposition or fire. Carbon entering the ocean may circulate for decades or centuries before reaching deeper waters. The budget tells us where the added carbon is going; it does not promise that every ton taken up this year is safely stored forever.

It is also easy to misuse the word natural. Human emissions have raised atmospheric CO2, and that increase drives much of the additional uptake by plants and seawater. The sinks are responding to the pollution. They are not an untouched reserve of removals that a company can claim simply by pointing at a forest or a coastline.

Land absorbs carbon, but not on command

Plants take in CO2 through photosynthesis. Some of that carbon becomes wood, roots and soil organic matter. Rising CO2 can increase plant growth under suitable conditions, while recovery from past land clearing can also add carbon to vegetation. These processes help explain why land, taken globally, has absorbed a substantial portion of our emissions.

But water, nutrients, temperature and disturbance all matter. A hotter, drier growing season can reduce uptake. Fire can release carbon accumulated over decades. Insect outbreaks and logging can alter storage, while warming soils can speed decomposition. A forest that remains standing may still absorb less carbon than it once did.

Land uptake is particularly variable from year to year. During some El Niño periods, heat and drought in tropical regions have weakened the land sink and increased fire emissions. Atmospheric CO2 has then risen faster, even without an equivalent jump in fossil-fuel emissions. That variability is one reason a single year of atmospheric growth cannot, by itself, tell us precisely how much people emitted.

There is an accounting trap here. The global land sink is usually reported alongside emissions from land-use change, such as deforestation. Both belong in the budget. Treating gross plant growth as a credit while ignoring clearing, decay or later fire produces a flattering number, not a net balance.

The ocean pays a chemical price

CO2 dissolves into surface seawater. Ocean circulation and chemical reactions then move or transform some of that added carbon, allowing the ocean to continue taking up a large share of human emissions. Without this uptake, atmospheric CO2 would be higher.

But dissolving CO2 changes seawater chemistry, increasing acidity and reducing the availability of carbonate ions used by many shell-building organisms. Ocean uptake is therefore not a cost-free disposal service. It shifts part of the burden from the atmosphere into marine systems.

Warming also complicates the future budget. Warmer surface water holds less dissolved CO2, and stronger layering can slow exchange between the surface and deeper ocean. Uptake will not suddenly switch off: it depends on the difference between atmospheric and ocean CO2, among other factors. But we cannot safely assume the ocean will keep absorbing the same fraction of emissions under every warming scenario.

A sink is not an offset

Suppose a business burns fossil fuel and notes that land and ocean absorb about half of global human emissions. It cannot subtract half its own emissions from its inventory. Those sinks are already part of the observed global response to everyone’s emissions. Claiming them again would count the same uptake twice.

A credible removal claim requires an additional, measurable increase in carbon storage beyond what would otherwise have happened, with careful treatment of reversal risks. Even then, storing carbon in a forest is not automatically equivalent to leaving fossil carbon underground. The fossil carbon adds to the active carbon cycle; the forest faces drought, fire, disease and future land-use decisions.

This distinction matters for national targets as well. If a country’s reported progress depends heavily on land uptake, changes in measurement methods or in the land sink itself can move the apparent balance. Transparent accounts should show fossil-fuel emissions, land-use emissions and land removals separately before combining them into a net figure. A single net number conceals which part of the result is under direct human control.

What happens if the sinks weaken?

The IPCC’s Sixth Assessment finds that land and ocean sinks are expected to keep taking up CO2 as concentrations rise, but their efficiency can decline as warming increases. That is not a forecast that all forests become carbon sources or that the ocean stops absorbing carbon. It is a warning against building climate plans on a fixed percentage of free uptake.

Permafrost makes the asymmetry clearer. Thawing can expose long-frozen organic matter to decomposition, releasing CO2 and methane. Such releases are a feedback from warming, not a substitute explanation for it: fossil-fuel emissions remain the principal driver we can directly cut. But additional carbon from thaw would make a given temperature goal harder to meet.

The most dependable way to reduce the load on both sinks is to emit less CO2. Protecting forests, restoring damaged ecosystems and monitoring ocean change all matter. None can turn continued fossil-fuel emissions into a balanced budget by assumption. Nature has been absorbing an extraordinary share of what we put into the air. The remaining share is enough to keep warming the planet.

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