CO₂ Sinks
Of the ~37 Gt CO₂ humans emit annually, only ~45% accumulates in the atmosphere. The rest is absorbed by oceans (~25%) and terrestrial ecosystems (~30%). These natural sinks have absorbed roughly half of all anthropogenic CO₂ emitted since 1750. Their future capacity is the largest single uncertainty in carbon budget calculations.
Key insights
The ocean is the largest single sink
Oceans contain ~38,000 Gt of carbon — about 50× the atmospheric reservoir. They absorb roughly 9.2 Gt CO₂/year from the atmosphere (latest Global Carbon Budget). Uptake mechanisms: physical solubility pump (cold water absorbs CO₂), biological pump (phytoplankton incorporate carbon, some sinks). Ocean uptake has risen as atmospheric CO₂ has risen — but the rate of uptake is slowing relative to emissions.
Land sinks are heterogeneous and stressed
Land ecosystems globally absorb ~3.4 GtC/year, but with huge variation: boreal forests, intact tropical forests, regrowing temperate forests are sinks; degraded tropical forests, drying soils, fire-prone systems are sources. The Amazon's eastern portions are now net carbon sources. The 2023-24 El Niño weakened the global land sink substantially. Saturation of mature forests is a long-term concern.
Sinks could weaken as climate changes
Ocean acidification (a consequence of CO₂ absorption) slows further uptake by changing carbonate chemistry. Warming oceans hold less dissolved CO₂. Land sink capacity is degraded by drought, fire, pest outbreaks. The IPCC AR6 projects 5-25% reduction in sink efficiency by 2100 under high-emission scenarios. A weakening sink means the same emissions produce more warming — a key consideration in future budgets.
Global carbon flows 2014–2023
Annual flows, Gt CO₂
Key Finding: Of ~37 GtCO₂ emitted, ~17 GtCO₂ remains in atmosphere; oceans absorb ~9 GtCO₂; land absorbs ~11 GtCO₂.
Land and ocean sinks 1960–2023
Carbon uptake rate, Gt CO₂/year
Key Finding: Both sinks have grown roughly in proportion to emissions. Year-to-year variability is dominated by ENSO and land conditions.
Methodology & caveats
Global Carbon Budget
Annual GCB publication reconciles emissions inventory, atmospheric CO₂ measurements, ocean uptake estimates, and land sink (residual) within accounting framework. The land sink is typically estimated as the residual: emissions minus atmospheric increase minus ocean uptake. This means it's the most uncertain term — and its variability reflects both real variation and accounting noise.
Airborne fraction
The 'airborne fraction' = fraction of emitted CO₂ remaining in atmosphere. Long-term average ~45%. Year-to-year varies 30-60% with ENSO and other variability. The remarkable stability of the long-term average has been an empirical regularity but is not guaranteed to continue — sink saturation could push it higher.
Natural variability
ENSO (El Niño/La Niña) cycles produce major year-on-year variability. El Niño years (warmer, drier tropical land): land sink weakens, drops 1-2 GtCO₂/year. La Niña years: land sink strengthens. The 2023-24 El Niño contributed to record atmospheric CO₂ growth (~3.4 ppm in 2024). Climate variability shapes the noise; emissions shape the signal.