Earth's Carbon Budget
Carbon cycles continuously between four major reservoirs: the atmosphere (~870 GtC), land biosphere (~2,000 GtC in vegetation and soils), oceans (~38,000 GtC), and lithosphere (~75,000,000 GtC in rocks and fossil fuels). Human activities — primarily fossil fuel combustion and deforestation — have transferred about 660 GtC from geological reservoirs to the atmosphere since 1750, raising CO₂ from 280 to over 420 ppm.
Sources and Sinks
Each year, about 10 GtC enter the atmosphere from fossil fuels and 1.1 GtC from deforestation. Natural sinks partially offset this: the ocean absorbs ~2.5 GtC through CO₂ dissolution and the biological pump, while land ecosystems absorb ~3.1 GtC through enhanced photosynthesis (CO₂ fertilization). The remaining ~45% accumulates in the atmosphere, driving climate change.
The Revelle Factor
The ocean's ability to absorb CO₂ is limited by carbonate chemistry. As dissolved CO₂ increases, the buffer capacity decreases — a relationship quantified by the Revelle factor. Currently around 10, the Revelle factor means only about 1/10 of added CO₂ is neutralized by carbonate ions. As the ocean absorbs more CO₂, the Revelle factor increases, making the ocean a progressively less effective sink.
Long-Term Fate
The timescale for CO₂ removal spans orders of magnitude. About half of emissions are absorbed within decades by ocean mixing and biological uptake. The remaining CO₂ decreases slowly over centuries through deep ocean circulation and over millennia through reaction with ocean sediments. Full removal requires 100,000+ years of silicate rock weathering. This means every ton of CO₂ emitted today will influence climate for thousands of years.