Environment · Simulator 088
Carbon moves between the air, the surface ocean, the deep ocean and plants and soils. Fossil fuel emissions add carbon to the air, and the ocean and land slowly take some of it back. Choose a future emissions path and change how quickly each reservoir exchanges carbon, then track CO₂ in the air and where the carbon ends up.
The model is a box model: four reservoirs with preindustrial stocks of 590 GtC (atmosphere), 900 (surface ocean), 37,100 (deep ocean) and 2,300 (vegetation and soils). Each pair exchanges carbon at rates that balance exactly before 1850 (60, 90 and 120 GtC per year). Emissions E enter the atmosphere:
dA/dt = E − F(air→ocean) + F(ocean→air) − F(photosynthesis) + F(respiration)
Three effects shape the uptake. The Revelle (buffer) factor R: seawater chemistry means a 1% rise in dissolved carbon raises the ocean's CO₂ pressure by about R%, so the surface ocean fills up quickly (R ≈ 10 today). Deep mixing slowly carries carbon down, where it stays for centuries. CO₂ fertilization: plants grow faster in richer air, so photosynthesis rises by β × ln(A/A₀).
Atmospheric CO₂ in ppm is A / 2.12. The airborne fraction is the share of all carbon ever emitted that is still in the air. Historical emissions (fossil fuels and land use) are modelled as a smooth rise reaching 11 GtC per year in 2020, about 700 GtC in total.
A teaching model: linear exchanges, no temperature feedbacks (warming oceans and soils release carbon in reality), no sediments or weathering, and only rough agreement with history. Complex Earth-system models differ in how quickly CO₂ falls after emissions stop.
In the constant-emissions scenario, the airborne fraction slowly falls after 2020, yet CO₂ keeps rising. How can both be true?
The airborne fraction is the carbon remaining in the air divided by everything ever emitted. Cumulative emissions keep growing by 11 GtC every year, and the sinks grow too as CO₂ rises, so the fraction drifts down (0.39 in 2020, about 0.34 in 2100). But the airborne amount still grows every year, because the sinks take up less than 11 GtC per year. CO₂ can only stabilize when emissions fall to the size of the sinks.