The global carbon cycle determines how much of our CO₂ emissions stay in the atmosphere and how much is absorbed by natural sinks. Understanding this cycle is essential for projecting future warming and setting emission reduction targets.
This simulator implements a box model with four reservoirs: atmosphere, ocean, land biosphere, and fossil fuels. Carbon flows between reservoirs at rates determined by physical and biological processes. The key equation is dC_atm/dt = emissions - ocean_uptake - land_uptake, where ocean uptake is proportional to the difference between atmospheric CO₂ and a pre-industrial equilibrium.
The airborne fraction — the share of emissions remaining in the atmosphere — is one of the most important numbers in climate science. Observations show it has been remarkably stable at ~44% for decades, meaning natural sinks have scaled up proportionally with emissions. But climate models suggest this may not continue: as the ocean warms and its surface layer saturates with CO₂, the ocean sink weakens. Land sinks face threats from drought, fire, and deforestation.
Hansen et al. (1988) used early versions of this carbon cycle framework in the seminal paper that brought climate change to public attention through testimony to the U.S. Congress. Their projections for atmospheric CO₂ growth have proven remarkably accurate.
Deforestation acts as both a source (releasing stored carbon) and a sink reducer (removing photosynthetic capacity). Tropical forests contain roughly 250 GtC in biomass — comparable to decades of fossil fuel emissions. Their preservation is both a climate and biodiversity imperative.