Carbon in Motion
Carbon cycles endlessly between four great reservoirs: the atmosphere (870 GtC), the ocean (38,000 GtC), land biosphere (2,000 GtC), and lithosphere (fossil fuels and rocks, >60 million GtC). Natural fluxes — photosynthesis, respiration, ocean gas exchange, and weathering — moved roughly equal amounts in and out of each reservoir for millennia. Since the Industrial Revolution, burning fossil fuels has injected an additional ~10 GtC per year into the atmosphere, disrupting this balance and driving global warming.
Sources and Sinks
Of the ~10 GtC emitted annually by fossil fuels (plus ~1 GtC from deforestation), about 25% is absorbed by the ocean and 30% by the land biosphere, leaving roughly 44% — the airborne fraction — accumulating in the atmosphere. The ocean sink operates through surface dissolution and the biological pump; the land sink works through enhanced photosynthesis (CO₂ fertilization) and regrowth of previously cleared forests. Both sinks show signs of approaching saturation as warming progresses.
Feedback Loops
The carbon cycle contains both negative and positive feedbacks. CO₂ fertilization and ocean uptake are negative feedbacks that partially offset emissions. But warming also releases carbon: thawing permafrost liberates ancient methane and CO₂, warming oceans hold less dissolved gas, and drought-stressed forests burn and die. This simulation models these competing effects, showing how the airborne fraction and atmospheric CO₂ respond to different emission scenarios over decades to centuries.
Pathways to Stabilization
Stabilizing atmospheric CO₂ at any level requires reducing net emissions to near zero — the natural sinks must balance remaining sources. The Paris Agreement target of 1.5°C implies cutting emissions roughly in half by 2030 and reaching net zero by 2050. Even after emissions stop, ocean circulation takes centuries to equilibrate, meaning committed warming and sea-level rise continue long after the last fossil fuel is burned.