Carbon Cycle

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the atmosphere, oceans, terrestrial biosphere, and geological reservoirs. It is the primary regulatory system governing Earth's climate over timescales from years to millions of years, and it is the cycle most directly perturbed by human activity.

The cycle operates through two distinct loops: the fast carbon cycle, driven by photosynthesis, respiration, and ocean-atmosphere exchange on timescales of years to centuries; and the slow carbon cycle, driven by rock weathering, volcanic outgassing, and sediment burial on timescales of millions of years. The fast cycle is a feedback system: higher CO₂ increases photosynthesis, which draws CO₂ down; warmer temperatures increase soil respiration, which releases CO₂. The net effect depends on which process dominates, and that dominance itself changes with temperature, moisture, and nutrient availability.

Human activity has perturbed the carbon cycle by approximately 40%. Atmospheric CO₂ has risen from 280 ppm in pre-industrial times to over 420 ppm today, a rate of increase unprecedented in at least the past 800,000 years. The capacity of natural sinks — oceans and vegetation — to absorb this excess is itself changing as the system warms, raising the possibility that the carbon cycle could transition from a net sink to a net source. This is not a distant risk. It is the central question of Earth system science in the 21st century.