The Earth’s carbon cycle has two natural segments — the slow carbon cycle, and the fast carbon cycle.
The fast carbon cycle involves Earth’s atmosphere, biosphere, and upper ocean. Carbon in this cycle circulates more quickly (on human and other organic timescales), and can be best understood as the flow of carbon through living ecosystems.
The slow carbon cycle consists of the movement of carbon via gravity, pressure, chemical weathering, and ocean overturning circulation on geological timescales. These processes move carbon from living ecosystems into geological and deep ocean reservoirs such as sediments, mineral deposits (oil, gas, coal), and deep waters.
The fast and slow carbon cycles are both part of Earth’s natural systems — and carbon can move between these fast and slow cycles in a number of ways. However, humans have disrupted and destabilized the natural cadence of these cycles.