The four major reservoirs
The carbon cycle moves carbon among four reservoirs: the atmosphere (as carbon dioxide and methane), the oceans (dissolved CO2, bicarbonate, and marine life), the biosphere (plants, animals, and soils), and the lithosphere (rocks, sediments, and fossil fuels), which is by far the largest but slowest-exchanging store.
Natural fluxes such as photosynthesis, respiration, and ocean-atmosphere gas exchange are large but were roughly balanced over the long term before industrialization.
How human activity unbalanced it
Burning fossil fuels transfers carbon from the slow lithospheric store into the atmosphere far faster than natural sinks can absorb it. Land and ocean sinks take up roughly half of human emissions, but the remainder accumulates, which is why atmospheric CO2 has risen from about 280 parts per million before the industrial era to over 420 today. This added forcing is the primary driver of modern climate change.