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Carbon is stored in four major places, called reservoirs: the atmosphere (as CO2), the oceans (as dissolved CO2 and bicarbonate), the biosphere (every living plant and animal), and the lithosphere (rocks, soils, shells, and fossil fuels like coal, oil, and natural gas). The atmosphere actually holds a relatively small fraction of Earth's total carbon. Most of it is locked up in the oceans, in living things, and in the ground.

Plants do absorb CO2 during photosynthesis and store the carbon in their tissues, but that storage is temporary. When the plant is eaten, the carbon moves into the animal that ate it. When the plant or animal dies, decomposers release the carbon back into the atmosphere as CO2 (and sometimes methane). When a forest burns, the stored carbon is released as CO2. The carbon cycle is a cycle precisely because nothing stays put forever.

The oceans are one of the largest carbon reservoirs on the planet. Seawater absorbs huge amounts of CO2 from the air at the surface, especially in cooler regions. Ocean plankton use that dissolved CO2 to photosynthesize. Marine animals build shells out of carbon-containing minerals like calcium carbonate, and when those animals die, their shells settle to the seafloor and eventually become rock. Without the ocean, the carbon cycle as we know it doesn't work.

Fossil fuels are carbon. Coal, oil, and natural gas formed from the buried, compressed remains of plants and microorganisms that lived millions of years ago. Those organisms originally pulled their carbon out of the atmosphere through photosynthesis. When humans burn fossil fuels today, that ancient carbon goes back into the atmosphere as CO2. Burning fossil fuels essentially shortcuts millions of years of the carbon cycle into a few decades, which is why it has such a noticeable effect on the air today.