Carbon Cycle: Largest Reservoir

The global carbon cycle involves the exchange of carbon atoms among the atmosphere, oceans, land (biosphere and soils), and geological reserves (rocks, fossil fuels). Carbon exists in various forms in these different parts of the Earth system, constantly moving between them through natural processes like photosynthesis, respiration, decomposition, combustion, and volcanic activity.

Understanding where most of the Earth's carbon is stored is key to comprehending the carbon cycle. Let's look at the main reservoirs:

• Atmosphere: Carbon is present primarily as carbon dioxide ($\text{CO}_2$). This is a relatively small, but very active, reservoir.
• Plant biomass on land: Carbon is stored in living vegetation, like trees, plants, and crops, as organic matter.
• Soils: This reservoir includes organic carbon from dead plants and animals, as well as inorganic carbon compounds. It's larger than the atmosphere and land biomass combined.
• The ocean: The ocean is the largest active reservoir of carbon. Carbon is stored in various forms, including dissolved inorganic carbon (like bicarbonate ions), dissolved organic carbon, and within marine organisms and sediments.
• Geological reservoirs: These include rocks (like carbonates) and fossil fuels (coal, oil, natural gas). This is by far the largest reservoir overall, but the carbon here is locked away for millions of years and is not actively cycling on human timescales unless disturbed by geological processes or human extraction and burning.

Comparing the sizes of the active reservoirs:

• Atmosphere: Approximately 800 gigatonnes of carbon (Gt C)
• Plant biomass on land: Approximately 550 Gt C
• Soils: Approximately 2,300 Gt C
• Ocean: Approximately 38,000 to 40,000 Gt C

Based on these figures, the ocean holds significantly more carbon than the atmosphere, land biomass, and soils combined, making it the largest active reservoir in the global carbon cycle.