Describe the carbon cycle and the role of the atmosphere, hydrosphere, biosphere, and geosphere in carbon storage and flux.

• A. Carbon only cycles between the atmosphere and oceans.
• B. Carbon moves among reservoirs via photosynthesis, respiration, decomposition, dissolution in water, weathering, and sedimentation; human activities boost atmospheric CO2 fluxes.
• C. Carbon is stored only in rocks and never released.
• D. The carbon cycle does not involve the biosphere.

Answer: (B)

Understanding the carbon cycle means seeing how carbon is stored in multiple reservoirs and moved between them through a set of biological, chemical, and geological processes. Carbon resides in the atmosphere as CO2 (and other carbon gases), in the oceans as dissolved inorganic carbon and carbonate minerals, in living and dead organisms and soils as organic carbon, and in rocks and sediments as carbonate minerals and fossil fuels. The movements among these stores are driven by key processes. Photosynthesis pulls CO2 from the atmosphere into plant and algal biomass, while respiration and decomposition release CO2 back to air or water. In the oceans, carbon exchanges with the atmosphere through gas transfer and chemistry that forms dissolved carbon species, linking the hydrosphere and atmosphere. Weathering of rocks consumes atmospheric CO2 over long timescales and moves carbon into the oceans and sediments, where it can be buried as organic matter or carbonate rocks. Sedimentation and long-term storage in sediments and rocks remove carbon from active cycling for extended periods, with tectonic processes eventually resetting the cycle. Human activities—burning fossil fuels, deforestation, and cement production—add more CO2 to the atmosphere, increasing the flux from the biosphere and geosphere to the air. This description captures how all four spheres participate in storage and flux, not just a subset of interactions.