Outline the carbon cycle, including major reservoirs and fluxes, and explain how human activities disrupt it.

• A. Carbon moves among atmosphere, biosphere, oceans, and sediments; human activities increase atmospheric CO2.
• B. Carbon remains permanently in rocks with no exchange.
• C. Carbon only cycles within the atmosphere.
• D. Humans remove carbon from the atmosphere completely.

Answer: (A)

The question is about how carbon moves through the Earth system and how human activity alters that movement. The best answer reflects the idea that carbon travels among the atmosphere, the biosphere (plants, animals, and soils), the oceans, and sediments/rocks, with several important fluxes tying these reservoirs together. Plants take up CO2 from the air during photosynthesis, storing carbon in biomass and soil organic matter. When plants respire, when they die and decompose, or when soils are disturbed, that carbon is released back to the atmosphere or to the soil. The oceans exchange CO2 with the atmosphere at the surface: CO2 dissolves in seawater, becomes dissolved inorganic carbon, and some is used by marine organisms to form shells and skeletons. When organisms die, some of that carbon becomes part of sediments, leading to long-term storage, while other carbon can be buried in sediment and rocks over geologic timescales. Weathering of rocks draws down atmospheric CO2 and, through geochemical processes, helps form carbonates that are stored long term; volcanism and tectonic processes later release some of that carbon back to the atmosphere, completing the cycle on long timescales. Human activities disrupt this balance mainly by adding large amounts of CO2 to the atmosphere through burning fossil fuels and cement production, reducing carbon storage in forests and soils through land-use change, and altering ocean chemistry and circulation, which can lessen the ocean’s capacity to absorb CO2. This combination raises atmospheric CO2 levels and shifts the natural fluxes among reservoirs.