Ecology
Carbon Cycle
Movement of carbon through atmosphere, ocean, biosphere, and lithosphere
The carbon cycle is the movement of carbon through Earth's systems: atmosphere (CO₂, CH₄), biosphere (organisms), oceans (dissolved CO₂, carbonate), lithosphere (rocks, fossil fuels). Key processes: photosynthesis (CO₂ → biomass), respiration (biomass → CO₂), decomposition, ocean absorption, weathering of rocks (slow), volcanic eruption, fossil fuel combustion. Pre-industrial: balanced cycle. Post-industrial: humans burning fossil fuels add ~10 Gt C/year — overwhelming natural balance; CO₂ accumulating in atmosphere. Driver of climate change. Critical for: climate, ecosystems, agriculture.
- Carbon reservoirsAtmosphere, biosphere, oceans, lithosphere
- Atmospheric CO₂280 ppm pre-industrial; 420+ ppm 2024
- PhotosynthesisMajor flux into biosphere
- RespirationMajor flux out of biosphere
- Anthropogenic~10 Gt C/year added (fossil fuels + deforestation)
- Oceans store~50× atmospheric carbon
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Why carbon cycle matters
- Climate change. CO₂ accumulation drives warming.
- Ecosystem function. Energy flow.
- Agriculture. CO₂ enrichment effects.
- Conservation. Forest protection.
- Carbon sequestration. Removing CO₂ from atmosphere.
- Ocean health. Acidification.
- Energy policy. Fossil fuel emissions.
Common misconceptions
- CO₂ is plant food only. Greenhouse gas; warming effect.
- Cycle balanced now. Humans disrupt; CO₂ accumulating.
- Trees solve climate. Help; not enough alone.
- Oceans unaffected. Acidification major problem.
- Cycle simple. Multiple reservoirs, fluxes, feedbacks.
- Volcanoes contribute most CO₂. Humans far exceed.
Frequently asked questions
What are carbon reservoirs?
Where carbon is stored. (1) Atmosphere: ~870 Gt C (mostly CO₂). (2) Oceans: ~38,000 Gt C (50× atmosphere; mostly dissolved + carbonates). (3) Land biosphere: ~2,300 Gt C (plants, soil, microbes). (4) Lithosphere: ~80,000,000 Gt C (rocks, fossil fuels — most carbon). Each reservoir cycles carbon at different rates. Atmosphere: small but central; affected by changes most.
What's the fast carbon cycle?
Photosynthesis-respiration loop. Plants take up CO₂ + sunlight → biomass. Respiration (animals, plants, decomposers): biomass → CO₂. Returns to atmosphere. Cycle: years to decades. Annual swing: ~120 Gt C through this cycle (much more than human emissions). Photosynthesis ~120 Gt C/year; respiration ~120 Gt C/year. Roughly balanced naturally.
What's the slow carbon cycle?
Geological cycle. (1) Weathering: rocks dissolve in rain (CO₂ in water → carbonic acid → erodes rock); calcium carbonate deposited in oceans. (2) Carbonate accumulation in marine sediments → eventually buried as rock. (3) Volcanic eruptions: release CO₂ from rocks back to atmosphere. (4) Tectonic subduction. Cycle: millions of years. Massive carbon storage.
How do humans disrupt the cycle?
Two main ways. (1) Fossil fuel burning: ~9-10 Gt C/year released. Burns ancient buried carbon (millions of years to deposit; released in seconds). (2) Deforestation: ~1 Gt C/year. Trees stored carbon; burned/decomposed → CO₂. Total: ~10 Gt C/year added. Atmosphere absorbs ~half; rest absorbed by oceans (acidification) and land (CO₂ fertilization).
What's ocean acidification?
Result of CO₂ uptake by oceans. CO₂ + H₂O → H₂CO₃ (carbonic acid) → H⁺ + HCO₃⁻. Lowers ocean pH. Pre-industrial pH ~8.2; now ~8.1; expected ~7.8 by 2100. Effects: dissolves calcium carbonate (corals, shellfish, plankton with shells); makes shell formation harder; ecosystem disruption. ~30% of human CO₂ emissions go to oceans.
What about feedback loops?
Climate-carbon feedbacks affect future. Some positive (amplifying): (1) Permafrost melt → release of methane and CO₂. (2) Forest fires → release stored carbon. (3) Reduced ocean uptake at higher T. (4) Tropical forests stressed → reduced sink. Some negative (dampening): (1) Plant growth boosted by CO₂ (CO₂ fertilization). (2) Ocean uptake initially. Complex; net effect: amplifying.
How is carbon measured?
Multiple methods. (1) Atmospheric: continuous monitoring at Mauna Loa (since 1958), other stations. (2) Biomass: forest inventories, satellite remote sensing. (3) Soil: sampling and analysis. (4) Oceans: chemistry of seawater; dissolved C measured. (5) Isotopes: ¹³C/¹²C ratio identifies source (fossil fuel C lighter due to plant origin). Sophisticated global modeling integrates data.