Carbon Cycle Calculators

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The carbon cycle describes the movement of carbon atoms between reservoirs: the atmosphere (CO₂, CH₄), terrestrial biosphere (plants, soil, organic matter), ocean (dissolved CO₂, marine organisms, sediments), and geosphere (fossil fuels, carbonate rocks). Key biological fluxes are: photosynthesis (CO₂ → organic carbon, ~120 Pg C/yr globally) and respiration and decomposition (organic carbon → CO₂). Human activities — primarily fossil fuel combustion (~10 Pg C/yr) and land use change (~1.5 Pg C/yr) — add carbon to the atmosphere faster than natural sinks can remove it, raising atmospheric CO₂ from 280 ppm (pre-industrial) to ~422 ppm (2024).

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Major Carbon Reservoirs

  • Atmosphere: ~870 Pg C as CO₂ (~422 ppm) + CH₄
  • Terrestrial biosphere: ~600 Pg C in living biomass; ~2,500 Pg C in soil organic matter
  • Ocean: ~38,000 Pg C total (surface ~900 Pg C; deep ocean ~37,000 Pg C)
  • Fossil fuels: ~3,700 Pg C (coal, oil, gas reserves)
  • Rocks (carbonate): ~100,000,000 Pg C (slowest reservoir)

Key Carbon Fluxes (Pg C/yr)

Gross photosynthesis (GPP): ~120. Plant respiration: ~60. Net primary production (NPP): ~60. Soil respiration + decomposition: ~60. Ocean uptake: ~2.6. Fossil fuel combustion: ~10. Land use change: ~1.5. Net atmospheric increase: ~5 Pg C/yr.

Ocean Carbon Chemistry

CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺ ⇌ CO₃²⁻ + 2H⁺. Ocean acidification: rising CO₂ → more H₂CO₃ → lower pH → threatens calcareous organisms (corals, shellfish, foraminifera). Ocean pH has dropped from 8.2 to ~8.1 since pre-industrial (0.1 pH unit = 26% increase in [H⁺]).

Glossary

Carbon Cycle
Movement of carbon between atmosphere, biosphere, ocean, and geosphere; photosynthesis fixes ~120 Pg C/yr; fossil fuels add ~10 Pg C/yr; net atmospheric accumulation ~5 Pg C/yr → rising CO₂.
Net Primary Production (NPP)
GPP − plant respiration; ~60 Pg C/yr globally; the organic carbon available to consumers and decomposers; drives ecosystem energy and determines how much carbon is available to soil and food webs.
Ocean Acidification
Reduction in ocean pH from rising CO₂ absorption; pH decreased 8.2→8.1 since pre-industrial (26% increase in [H⁺]); threatens calcareous organisms (corals, shellfish) by reducing carbonate ion availability.

Frequently Asked Questions

Carbon cycles between four major reservoirs: Atmosphere: ~870 Pg C (mostly CO₂ at ~422 ppm; small amount as CH₄). Terrestrial biosphere: ~600 Pg C in living vegetation; ~2,500 Pg C in soil organic matter; ~500 Pg C in permafrost (increasingly thawing). Ocean: ~38,000 Pg C total; surface ocean exchanges rapidly with atmosphere; deep ocean stores ancient carbon. Geosphere: fossil fuels (~3,700 Pg C recoverable); carbonate rocks (~100,000,000 Pg C — very slow cycle). Key fluxes: GPP (photosynthesis) ≈ 120 Pg C/yr fixes CO₂; respiration and decomposition return ~115 Pg C/yr to atmosphere; ocean absorbs ~2.6 Pg C/yr; fossil fuels add ~10 Pg C/yr.

Photosynthesis is the dominant carbon fixation process: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Global GPP ≈ 120 Pg C/yr (about 30% removed from the atmosphere annually — but the same amount returned by respiration and decomposition, so the biosphere is roughly carbon neutral in the absence of fossil fuel additions). NPP = GPP − plant respiration ≈ 60 Pg C/yr: this organic carbon is available to heterotrophs (animals, decomposers). Soil respiration (microbial decomposition) returns most NPP to the atmosphere as CO₂. Small fraction is buried in sediments → may form fossil fuels over millions of years.

Fossil fuel combustion adds ~10 Pg C/yr to the atmosphere — carbon that was sequestered from the atmosphere over millions of years by photosynthesis and buried as organic sediment. This is ~10× faster than natural geological carbon cycling. Of the ~11.5 Pg C/yr total anthropogenic emissions: ~2.6 Pg C/yr is absorbed by the ocean (causing acidification). ~3.1 Pg C/yr is absorbed by land vegetation (net land sink). ~5.8 Pg C/yr remains in the atmosphere → increasing CO₂ at ~2.4 ppm/yr. Cumulative effect: atmospheric CO₂ has risen from 280 ppm (pre-industrial) to ~422 ppm (2024) — a 51% increase, raising global average temperature ~1.2°C above pre-industrial baseline.

Ocean acidification: as atmospheric CO₂ increases, more dissolves in the ocean: CO₂ + H₂O → H₂CO₃ → HCO₃⁻ + H⁺. More H⁺ → lower pH. Ocean pH has decreased from 8.2 to ~8.1 since pre-industrial. This 0.1 unit decrease represents a 26% increase in [H⁺] (pH is logarithmic). Consequences: Reduced carbonate ion (CO₃²⁻) availability → harder for marine organisms to build calcium carbonate (CaCO₃) shells and skeletons. Most vulnerable: corals (tropical and cold-water), oysters and clams, pteropods (sea snails), foraminifera, sea urchins. Coral bleaching is also driven by warming — ocean acidification is a compounding stressor. At current emission trajectories: pH projected to fall to ~7.95 by 2100 — beyond the range experienced for millions of years.