3.1 Global Carbon Reservoirs and Fluxes

Carbon, the building block of life, cycles through Earth's major reservoirs: atmosphere, biosphere, oceans, and lithosphere. This dynamic process involves complex exchanges and transformations, shaping our planet's climate and ecosystems.

Understanding carbon cycle dynamics is crucial for grasping global environmental changes. From rapid atmospheric turnover to long-term storage in rocks, carbon's journey through various reservoirs impacts everything from short-term weather patterns to long-term climate stability.

Global Carbon Reservoirs

Major global carbon reservoirs

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• Atmosphere stores ~750 Gt carbon primarily as CO2 impacts global climate and weather patterns
• Terrestrial biosphere contains 2,000-3,000 Gt carbon in living biomass and soil organic matter influences ecosystem productivity
• Oceans hold largest active reservoir ~38,000 Gt carbon mostly as dissolved inorganic carbon regulates atmospheric CO2 levels
• Lithosphere harbors 75,000,000 Gt carbon in sedimentary rocks and 5,000 Gt in fossil fuels (coal, oil, natural gas) drives long-term carbon cycle

Carbon fluxes between reservoirs

• Atmosphere-terrestrial biosphere exchange through photosynthesis removes CO2 while respiration and decomposition release it back affects seasonal CO2 variations
• Atmosphere-ocean exchange via CO2 dissolution and outgassing at air-sea interface driven by partial pressure differences and solubility influences ocean pH
• Terrestrial-ocean transfer through river transport of dissolved and particulate organic carbon and coastal erosion impacts marine ecosystems
• Lithosphere-atmosphere exchange from volcanic emissions releases CO2 while weathering of carbonate rocks consumes it affects long-term climate stability

Carbon Cycle Dynamics

Carbon residence times

• Atmosphere: 5-7 years rapid turnover impacts short-term climate variability
• Terrestrial biosphere:
  • Living biomass: 1-100 years varies by ecosystem type (grasslands vs forests)
  • Soil organic matter: 10-10,000 years depends on depth and environmental conditions
• Oceans:
  • Surface waters: 5-10 years influences air-sea gas exchange
  • Deep ocean: 200-1000 years affects long-term carbon storage
• Lithosphere:
  • Sedimentary rocks: millions of years controls long-term climate regulation
  • Fossil fuels: millions of years formation time exceeds human timescales

Balance in global carbon cycle

• Natural carbon cycle maintained roughly balanced fluxes in pre-industrial era with seasonal variations due to terrestrial biosphere activity (leaf growth, senescence)
• Anthropogenic perturbations disrupt balance:
  1. Fossil fuel combustion adds 9-10 Gt C/year to atmosphere
  2. Deforestation releases 1-2 Gt C/year reducing carbon sink capacity
• Carbon sinks mitigate some anthropogenic emissions:
  • Oceans absorb ~25% of anthropogenic CO2 leads to ocean acidification
  • Terrestrial biosphere uptake varies annually depends on climate conditions
• Atmospheric CO2 increase:
  • ~45% of emissions remain in atmosphere not absorbed by natural sinks
  • Current rate ~2 ppm per year accelerating compared to geological past
• Implications of imbalance:
  • Climate change due to enhanced greenhouse effect alters global temperature patterns
  • Ocean acidification from increased CO2 dissolution threatens marine ecosystems (coral reefs)