9.1 Global Carbon Cycle and Climate Change Implications

The global carbon cycle is a complex system of carbon exchange between Earth's reservoirs. It involves atmospheric CO2, ocean carbon, terrestrial biosphere, and lithosphere. Understanding this cycle is crucial for grasping climate change and its impacts.

Human activities have significantly disrupted the natural carbon balance. Fossil fuel burning, deforestation, and industrial processes release excess CO2, leading to global warming and ocean acidification. Mitigation strategies and adaptation measures are essential to address these challenges.

Global Carbon Cycle

Reservoirs and fluxes of carbon

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• Carbon reservoirs store carbon in various forms
  • Atmosphere contains CO2, CH4, and other greenhouse gases
  • Oceans hold dissolved inorganic carbon, organic matter, and marine life
  • Terrestrial biosphere encompasses plants, animals, and soil organic matter
  • Lithosphere includes fossil fuels, sedimentary rocks, and minerals
• Major carbon fluxes transfer carbon between reservoirs
  • Atmosphere-ocean exchange involves CO2 absorption and release
  • Photosynthesis and respiration cycle carbon between atmosphere and biosphere
  • Weathering of rocks slowly removes atmospheric CO2 over geological timescales
  • Volcanic emissions release CO2 from Earth's interior to the atmosphere
• Quantification of reservoirs and fluxes measured in gigatons of carbon (GtC)
  • Atmosphere: ~830 GtC
  • Oceans: ~38,000 GtC
  • Terrestrial biosphere: ~2,000 GtC
  • Lithosphere: >60,000,000 GtC
• Timescales of carbon cycling vary widely
  • Short-term cycles occur within days to years (seasonal plant growth)
  • Long-term cycles span centuries to millennia (ocean circulation, rock weathering)

Photosynthesis and respiration in carbon cycling

• Photosynthesis captures atmospheric CO2 and converts it to organic matter
  • Carbon dioxide fixation occurs in chloroplasts
  • Light-dependent reactions generate ATP and NADPH
  • Calvin cycle uses energy from light reactions to produce glucose
  • Net primary production represents total carbon fixed minus plant respiration
• Respiration releases CO2 back to the atmosphere
  • Cellular respiration breaks down organic molecules for energy
  • Aerobic respiration uses oxygen, while anaerobic respiration doesn't
  • Autotrophic respiration by plants, heterotrophic respiration by animals and decomposers
• Carbon flux balance determines net ecosystem carbon exchange
  • Gross primary production measures total carbon fixed by photosynthesis
  • Net ecosystem production equals gross primary production minus ecosystem respiration
• Seasonal variations in CO2 levels reflect changing balance of photosynthesis and respiration
  • Lower atmospheric CO2 in Northern Hemisphere summer due to increased plant growth
  • Higher CO2 in winter when respiration dominates over reduced photosynthesis
• Factors affecting photosynthesis and respiration rates impact carbon cycling
  • Temperature influences enzyme activity and metabolic rates
  • Water availability affects stomatal opening and cellular processes
  • Nutrient availability, especially nitrogen and phosphorus, limits plant growth

Human Impacts and Climate Change

Human impact on carbon cycle

• Anthropogenic CO2 emissions disrupt natural carbon balance
  • Fossil fuel combustion releases ~9 GtC/year
  • Deforestation and land-use changes contribute ~1.5 GtC/year
  • Industrial processes like cement production add ~0.5 GtC/year
• Perturbations to natural carbon fluxes alter ecosystem functioning
  • Ocean acidification threatens marine ecosystems and calcifying organisms
  • Changes in terrestrial carbon storage affect soil fertility and biodiversity
• Greenhouse effect enhancement amplifies global warming
  • Radiative forcing measures climate impact of increased greenhouse gases
  • Feedback mechanisms (albedo changes, water vapor) can amplify or dampen warming
• Observed climate change impacts manifest globally
  • Global temperature increase of ~1℃ since pre-industrial times
  • Sea level rise of ~3.3 mm/year threatens coastal areas
  • Extreme weather events become more frequent and intense (hurricanes, droughts)
• Carbon cycle-climate feedbacks potentially accelerate warming
  • Permafrost thawing releases stored carbon as CO2 and CH4
  • Ocean circulation changes alter carbon uptake and heat distribution
  • Forest dieback reduces carbon storage capacity and alters albedo

Strategies for carbon-related climate change

• Mitigation strategies aim to reduce greenhouse gas emissions
  • Renewable energy adoption (solar, wind, geothermal) decreases fossil fuel dependence
  • Carbon capture and storage technologies remove CO2 from point sources or atmosphere
  • Reforestation and afforestation increase terrestrial carbon sinks
  • Improved agricultural practices reduce emissions and enhance soil carbon sequestration
• Adaptation strategies help cope with unavoidable climate impacts
  • Coastal protection measures (sea walls, mangrove restoration) guard against sea level rise
  • Water resource management addresses changing precipitation patterns
  • Agricultural adaptations include drought-resistant crops and improved irrigation
  • Urban planning and infrastructure design consider future climate scenarios
• Policy approaches guide collective action on climate change
  • International agreements (Paris Agreement) set global emission reduction targets
  • Carbon pricing mechanisms internalize environmental costs of emissions
  • Emissions trading systems create market incentives for reducing greenhouse gases
• Technological innovations offer potential solutions
  • Negative emission technologies actively remove CO2 from the atmosphere
  • Enhanced weathering accelerates natural CO2 removal by rock weathering
  • Ocean iron fertilization stimulates phytoplankton growth to increase carbon uptake
• Challenges and limitations complicate implementation of strategies
  • Economic considerations include costs of transition and potential job losses
  • Technological feasibility varies for different mitigation and adaptation approaches
  • Social and political barriers hinder adoption of climate policies
• Integrated assessment models inform decision-making
  • Projecting future scenarios helps anticipate climate impacts and policy outcomes
  • Cost-benefit analysis of mitigation strategies guides resource allocation