7.3 Global carbon cycle

The global carbon cycle is a crucial component of Earth's climate system, regulating atmospheric CO2 levels and influencing long-term temperature trends. It involves complex interactions between terrestrial, oceanic, and atmospheric reservoirs, with carbon constantly moving between these components through various processes.

Understanding carbon fluxes, reservoirs, and feedback mechanisms is essential for predicting future climate scenarios and developing effective mitigation strategies. Human activities have significantly altered the natural carbon cycle, leading to increased atmospheric CO2 concentrations and associated climate impacts.

Components of carbon cycle

• Carbon cycle plays a crucial role in atmospheric physics by regulating Earth's climate through greenhouse gas dynamics
• Understanding carbon reservoirs provides insights into carbon storage and exchange processes that impact atmospheric composition
• Carbon cycle components interact with other Earth systems, influencing weather patterns and long-term climate trends

Terrestrial carbon reservoirs

Top images from around the web for Terrestrial carbon reservoirs

• 

  Biogeochemical Cycles · Microbiology View original

  Is this image relevant?

• 

  Soil carbon | Environment, land and water | Queensland Government View original

  Is this image relevant?

• 

  Biogeochemical Cycles and the Flow of Energy in the Earth System | Sustainability: A ... View original

  Is this image relevant?

• 

  Biogeochemical Cycles · Microbiology View original

  Is this image relevant?

• 

  Soil carbon | Environment, land and water | Queensland Government View original

  Is this image relevant?

1 of 3

Top images from around the web for Terrestrial carbon reservoirs

• 

  Biogeochemical Cycles · Microbiology View original

  Is this image relevant?

• 

  Soil carbon | Environment, land and water | Queensland Government View original

  Is this image relevant?

• 

  Biogeochemical Cycles and the Flow of Energy in the Earth System | Sustainability: A ... View original

  Is this image relevant?

• 

  Biogeochemical Cycles · Microbiology View original

  Is this image relevant?

• 

  Soil carbon | Environment, land and water | Queensland Government View original

  Is this image relevant?

1 of 3

• Soil organic matter stores approximately 1500-2400 gigatons of carbon
• Vegetation biomass contains roughly 450-650 gigatons of carbon
• Permafrost regions hold an estimated 1300-1600 gigatons of carbon
  • Thawing permafrost can release significant amounts of greenhouse gases
• Terrestrial reservoirs exchange carbon with the atmosphere through processes like photosynthesis and decomposition

Oceanic carbon reservoirs

• Surface ocean contains about 900 gigatons of dissolved inorganic carbon
• Deep ocean stores approximately 37,000 gigatons of carbon
  • Largest carbon reservoir in the fast carbon cycle
• Marine sediments hold vast amounts of carbon in the form of calcium carbonate
• Oceanic carbon uptake influences atmospheric CO2 concentrations and ocean acidification

Atmospheric carbon reservoir

• Atmosphere contains around 750 gigatons of carbon, primarily as CO2
• Atmospheric carbon concentration has increased from ~280 ppm to over 410 ppm since pre-industrial times
• Serves as a central hub for carbon exchange between terrestrial and oceanic reservoirs
• Changes in atmospheric carbon directly impact Earth's radiative balance and climate

Carbon fluxes

• Carbon fluxes represent the movement of carbon between different reservoirs in the Earth system
• Understanding carbon fluxes helps predict future atmospheric CO2 concentrations and climate change impacts
• Fluxes vary in magnitude and direction, influencing the overall carbon balance of the planet

Natural carbon exchanges

• Terrestrial photosynthesis removes about 123 gigatons of carbon from the atmosphere annually
• Plant and soil respiration release approximately 119 gigatons of carbon back to the atmosphere each year
• Ocean-atmosphere gas exchange results in a net absorption of about 2 gigatons of carbon by the oceans annually
• Volcanic emissions contribute a relatively small amount of carbon to the atmosphere (0.1-0.3 gigatons per year)

Anthropogenic carbon emissions

• Fossil fuel combustion and cement production release about 9.5 gigatons of carbon annually
• Land-use changes (deforestation) contribute approximately 1.5 gigatons of carbon to the atmosphere each year
• Industrial processes (steel production) emit additional carbon dioxide
• Anthropogenic emissions have significantly altered the natural carbon cycle balance

Carbon sinks vs sources

• Carbon sinks absorb more carbon than they release (oceans, growing forests)
• Carbon sources emit more carbon than they absorb (fossil fuel burning, deforestation)
• Net carbon flux determines whether a reservoir acts as a sink or source
• Some reservoirs can switch between sink and source depending on environmental conditions (El Niño events)

Carbon cycle processes

• Carbon cycle processes drive the movement of carbon between reservoirs in the Earth system
• These processes operate on various timescales, from seconds to millions of years
• Understanding these processes helps predict future changes in atmospheric CO2 and climate

Photosynthesis and respiration

• Photosynthesis converts atmospheric CO2 into organic compounds using sunlight energy
  • $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$
• Cellular respiration breaks down organic compounds to release energy and CO2
  • $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$
• Terrestrial ecosystems sequester carbon through net primary production
• Microbial decomposition in soils releases CO2 back to the atmosphere

Ocean-atmosphere gas exchange

• CO2 dissolves in seawater forming carbonic acid, bicarbonate, and carbonate ions
  • $CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow HCO_3^- + H^+ \leftrightarrow CO_3^{2-} + 2H^+$
• Solubility pump drives CO2 absorption in cold, high-latitude waters
• Biological pump transfers carbon from surface to deep ocean through marine organism activity
• Upwelling brings carbon-rich deep waters to the surface, potentially releasing CO2

Weathering and sedimentation

• Chemical weathering of silicate rocks consumes atmospheric CO2 over geological timescales
  • $CaSiO_3 + 2CO_2 + H_2O \rightarrow Ca^{2+} + 2HCO_3^- + SiO_2$
• Carbonate rock formation in oceans sequesters carbon for long periods
  • $Ca^{2+} + 2HCO_3^- \rightarrow CaCO_3 + CO_2 + H_2O$
• Tectonic processes return buried carbon to the surface through volcanism
• Weathering rates influence long-term atmospheric CO2 concentrations and climate stability

Biogeochemical cycles

• Biogeochemical cycles describe the movement of elements through Earth's systems
• Carbon cycle interactions with other elemental cycles affect atmospheric composition and climate
• Understanding these interactions helps predict ecosystem responses to environmental changes

Carbon-nitrogen interactions

• Nitrogen availability limits terrestrial carbon uptake in many ecosystems
• Increased nitrogen deposition can enhance plant growth and carbon sequestration
• Nitrogen fertilization practices impact soil carbon storage and greenhouse gas emissions
• Nitrification and denitrification processes in soils affect carbon and nitrogen cycling

Carbon-oxygen interactions

• Photosynthesis and respiration couple carbon and oxygen cycles
• Oceanic primary production influences dissolved oxygen concentrations
• Oxygen availability affects organic matter decomposition rates in soils and sediments
• Changes in atmospheric oxygen levels impact fossil fuel combustion efficiency

Carbon-phosphorus interactions

• Phosphorus often limits marine primary production, affecting carbon uptake
• Terrestrial phosphorus cycling influences plant growth and carbon sequestration
• Weathering of phosphate-containing rocks affects long-term carbon cycle dynamics
• Anthropogenic phosphorus inputs alter aquatic ecosystem carbon cycling

Carbon cycle feedbacks

• Carbon cycle feedbacks can amplify or dampen the effects of climate change
• Understanding these feedbacks helps improve climate model predictions
• Some feedbacks operate on short timescales, while others influence long-term climate trends

Climate-carbon feedbacks

• Warming-induced permafrost thaw releases stored carbon, potentially accelerating climate change
• Increased atmospheric CO2 enhances plant growth, leading to greater carbon uptake (CO2 fertilization effect)
• Ocean warming reduces CO2 solubility, potentially decreasing oceanic carbon uptake
• Changes in precipitation patterns affect terrestrial ecosystem carbon storage capacity

Biological feedbacks

• Shifts in species distributions and community composition alter ecosystem carbon dynamics
• Changes in growing season length impact terrestrial carbon uptake
• Ocean acidification affects marine calcifying organisms, influencing the oceanic carbon pump
• Soil microbial activity responds to temperature and moisture changes, affecting decomposition rates

Oceanic feedbacks

• Weakening of the Atlantic Meridional Overturning Circulation could reduce oceanic carbon uptake
• Changes in ocean stratification affect nutrient availability and marine primary production
• Alterations in sea ice cover impact air-sea gas exchange and marine ecosystem productivity
• Potential release of methane hydrates from ocean sediments could amplify warming

Carbon cycle modeling

• Carbon cycle models help simulate and predict carbon dynamics in the Earth system
• Models range from simple conceptual representations to complex Earth system models
• Improving model accuracy enhances our ability to project future climate change scenarios

Box models

• Represent carbon reservoirs as interconnected boxes with specified fluxes
• Useful for understanding basic carbon cycle dynamics and timescales
• Can be easily manipulated to explore different scenarios and sensitivities
• Limited in their ability to capture spatial heterogeneity and complex feedbacks

Earth system models

• Integrate carbon cycle processes with atmospheric, oceanic, and terrestrial components
• Include detailed representations of biogeochemical cycles and climate feedbacks
• Capable of simulating complex interactions between carbon cycle and climate system
• Require significant computational resources and extensive parameterization

Model uncertainties

• Incomplete understanding of some carbon cycle processes leads to parameterization uncertainties
• Limited observational data for model validation, especially for long-term processes
• Challenges in representing sub-grid scale processes and heterogeneity
• Uncertainties in future anthropogenic emissions scenarios affect model projections

Carbon cycle perturbations

• Human activities have significantly altered the natural carbon cycle
• Understanding these perturbations helps assess their impacts on climate and ecosystems
• Studying past perturbations provides insights into potential future changes

Fossil fuel combustion

• Releases approximately 9.5 gigatons of carbon into the atmosphere annually
• Increases atmospheric CO2 concentration at a rate of about 2.5 ppm per year
• Alters the isotopic composition of atmospheric CO2 (Suess effect)
• Leads to redistribution of carbon between atmosphere, land, and ocean reservoirs

Land use changes

• Deforestation releases stored carbon and reduces terrestrial carbon uptake capacity
• Agricultural practices affect soil carbon storage and greenhouse gas emissions
• Urbanization alters local carbon dynamics and creates urban heat islands
• Afforestation and reforestation efforts can enhance terrestrial carbon sequestration

Ocean acidification

• Approximately 25% of anthropogenic CO2 emissions are absorbed by the oceans
• Decreases ocean pH, affecting marine ecosystems and calcifying organisms
• Reduces the ocean's future capacity to absorb atmospheric CO2
• Alters marine biogeochemical cycles and ecosystem functioning

Carbon cycle measurements

• Accurate measurements are crucial for understanding carbon cycle dynamics
• Various techniques are employed to quantify carbon fluxes and reservoir sizes
• Continuous monitoring helps detect changes and trends in the carbon cycle

Atmospheric CO2 monitoring

• Mauna Loa Observatory provides the longest continuous record of atmospheric CO2 (Keeling Curve)
• Global network of monitoring stations (NOAA's Global Greenhouse Gas Reference Network)
• Flask sampling programs collect air samples for laboratory analysis
• Satellite-based measurements (OCO-2, GOSAT) provide global coverage of atmospheric CO2

Isotope tracers

• Carbon-13 (¹³C) measurements help distinguish between terrestrial and oceanic carbon sources
• Carbon-14 (¹⁴C) used to determine the age of carbon and identify fossil fuel contributions
• Oxygen isotopes (¹⁸O) provide information on photosynthesis and respiration processes
• Multiple isotope analyses improve understanding of carbon cycle processes and fluxes

Remote sensing techniques

• Satellite-based vegetation indices (NDVI) estimate terrestrial primary production
• LiDAR technology measures forest biomass and carbon storage
• Ocean color sensors assess marine primary productivity
• Gravity measurements (GRACE mission) detect changes in terrestrial water storage affecting carbon cycling

Carbon cycle timescales

• Carbon cycle processes operate on various timescales, from seconds to millions of years
• Understanding these timescales helps interpret past climate changes and predict future scenarios
• Different carbon reservoirs have distinct residence times and response rates to perturbations

Short-term carbon dynamics

• Diurnal cycles of photosynthesis and respiration affect atmospheric CO2 concentrations
• Seasonal variations in terrestrial vegetation drive annual atmospheric CO2 oscillations
• El Niño-Southern Oscillation influences interannual variability in carbon fluxes
• Decadal climate oscillations (PDO) impact ocean-atmosphere carbon exchange

Long-term carbon storage

• Deep ocean carbon has a residence time of about 1000 years
• Soil organic matter can store carbon for centuries to millennia
• Permafrost carbon represents a long-term storage vulnerable to climate warming
• Peatlands accumulate carbon over thousands of years, storing significant amounts

Geological carbon cycle

• Weathering of silicate rocks consumes CO2 over millions of years
• Carbonate rock formation sequesters carbon for long geological periods
• Plate tectonics and volcanism return buried carbon to the surface over millions of years
• Fossil fuel deposits represent carbon stored over hundreds of millions of years

Carbon cycle and climate

• Carbon cycle plays a fundamental role in regulating Earth's climate system
• Understanding carbon-climate interactions helps interpret past climate changes
• Projecting future carbon cycle changes is crucial for climate change predictions

Greenhouse effect

• CO2 absorbs and re-emits longwave radiation, trapping heat in the atmosphere
• Increasing atmospheric CO2 concentrations enhance the greenhouse effect
• Water vapor feedback amplifies the warming caused by CO2
• Other greenhouse gases (methane) also contribute to the overall greenhouse effect

Historical climate variations

• Ice core records reveal CO2-temperature coupling over glacial-interglacial cycles
• Paleoclimate proxies provide insights into past carbon cycle perturbations (PETM)
• Natural carbon cycle variations have driven climate changes throughout Earth's history
• Understanding past carbon-climate interactions helps contextualize current changes

Future climate projections

• Earth system models simulate future carbon cycle changes under various emission scenarios
• Projections consider potential carbon cycle feedbacks and their impacts on climate
• Uncertainties in carbon cycle responses contribute to ranges in climate projections
• Tipping points in the carbon cycle could lead to abrupt climate changes

Carbon cycle management

• Managing the carbon cycle is crucial for mitigating climate change impacts
• Various strategies aim to reduce atmospheric CO2 concentrations or enhance carbon sinks
• Balancing effectiveness, feasibility, and potential side effects is essential for carbon management

Carbon sequestration strategies

• Afforestation and reforestation enhance terrestrial carbon storage
• Soil carbon management practices improve agricultural carbon sequestration
• Ocean iron fertilization proposed to enhance marine carbon uptake
• Geological carbon capture and storage injects CO2 into underground reservoirs

Emission reduction policies

• Carbon pricing mechanisms (carbon tax) incentivize emissions reductions
• Cap-and-trade systems establish markets for carbon emissions allowances
• Renewable energy adoption reduces fossil fuel-related carbon emissions
• Energy efficiency improvements decrease overall carbon footprint

Geoengineering proposals

• Solar radiation management techniques aim to reduce incoming solar radiation
• Enhanced weathering accelerates natural CO2 consumption processes
• Direct air capture technologies remove CO2 from the atmosphere
• Ocean alkalinization proposed to enhance oceanic CO2 uptake and mitigate acidification