The Carbon Cycle (College Board AP® Environmental Science): Study Guide

What is the carbon cycle?

• Many different materials cycle through the abiotic and biotic components of ecosystems

  • All materials in the living world are recycled to provide the building blocks for future organisms

• Elements such as carbon are not limitless resources

  • There is a finite amount of each element on the planet

  • Elements need to be recycled in order to allow new organisms to be made and grow

• Carbon is constantly being recycled around the biosphere so that the total amount of carbon in the biosphere is essentially constant

  • Carbon is transferred from one form to another by the various processes in the carbon cycle

• The carbon cycle is the set of processes by which carbon atoms and carbon-containing molecules move between Earth’s atmosphere, oceans, land, and living organisms

  • Carbon moves through sources (which release carbon) and sinks (which absorb carbon)

  • This cycle is vital for maintaining the balance of carbon, regulating global temperatures, and supporting life

Carbon sources

• Carbon sources release carbon

• Examples include:

  • Burning fossil fuels

  • Respiration by plants and animals

  • Volcanic eruptions

  • Decay of organic material

Carbon sinks

• Carbon sinks absorb or remove carbon from another part of the carbon cycle

• Examples include:

  • Forests (terrestrial biomass)

  • Oceans (dissolved CO₂, phytoplankton)

  • Soil (organic matter and carbonates)

Fast and slow stages of the carbon cycle

• The carbon cycle consists of fast and slow stages that regulate the movement of carbon through the Earth’s systems:

Fast stages of the carbon cycle

• Photosynthesis:

  • Plants absorb carbon dioxide (CO₂) from the atmosphere to produce glucose

• Respiration:

  • Animals and plants release CO₂ back into the atmosphere through metabolic processes

• Decomposition:

  • Microorganisms break down dead organisms, releasing CO₂ or methane (CH₄) into the atmosphere

• Combustion:

  • Burning of organic matter (e.g., fossil fuels or biomass) releases stored carbon as CO₂

Slow stages of the carbon cycle

• Weathering and erosion:

  • Carbon dioxide reacts with water to form carbonic acid, which dissolves rocks, releasing bicarbonates into the ocean

• Sedimentation:

  • Bicarbonates are used by marine organisms to form shells, which accumulate as sediment and eventually become limestone

• Subduction and volcanic activity:

  • Tectonic movements subduct carbon-rich rocks into the mantle, where it is released back into the atmosphere through volcanic eruptions

Carbon reservoirs

• Carbon reservoirs are storage locations for carbon in different forms, including:

  • Carbon dioxide (CO₂)

  • Methane (CH₄)

  • Organic material

Types of carbon reservoirs

Long-term reservoirs

• Store carbon for millions of years

• Examples include:

  • Fossil fuels: Coal, oil, and natural gas buried underground

  • Carbonate rocks: Limestone formed from marine organisms' skeletons

• Long-term reservoirs are critical in maintaining stability over geological time

Short-term reservoirs

• Hold carbon for shorter timescales (days to decades)

• Examples include:

  • Atmosphere: Stores CO₂ and CH₄

  • Living organisms: Plants and animals store carbon in organic forms

  • Soil: Stores carbon from decaying plant and animal material.

• These reservoirs are highly dynamic and quickly influenced by processes like photosynthesis and respiration

Ocean carbon storage

• Surface ocean (short-term reservoir):

  • The upper layer of the ocean, where CO₂ exchanges with the atmosphere, is considered a short-term reservoir

  • Carbon cycles here on the scale of days to years

• Deep ocean (longer-term storage):

  • Carbon that sinks into deeper layers (via biological and physical processes like the biological pump) is stored for hundreds to thousands of years

  • This can classify parts of the ocean as a medium- to long-term reservoir

    • For example, carbon in deep-sea sediments or dissolved inorganic carbon is effectively removed from the atmosphere for extended periods

Carbon cycling between photosynthesis & respiration

Photosynthesis

• Plants, algae, and some bacteria absorb CO₂ from the atmosphere or water

• They use sunlight to convert CO₂ and water into glucose and oxygen

• Examples:

  • Terrestrial ecosystems: Forests like the Amazon rainforest absorb vast amounts of CO₂

  • Marine ecosystems: Phytoplankton in oceans play a critical role in removing CO₂ from the atmosphere

Respiration

• All living organisms release CO₂ back into the atmosphere or water through cellular respiration

• This process converts glucose and oxygen into energy

  • CO₂ is produced as a byproduct

• Example:

  • Animals, fungi, and decomposing microbes all contribute to respiration

• Photosynthesis and respiration form a short-term carbon cycle, creating a continuous exchange of carbon between living organisms and the environment

Atmospheric carbon reservoir

• Carbon is stored in the atmosphere primarily as CO₂ and CH₄ (methane)

Natural carbon fluxes

• Carbon enters the atmosphere from natural sources such as respiration, volcanic eruptions, and the decay of organic matter

• Carbon leaves the atmosphere when absorbed by plants, oceans, or soil.

Decomposition and carbon storage

• Plant and animal decomposition transfers carbon to the soil

• Over millions of years, some carbon becomes fossil fuels

Human impacts

• The burning of fossil fuels is an unnatural (antrhopogenic) source of carbon entering the atmosphere

• This activity rapidly releases large amounts of carbon, previously stored in long-term reservoirs, into the atmosphere as CO₂