🌿 APES Topic 1.4: The Carbon Cycle

  1. What Is the Carbon Cycle?The carbon cycle describes how carbon moves through Earth's different parts: the air (atmosphere), living things (biosphere), water (hydrosphere), and rocks/soil (geosphere). Carbon is a key building block for all life. It details the movement of carbon atoms and carbon-containing molecules between various sources (where carbon is released) and sinks (where carbon is absorbed and stored).

Carbon is held in "reservoirs" (places where it stays for a while) and moves between them through various "processes" that either release or take in carbon. There are "fast" cycles, which involve rapid movement (like between living things and the air), and "slow" cycles, which involve long-term storage (like in rocks and fossil fuels).

  1. Key Components: Sources, Sinks, & Reservoirs

A carbon sink is a natural reservoir that absorbs more carbon than it releases. Examples include vast forests (which absorb \text{CO}2 through photosynthesis) and the oceans (which dissolve large amounts of \text{CO}2 ).

A carbon source is something that releases carbon into the atmosphere or other parts of the cycle. Examples of natural sources include volcanic eruptions, and anthropogenically (human-caused) include deforestation (which removes carbon stored in trees and soil, releasing it) and raising cattle (due to methane, \text{CH}_4 , production).

Reservoirs also differ by residence time (how long carbon stays there): Depending on the reservoir, carbon can be held for a short period of time or a long period of time.

  • Short-term reservoirs: Carbon stays here for years to decades. Examples: air, living plants and animals, surface ocean, soil organic matter.

  • Long-term reservoirs: Carbon can stay here for thousands to millions of years. Examples: fossil fuels, deep ocean, carbonate rocks (like limestone).

  1. Main Processes / Fluxes

These are the ways carbon moves between reservoirs:

  • Photosynthesis: Plants, algae, and some bacteria take \text{CO}_2 from the air (or water) and turn it into food (sugars), storing carbon in their bodies.

  • Respiration: Plants, animals, and microbes break down food for energy, releasing \text{CO}_2 back into the air or water. Carbon cycles between photosynthesis and cellular respiration in living things.

  • Decomposition: After living things die, decomposers break down their remains, releasing \text{CO}2 (or methane, \text{CH}4 , in places without much oxygen). Incomplete plant and animal decomposition over millions of years can lead to carbon storage in fossil fuels.

  • Combustion: Burning things like wood or fossil fuels quickly releases carbon as \text{CO}2 into the air. Burning fossil fuels removes that carbon from long-term geological storage, emitting it into the atmosphere as carbon dioxide ( \text{CO}2 ). Human burning of fossil fuels is a major contributor to current atmospheric \text{CO}_2 levels.

  • Oceanic Uptake & Exchange: \text{CO}2 dissolves into ocean water. Marine life uses it, and the ocean also exchanges \text{CO}2 with the atmosphere.

  • Sedimentation / Rock Formation: Carbon from dead marine organisms (like shells) settles on the ocean floor and over millions of years forms rocks (like limestone).

  • Volcanism & Geological Processes: Carbon stored in rocks can be released slowly over long periods by volcanoes or other geological activity.

  1. Fast vs Slow Carbon Cycles

  • Fast Carbon Cycle: Happens quickly, from days to decades. It involves living things, the air, and the surface ocean (e.g., photosynthesis, respiration, decomposition). This cycle responds quickly to changes.

  • Slow Carbon Cycle: Happens over very long geological timescales (thousands to millions of years). It involves carbon being buried in sediments, forming fossil fuels, creating rocks, and being released by volcanoes.

  1. Human Impacts & Why It Matters

Human actions, especially burning fossil fuels (releasing long-stored carbon) and cutting down forests (removing carbon sinks and releasing stored carbon through decomposition or burning), act as significant carbon sources, putting carbon into the atmosphere much faster than natural systems can remove it. This unbalanced carbon cycle causes problems:

  • Increased atmospheric \text{CO}_2 leads to the greenhouse effect, causing global warming and climate change.

  • More \text{CO}_2 dissolving into the ocean makes it more acidic (ocean acidification), which harms marine life, especially those with shells.

  • Some natural carbon sinks (like forests) are being destroyed, reducing their ability to absorb carbon. Also, melting permafrost (frozen ground) can release more stored carbon.

  1. Important Terms / Things to Know for APES

  • Reservoir / Sink vs Source: Know which store carbon and which release it.

  • Residence time: Understand short vs. long terms and examples.

  • Fluxes: Photosynthesis, respiration, decomposition, combustion.

  • Ocean's role: Its interaction with the atmosphere and its changing chemistry.

  • Geologic portion (slow cycle) vs. biological portion (fast cycle).

  • Anthropogenic changes: Human-caused changes to the carbon cycle and their effects.