AP Syllabus focus:
‘Carbon dioxide occurs naturally from processes such as respiration, decomposition, and volcanic eruptions.’
Atmospheric carbon dioxide (CO2) is a naturally occurring trace gas that cycles continuously among living organisms, soils, oceans, and Earth’s crust. Understanding its natural sources helps explain background CO2 levels and why they vary by location and season.
Graph comparing atmospheric across long time intervals, using ice-core reconstructions for the deeper past and instrumental measurements for the modern era. It contextualizes present-day concentrations against natural variability observed in paleoclimate records and highlights how the modern record is measured and displayed as a time series. Source
What “natural sources” means in APES
Natural sources are processes that add CO2 to the atmosphere without direct human control. They are part of the carbon cycle and are typically balanced (over long timescales) by natural sinks such as photosynthesis and ocean uptake.
Atmospheric carbon dioxide (CO2): A naturally occurring gas in the atmosphere composed of one carbon atom and two oxygen atoms, exchanged among Earth system reservoirs through physical, chemical, and biological processes.
Natural sources can be:
Biological (driven by organisms and microbes)
Geologic (driven by Earth’s internal heat and plate tectonics)
Diagram of carbon cycling among major Earth system reservoirs (atmosphere, biosphere, hydrosphere, and geosphere), with arrows showing key processes that move carbon between them. It helps distinguish fast biological fluxes (photosynthesis, respiration, decomposition) from longer-term storage in sinks such as oceans and rocks, framing “natural sources” within the broader carbon cycle. Source
Respiration as a natural CO2 source
Respiration releases CO2 when organisms break down organic molecules to obtain energy. It occurs in:
Animals (including humans, as part of natural biology)
Plants (plants both photosynthesise and respire; respiration occurs day and night)
Microorganisms (especially abundant in soils)
Key ideas for APES:
Ecosystem respiration (plants + animals + microbes) is a major natural CO2 flux.
Soil respiration is often dominated by microbes and plant roots; it can increase when conditions favour biological activity (e.g., warm, moist soils).
Respiration returns carbon to the atmosphere that was recently stored in biomass through photosynthesis, helping drive short-term (daily/seasonal) CO2 patterns.
Decomposition as a natural CO2 source
Decomposition releases CO2 as bacteria and fungi break down dead biomass and organic wastes. It is closely linked to detritus-based food webs and is especially important in soils and sediments.
Main pathways:
Aerobic decomposition (with oxygen): produces CO2 efficiently as organic matter is oxidised.
Low-oxygen decomposition (limited oxygen): produces less CO2 directly and can shift carbon into other forms in the environment; however, CO2 is still produced through microbial metabolism and later oxidation of decomposition products.
Environmental controls that change how much CO2 is released include:
Temperature: microbial activity generally rises with warmth, increasing CO2 release.
Moisture: very dry soils limit microbes; waterlogged soils limit oxygen, changing decomposition dynamics.
Organic matter availability: more leaf litter or dead plant material can increase decomposition-driven CO2 emissions.
Volcanic eruptions and geologic CO2
Volcanic eruptions release CO2 from Earth’s interior to the atmosphere. This CO2 originates from:
Magma degassing (CO2 dissolved in molten rock is released as pressure drops)
Metamorphism and subduction-related processes (carbon-bearing rocks are heated and transformed, releasing CO2 that can later vent through volcanoes)
Important APES framing:
Geologic CO2 emissions are part of the long-term carbon cycle, moving carbon from rocks to the atmosphere over thousands to millions of years.
Volcanic CO2 release can be episodic (eruptions) and also continuous (persistent degassing from volcanic regions).
How natural CO2 sources vary across place and time
Natural CO2 inputs are not uniform; they shift with ecosystem type and Earth processes:
Forests and productive soils: high respiration and decomposition can create strong local CO2 release, especially at night and during warm seasons.
Cold or dry regions: lower biological activity generally reduces natural CO2 emissions.
Tectonically active regions: more opportunities for geologic CO2 degassing.
These variations help explain why atmospheric CO2 is dynamic even without human influence, as natural systems continuously emit and absorb CO2 on different timescales.
Plot of atmospheric showing a pronounced annual (seasonal) oscillation superimposed on a longer-term rise, illustrating that is dynamic on multiple timescales. The seasonal “wiggles” align with biosphere-driven uptake and release (especially Northern Hemisphere vegetation), connecting directly to respiration and decomposition as natural sources. Source
FAQ
They often use carbon isotopes, especially the ratio of $^{13}C$ to $^{12}C$.
Biological processes tend to favour lighter $^{12}C$, producing a distinctive isotopic signature compared with many geologic sources.
Seasonal plant growth and dormancy shift net carbon exchange.
In many regions, photosynthetic uptake rises during the growing season and falls in winter, while respiration and decomposition continue, creating predictable seasonal oscillations.
Warmer conditions generally increase enzyme activity and microbial metabolism.
If soils are warm and sufficiently moist, respiration and decomposition can accelerate; extreme heat or drought can suppress activity by limiting water and stressing organisms.
Yes—oceans can release CO2 through outgassing when surface waters are warmer or when deep, CO2-rich waters upwell.
Whether the ocean is a net source or sink depends on temperature, circulation, and carbonate chemistry.
Both occur.
CO2 can be released in bursts during eruptions and also continuously through fumaroles, diffuse soil degassing, and persistent volcanic plumes, even when a volcano is not erupting.
Practice Questions
State two natural processes that release carbon dioxide into the atmosphere. (2 marks)
Respiration (1)
Decomposition (1)
Volcanic eruptions/volcanic degassing (1) (credit any two)
Explain how respiration, decomposition, and volcanic eruptions act as natural sources of atmospheric . Include one factor that can change the rate of CO2 release for either respiration or decomposition. (5 marks)
Respiration releases as organisms break down organic molecules for energy (1)
Occurs in animals/plants/microbes and contributes to ecosystem/soil CO2 flux (1)
Decomposition releases as microbes break down dead organic matter (1)
Volcanic activity releases geologic via degassing from magma/Earth’s interior (1)
One valid factor affecting respiration or decomposition rate (e.g., temperature, moisture, oxygen availability, organic matter supply) with a brief link to increased/decreased release (1)
