Syllabus Edition
First teaching 2015
Last exams 2025
What is the Carbon Cycle?
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
The Carbon Cycle
What is the carbon cycle?
Carbon is constantly being recycled around the biosphere so that the number of carbon atoms in the biosphere is essentially constant; carbon atoms merely swap from one compound to another by the various processes in the carbon cycle
Carbon cycle diagram
Storage and Flows in the Carbon Cycle
Storages in the carbon cycle include:
The atmosphere (as CO2)
Sedimentary rocks
Fossil fuels like coal, oil, and gas; coal is largely carbon
Soil and other organic matter
Vegetation (e.g. as cellulose)
Animals
Dissolved in the oceans (as CO2)
Flows in the carbon cycle include:
Consumption (feeding)
Death and decomposition
Photosynthesis
Respiration
Dissolving
Fossilisation
Photosynthesis
Autotrophs use the energy of sunlight to 'fix' carbon dioxide, turning its carbon into sugars and other organic molecules
This removes carbon from the atmosphere
Terrestrial plants use gaseous CO2 directly from the air
Aquatic organisms use CO2 dissolved in water
As much CO2 is fixed from ocean microorganisms, as from terrestrial plants
Sedimentation
Plants that die are not fully decomposed by saprobionts; their bodies form layers of sediment that can accumulate over millions of years, locking carbon into the ground
This sediment is a store of energy and can form fossil fuels like peat and coal
Aquatic organisms that die also form sediments on the sea bed; these can go on to form other fossil fuels like oil and gas
Shells and other calcium-containing body parts can form sedimentary rocks such as limestone
The existence of life forms over billions of years has shaped the biosphere, in that their remains are still being recycled
Respiration
All life forms respire, including autotrophs
Heterotrophs rely on respiration for all their energy needs
Respiration puts CO2 into the atmosphere, in the opposite direction to photosynthesis
Anaerobic respiration also releases CO2 into the atmosphere, via fermentation by yeast, moulds and bacteria
Feeding
Carbon is passed from autotroph to heterotroph during feeding
Carbon is also passed from primary consumer to secondary consumer
Biomass transfer always includes the transfer of carbon, the main element in biomass
Decay & Decomposition
Dead plants and animals are fed upon by detritivores and decayed by saprophytes
Releasing carbon into the surroundings
Supplying carbon to the detritivores
Supplying carbon to the saprophytes
Waste matter such as faeces and urine is used by decaying saprobionts
Such processes can release CO2 back into the air
Human Impacts on the Carbon Cycle
What is the human impact on the carbon cycle?
Human activities such as burning fossil fuels, burning, deforestation, urbanisation and agriculture impact the balance of storages and flow within the carbon cycle
Burning fossil fuels
Since the mid 19th century, humans have extracted and burned increasing amounts of fossil fuels from the Earth
CO2 is being returned to the atmosphere faster than it can be absorbed by plants and aquatic producers
The CO2 level in the atmosphere is approximately double that of 800,000 years ago
Warmer temperatures mean that less CO2 can be dissolved in the oceans, so is released into the air
This has caused dramatic climate change and affected many other species, mainly through changing habitats
Photo by Renaldo Matamoro on Unsplash
Deforestation reduces the ability of forests to act as carbon sinks
Deforestation
Mass deforestation is reducing the amount of producers available to take carbon dioxide out of the atmosphere by photosynthesis
In many areas of the world, deforestation is taking place for land rather than for timber, and as such these forest are simply burnt down, releasing yet more carbon dioxide into the atmosphere
Urbanisation
As well as increasing human activities such as transportation, energy use, and industrial activities in urban areas, urbanisation also affects the carbon cycle by altering land use patterns
Forests, wetlands, and other natural ecosystems are often replaced with buildings, roads, and other infrastructure
This reduces the amount of carbon that can be sequestered in plants and soil, and the carbon storage capacity of the land is decreased.
Worked Example
Discuss human impacts on the carbon cycle.
Answer
Humans have a significant impact on the carbon cycle, mainly through the combustion of fossil fuels and deforestation. Fossil fuels such as coal, oil, and natural gas contain carbon that has been stored underground for millions of years. When these fuels are burned, carbon is released into the atmosphere as carbon dioxide (CO2), contributing to the greenhouse effect and climate change.
Deforestation, especially in tropical regions, also affects the carbon cycle. Trees and other plants absorb CO2 from the atmosphere during photosynthesis, storing carbon in their biomass. When trees are cut down and burned or left to decay, the stored carbon is released back into the atmosphere as CO2.
Other human activities that contribute to carbon emissions include agriculture, transportation, and industry. For example, livestock farming produces methane, a potent greenhouse gas, through enteric fermentation in cows, sheep, and other ruminants. Transportation, especially cars and trucks, burns fossil fuels and releases CO2 into the atmosphere. Industry and manufacturing processes also contribute to carbon emissions through the burning of fossil fuels and other energy-intensive processes.
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