Key Terms: The Carbon Cycle (AQA A Level Geography): Revision Note

Carbon cycle stores - key terms

Atmosphere – The layer of gases surrounding Earth that stores carbon mainly as carbon dioxide and methane for about 6 years.

Biosphere – The global sum of all ecosystems, where living organisms store carbon for an average of 18 years.

Carbon biomolecules - Organic molecules including carbohydrates, fats, proteins etc., and form 50% of the total dry mass of living things

Cryosphere – Frozen regions of Earth such as permafrost in tundra areas, which store carbon for thousands of years.

Flux - also referred to as a flow when carbon is transferred from one place to another.

Hydrosphere – All water bodies on Earth, including oceans which store dissolved carbon such as bicarbonates and carbonates. Surface storage lasts around 25 years; deep storage can last over a thousand.

Lithosphere – The outermost layer of Earth where the majority of carbon is stored long-term in sedimentary rocks and fossil fuels.

Pedosphere – The soil layer of Earth, storing organic carbon from dead plants and animals for periods ranging from days to thousands of years.

Carbon cycle transfers - key terms

Burial and compaction – The process by which dead marine organisms settle on the ocean floor and are compressed into sedimentary rock like limestone over millions of years.

Carbon sequestration – The natural or artificial process of capturing and storing carbon, particularly in vegetation, oceans, or rock formations.

Climatic climax community - The end nature of the vegetation found at the end of a succession. In the UK, the climatic climax for a lithosere is deciduous woodland.

Combustion – The burning of organic material or fossil fuels that releases carbon back into the atmosphere.

Decomposition – The breakdown of organic material by bacteria, fungi and other organisms, returning carbon to the soil and atmosphere.

Halosere - Succession of vegetation occurring in salt-rich conditions.

Hydrosere - Succession of vegetation that occurs in freshwater.

Lithosere - Vegetation succession that occurs on bare rock.

Photosynthesis – The process by which plants convert atmospheric CO₂ into carbohydrates using sunlight, storing carbon in their tissues.

Psammosere - vegetation succession in sandy areas e.g. sand dunes.

Respiration – The process by which living organisms break down carbohydrates and release carbon dioxide back into the atmosphere.

Solubility pump - When carbon is transported as dissolved inorganic carbon from the ocean surface to the ocean depths.

Thermohaline circulation - The mechanism driving deep ocean currents. The density of seawater is determined through its temperature (thermo) and saltiness/salinity (haline) at a particular location. The difference in density between one location and another drives the thermohaline circulation which mixes and transports the waters of the oceans.

Weathering – A chemical process where carbon dioxide in rainwater forms carbonic acid that breaks down carbonate rocks like limestone.

Physical impacts on the carbon cycle - key terms

Interglacial periods - Warmer phases where temperatures are above average, such as the Medieval warm period.

Glacial periods – Extended cold phases that reduce atmospheric CO₂ and limit plant growth, weathering, and decomposition.

Permafrost thaw – The melting of permanently frozen ground, which releases stored carbon and methane into the atmosphere.

Volcanic activity – Eruptions that release carbon stored in rocks into the atmosphere, though this contributes far less CO₂ than human activities..

Wildfires – Natural or human-triggered events that rapidly transfer stored carbon in vegetation to the atmosphere.

Human impacts on the carbon cycle - key terms

Deforestation – The removal of forests reduces carbon storage and increases atmospheric CO₂ through burning or decomposition of plant material.

Fossil fuel burning – The combustion of coal, oil, and gas for energy that releases large amounts of CO₂ into the atmosphere, driving the enhanced greenhouse effect.

Hydrocarbons - Hydrocarbons are a compound of hydrogen and carbon, which are the main components of oil and natural gas. Fossil fuels are types of hydrocarbon which were formed from the compressed remains of organic matter (plant or animal) at the bottom of oceans over millions of years.

Land use change – Human alterations to land for agriculture, settlements or industry that reduce natural carbon sinks and increase emissions.

Urbanisation – The expansion of built environments that decreases vegetation cover, increases emissions from transport and industry, and contributes to the urban carbon footprint.

The carbon budget - key terms

Carbon budget – The balance between carbon inputs and outputs across Earth systems; it measures carbon emissions and removals globally or locally.

Carbon footprint – The total amount of greenhouse gases, especially CO₂, produced by human activities and expressed as GtC/yr.

Gigatonne of carbon (GtC) – A unit of measurement equal to one billion tonnes of carbon dioxide equivalent, used in carbon accounting.

Greenhouse effect – A natural process where gases like CO₂ trap heat in Earth’s atmosphere, maintaining a habitable temperature. Enhanced by excess emissions.

Impacts of carbon cycle – Effects of carbon transfers and storage on Earth’s systems, including atmospheric warming, changes to soil fertility, and ocean acidity.