AQA Specification focus:
‘Global distribution and size of major stores of carbon – lithosphere, hydrosphere, cryosphere, biosphere, atmosphere.’
The carbon cycle involves the movement of carbon through Earth's systems. Understanding where carbon is stored and how much exists in each store is essential.
Global Carbon Stores
Overview of Carbon Distribution
Carbon is a fundamental element in all living things and plays a critical role in regulating Earth’s climate through its presence in key carbon stores. These stores vary significantly in size, stability, and rate of exchange with other parts of the cycle.
This block diagram quantifies carbon stocks in GtC for atmosphere, land biota, soil, surface and deep oceans, sedimentary rocks, and mantle; the inclusion of mantle and sedimentary rocks provides extra depth beyond the syllabus requirement. Source
There are five major global carbon stores:
Lithosphere
Hydrosphere
Cryosphere
Biosphere
Atmosphere
This diagram shows the size of major carbon reservoirs (in GtC) and the annual fluxes (GtC/yr) between them, highlighting atmosphere, vegetation, soils, oceans, and fossil fuel stores; note that flows associated with human activities (red arrows) are also indicated, which goes beyond the syllabus focus. Source
Each store holds carbon in different forms and quantities.
The Lithosphere
The lithosphere refers to the solid outer part of the Earth, including the crust and upper mantle.
Lithosphere: The rigid outer layer of the Earth, including the crust and upper mantle, where most of Earth's carbon is stored as sedimentary rock.
Largest global store of carbon, containing around 99.9% of all carbon on Earth.
Stored primarily in carbonate rocks (e.g. limestone) and fossil fuels (coal, oil, natural gas).
Carbon here is locked away for millions of years, making it a long-term store.
Released slowly through processes such as volcanic activity and weathering.
Carbon in Sedimentary Rocks
Rocks such as limestone and chalk form from compacted marine organisms rich in calcium carbonate.
These geological processes act over geological timescales.
Fossil Fuels
Formed from the remains of ancient organisms over millions of years under pressure and heat.
Represent a relatively small proportion of lithospheric carbon, but significant due to human extraction and combustion.
The Hydrosphere
The hydrosphere consists of all water on Earth — oceans, rivers, lakes, and groundwater.
Hydrosphere: The total amount of water on the planet, including surface water, groundwater, and atmospheric water vapour.
Second-largest carbon store.
Oceans absorb atmospheric CO₂ through diffusion.
Carbon is stored in:
Surface waters as dissolved carbon dioxide.
Deep ocean waters where carbon is transported via the biological pump.
Marine organisms which extract carbon to form calcium carbonate shells.
Around 38,000 gigatonnes (Gt) of carbon is stored in the ocean, mostly in deep waters.
Ocean-Atmosphere Exchange
The oceanic carbon pump moves carbon between the atmosphere and the deep ocean.
Cold waters absorb more CO₂, and carbon-rich waters sink in high-latitude regions (thermohaline circulation).
The Cryosphere
The cryosphere includes all frozen water — glaciers, permafrost, and sea ice.
Cryosphere: The frozen water part of the Earth system, including glaciers, ice caps, and permafrost.
Contains large amounts of carbon-rich organic material trapped in permafrost.
Permafrost regions (e.g. Siberia, Alaska) hold an estimated 1,400–1,600 Gt of carbon.
Warming temperatures may thaw permafrost, releasing CO₂ and methane (CH₄) — potent greenhouse gases — into the atmosphere.
The Biosphere
The biosphere consists of all living and recently dead organisms — plants, animals, and microorganisms.
Biosphere: The global sum of all ecosystems; it includes all living organisms and the organic matter they produce.
Stores approximately 560 Gt of carbon globally.
Carbon is stored in:
Vegetation (e.g. forests, grasslands).
Soils (as organic matter and humus).
Animals and microorganisms.
Carbon moves rapidly in and out of this store via:
Photosynthesis (carbon absorbed).
Respiration (carbon released).
Decomposition of dead organic matter.
Terrestrial vs. Soil Carbon
Soils contain more carbon than living vegetation due to accumulated dead organic matter.
Tropical rainforests are major biospheric carbon stores, but soil carbon is especially significant in peatlands and cold regions.
The Atmosphere
The atmosphere contains carbon primarily in the form of carbon dioxide (CO₂) and methane (CH₄).
Atmosphere: The layer of gases surrounding Earth, where carbon is stored as greenhouse gases such as CO₂ and CH₄.
Contains around 870 Gt of carbon.
Levels of atmospheric CO₂ have increased from 280 ppm (pre-industrial) to over 420 ppm (current) due to anthropogenic emissions.
Atmospheric carbon plays a crucial role in regulating Earth's radiative balance and climate.
Greenhouse Effect
Carbon gases trap longwave radiation, contributing to global warming.
The atmosphere is a short-term, rapidly cycling store, with continuous exchanges with the biosphere, hydrosphere, and cryosphere.
Relative Size and Stability of Carbon Stores
Lithosphere: Largest and most stable over geological time (millions of years).
Hydrosphere: Large store, actively cycling, especially in the surface layers.
Cryosphere: Vulnerable to climate change; potentially large emissions if thawed.
Biosphere: Medium-sized and dynamic, sensitive to deforestation, land use, and climate.
Atmosphere: Smallest active store but highly significant due to influence on climate and fast response to change.
Importance of Store Size
The size of a store affects its ability to buffer or amplify changes.
Fluxes (flows of carbon) between stores control the carbon budget and are central to understanding climate systems.
FAQ
Geological timescales span millions of years, during which carbon accumulates slowly in sedimentary rocks through processes such as burial and lithification.
Over time, tectonic activity may expose these rocks, leading to weathering and gradual carbon release. Because of this slow turnover, the lithosphere acts as a long-term, relatively inert carbon store compared to others.
Permafrost stores vast amounts of organic carbon that has been frozen for thousands of years.
When permafrost thaws due to rising temperatures:
Microbial activity increases.
Organic matter decomposes.
Greenhouse gases such as CO₂ and CH₄ are released.
This makes the cryosphere a potentially significant feedback mechanism in climate change.
Marine organisms absorb CO₂ during photosynthesis and use carbon to build calcium carbonate shells.
When they die:
Shells sink to the ocean floor.
Some dissolve; others form sediments.
This biological pump transfers carbon from surface waters to deep ocean stores, supporting long-term storage.
Soil rich in organic matter (e.g., humus) stores more carbon.
Factors affecting storage include:
Soil type (e.g., peat soils store more carbon).
Temperature and moisture (which influence decomposition).
Vegetation cover (which adds organic input).
Soil disturbance or erosion can rapidly release stored carbon back into the atmosphere.
Atmospheric carbon levels respond quickly to both natural processes and human activities.
Rapid fluxes occur due to:
Seasonal plant growth and decay.
Volcanic eruptions or wildfires.
Fossil fuel combustion and land-use changes.
This makes the atmosphere the most sensitive and volatile of the active carbon stores.
Practice Questions
Identify three of the five major global carbon stores and briefly describe one characteristic of each. (3 marks)
Mark Scheme for Question 1 (3 marks):
1 mark for correctly identifying each store (maximum 3 marks).
Up to 1 mark for a relevant characteristic per store, only if the store is correctly named.
Example answer (3/3):
Lithosphere – contains carbon stored in rocks and fossil fuels. (1 mark)
Atmosphere – contains carbon as carbon dioxide and methane. (1 mark)
Hydrosphere – stores carbon dissolved in ocean water. (1 mark)
Explain how the size and stability of carbon stores vary across the lithosphere, biosphere and atmosphere. (6 marks)
Mark Scheme for Question 2 (6 marks):
Level 1 (1–2 marks):
Basic explanation with limited structure or detail.
May list differences with minimal development.
Level 2 (3–4 marks):
Clear explanation showing some understanding of differences in store size and stability.
Some use of appropriate terminology.
Partial reference to at least two stores.
Level 3 (5–6 marks):
Detailed and well-structured explanation.
Clear comparison of all three stores.
Uses accurate terminology such as "long-term store", "dynamic", or "volatile".
May include numerical data for further support (e.g. gigatonnes).
Example answer (Level 3 – 6/6):
The lithosphere is the largest carbon store, holding around 99.9% of global carbon in rocks and fossil fuels. It is very stable over millions of years. In contrast, the biosphere is a medium-sized store, holding around 560 Gt of carbon in living organisms and soils, and is more dynamic, with regular carbon exchange through photosynthesis and respiration. The atmosphere is the smallest active store at around 870 Gt, but is very unstable, with carbon levels changing rapidly due to human activity like fossil fuel combustion.
