Earth’s Life Support Systems
Water and Carbon Cycles
The water and carbon cycles are global systems that transfer and store essential elements supporting life on Earth. These cycles operate between the land, oceans, atmosphere, and biosphere, and are closely linked through processes such as photosynthesis, respiration, and climate regulation.
Both cycles can be viewed as open systems at local and regional scales, but closed systems at a global scale. The total amount of water and carbon on Earth remains broadly constant, though energy from the Sun drives continuous movement between stores.
1. Understanding Open and Closed Systems
Systems theory in geography
- A system is a set of interrelated components working together.
- All systems have inputs, stores, flows (transfers), and outputs.
Open systems
- Matter and energy can enter or leave the system.
- Examples include a drainage basin (where water enters as precipitation and leaves as runoff) or a forest ecosystem (where carbon enters via photosynthesis and exits through respiration or decomposition).
Closed systems
- Energy can enter and leave, but matter remains constant.
- The global water and carbon cycles are often treated as closed systems because the total amounts of water and carbon on Earth remain roughly constant.
- Solar energy drives movement between stores and sustains life processes.
2. The Characteristics of Systems
Both the water and carbon cycles operate through inputs, outputs, stores, and transfers, which together regulate Earth’s climate and maintain the conditions necessary for life.
| Component | Description |
|---|---|
| Inputs | Energy or matter entering a system to be processed. |
| Stores | Where energy or matter is held (before processes are in place to move them on). |
| Transfers (flows) | Movement of energy or matter between stores that allow inputs to become outputs. |
| Outputs | Outcomes of processing within a the system. |
3. Overview of the Global Water Cycle
The global water cycle describes the continuous circulation of water between the atmosphere, oceans, cryosphere, and land.
- Globally, it operates as a closed system — the total volume of water remains constant.
- Locally, such as within a drainage basin, it acts as an open system with inflows and outflows of water.
Key stores:
- Oceans – the largest store of water on Earth (~97%).
- Cryosphere – stores freshwater as ice (mainly in Antarctica and Greenland).
- Groundwater – around 30% of the world’s freshwater stored in aquifers.
- Atmosphere – contains only ~0.001% of global water as vapour and clouds.
- Biosphere – water within living organisms.
Key transfers:
- Evaporation and transpiration move water to the atmosphere.
- Condensation forms clouds.
- Precipitation returns water to Earth’s surface.
- Infiltration, percolation, and runoff move water through and across the land.
- The hydrological cycle plays a vital role in energy transfer, climate regulation, and ecosystem productivity.
For detailed notes on the distribution and size of water stores, see The Distribution and Size of the Major Stores in the Water Cycle.
4. Overview of the Global Carbon Cycle
The carbon cycle describes the movement of carbon between the atmosphere, oceans, lithosphere, biosphere, and cryosphere.
- Globally, the total amount of carbon is constant — making it a closed system.
- At local or regional scales, carbon moves freely in open systems (for example, within a forest or soil profile).
Key stores:
- Lithosphere – the largest store, with carbon locked in rocks and fossil fuels (~100 million GtC).
- Oceans – the second largest store, containing dissolved CO₂ and marine biomass (~38,000 GtC).
- Biosphere – carbon stored in living and dead organic matter (~2,000 GtC).
- Atmosphere – CO₂ and CH₄ gases (~830 GtC).
- Cryosphere – frozen organic carbon trapped in permafrost.
Key transfers:
- Photosynthesis – removes CO₂ from the atmosphere and stores it in plant biomass.
- Respiration and decomposition – return CO₂ to the air.
- Combustion – releases stored carbon from fossil fuels or biomass.
- Ocean–atmosphere exchange – CO₂ dissolves into or out of seawater.
- Weathering and sedimentation – move carbon between rocks, oceans, and the atmosphere over geological timescales.
- Carbon fluxes operate over short (biological) and long (geological) timescales, regulating climate and sustaining life on Earth
For more details on global carbon stores, visit The Distribution and Size of the Major Stores in the Carbon Cycle.
5. Links Between the Water and Carbon Cycles
The two cycles are interconnected and mutually influential:
- Photosynthesis and respiration link water and carbon through plant growth and energy transfer.
- Oceans store both CO₂ and water, exchanging gases with the atmosphere.
- Climate regulation depends on both: water vapour and CO₂ are key greenhouse gases.
- Cryosphere changes (melting ice) release both freshwater and trapped carbon, influencing global feedbacks.
Together, the water and carbon cycles form the foundation of Earth’s life support systems.
Earth’s Life Support Systems
Water and Carbon Cycles
The water and carbon cycles are global systems that transfer and store essential elements supporting life on Earth. These cycles operate across the land, oceans, and atmosphere and can be understood as a series of open and closed systems that are interconnected and dynamic.
1. Understanding Open and Closed Systems
1.1 Systems theory in geography
- A system is a set of interrelated components working together.
- Systems have inputs, stores, flows (or transfers), and outputs.
1.2 Open systems
- Matter and energy can enter or leave the system.
- Most local or regional water and carbon systems (such as a drainage basin or forest ecosystem) are open systems because they exchange materials with their surroundings.
1.3 Closed systems
- Only energy enters and leaves; matter remains within the system.
- The global water cycle and global carbon cycle are often viewed as closed systems because the total amount of water and carbon on Earth stays broadly constant, though energy from the Sun drives continuous movement between stores.
2. The Global Water Cycle
Global water cycle – stores and annual flows
2.1 Key stores
- Oceans: Contain about 97% of all water on Earth.
- Cryosphere: Stores water as ice in glaciers and ice sheets. Almost 80% of the Earth’s fresh water is locked up in ice and glaciers. Almost 90% of the global ice mass is found in Antarctica. The Greenland ice cap holds about 10% of global ice mass.
- Atmosphere: Contains water vapour and clouds. This is an exceptionally small store of water, accounting for only 0.001% of Earth’s water.
- Terrestrial stores: Include rivers, lakes, soil moisture, and groundwater. Surface fresh water accounts for about 2.5% of all water on Earth and is found in glaciers, lakes, reservoirs, wetlands, rivers, and streams. 20% of the world’s surface freshwater is in Lake Baikal, Russia. A further 20% is contained within the Great Lakes of North America. Only 0.006% of the world’s fresh water reserves are held in rivers. Around 30% of global fresh water is stored underground in aquifers.
- Biosphere: Water within living organisms.
2.2 Transfers (fluxes)
- Evaporation and transpiration move water from land and ocean surfaces into the atmosphere. Oceans account for around 90% of the water evaporated in the water cycle.
- Condensation forms clouds as water vapour cools.
- Precipitation returns water to the Earth’s surface.
- Infiltration, percolation, surface runoff, and groundwater flow move water through and across the land.
2.3 Global balance
The water cycle is powered by solar energy and gravitational potential energy.
Although water is constantly moving between stores, the overall volume on Earth remains the same, showing how the cycle operates as a closed system globally, but as an open system locally.
3. The Global Carbon Cycle
3.1 Key stores (reservoirs)
| Store | Approximate carbon (GtC) | Notes |
|---|---|---|
| Lithosphere | 100,000,000 | Rocks and fossil fuels – largest store |
| Oceans | 38,000 | Dissolved CO₂, marine organisms, sediments |
| Biosphere | 2,000 | Living and dead organic matter |
| Atmosphere | 830 | Carbon dioxide and methane |
| Cryosphere | Small, but significant | Frozen organic carbon in permafrost |
3.2 Transfers (fluxes)
- Photosynthesis: Plants absorb CO₂ from the atmosphere.
- Respiration: Organisms release CO₂ back to the atmosphere.
- Decomposition: Breaks down organic matter, releasing carbon.
- Combustion: Burning of biomass or fossil fuels releases CO₂.
- Ocean–atmosphere exchange: CO₂ dissolves in or is released from ocean water.
- Weathering and sedimentation: Carbon moves between rocks, oceans, and the atmosphere over geological timescales.
3.3 Global balance
- Like water, total carbon on Earth is fixed, so the global cycle is a closed system.
- However, local and regional carbon cycles (e.g. within a forest or soil profile) are open systems, exchanging carbon with the atmosphere and oceans.
4. Interactions Between the Water and Carbon Cycles
- Photosynthesis and respiration link both cycles as plants use water and CO₂ to produce biomass.
- Ocean–atmosphere interactions exchange both water vapour and carbon dioxide, influencing global temperature.
- Climate regulation: Increased atmospheric CO₂ enhances the greenhouse effect, raising temperatures and influencing evaporation and precipitation.
- Cryosphere feedbacks: Melting ice releases freshwater and trapped carbon, affecting both cycles.
5. Human Influences on the Cycles
5.1 Water cycle impacts
- Deforestation reduces interception and transpiration, altering runoff and infiltration.
- Urbanisation increases surface runoff and reduces groundwater recharge.
- Climate change is intensifying evaporation and altering precipitation patterns.
5.2 Carbon cycle impacts
- Burning fossil fuels adds excess CO₂ to the atmosphere.
- Land-use change reduces carbon storage in vegetation and soils.
- Ocean acidification results from increased CO₂ absorption by oceans, affecting marine ecosystems.
5.3 Interconnected consequences
Changes in one cycle can amplify changes in the other, for example, warming oceans release more CO₂, which further enhances global warming.