Water and Carbon Cycles
Water and Carbon Cycles as Natural Systems
Introduction
The water and carbon cycles are two of the most critical natural systems on Earth. They help regulate climate, influence weather patterns, and support life. Geographers study these cycles using a systems approach, which helps to understand how energy and matter move between stores and how these movements maintain or disturb balance within the Earth system.
What Is a System?
A system is a set of interrelated components that work together through flows or transfers of energy and matter. Systems have inputs, outputs, stores (or components), and processes that link them.
Systems in physical geography can be:
| System Type | Description | Example |
|---|---|---|
| Open | Energy and matter can enter and leave the system. | A drainage basin – receives water as precipitation and loses it as runoff and evapotranspiration.
Forest ecosystem – takes in carbon through photosynthesis and releases it via respiration, decomposition, and combustion. |
| Closed | Energy can enter or leave, but matter stays within the system. | Global water cycle – total water remains constant, though it changes state and location.
Global carbon cycle – total carbon remains constant but transfers between stores (atmosphere, oceans, biosphere, lithosphere). |
| Isolated | No transfer of energy or matter with surroundings. These rarely exist in nature. | The universe (no exchange of matter or energy beyond its boundary). |
Closed and Open Systems in Geography
Systems Within Systems
The Earth is a closed system that contains four major subsystems, each interacting with the others:
- Atmosphere – gases surrounding the planet.
- Hydrosphere – all water on, above, and below the Earth’s surface.
- Lithosphere – the solid rock, soils, and crust.
- Biosphere – all living organisms.
Each subsystem contains smaller open systems such as drainage basins, forests, or glaciers. These are known as cascading systems, where the output of one subsystem becomes the input to another. For example, sediment from a river system (fluvial) may become input for a coastal system when deposited at the mouth of a river.
| Component | Definition | Example from the Water Cycle | Example from the Carbon Cycle |
|---|---|---|---|
| Inputs | Energy or matter entering the system. | Precipitation entering a drainage basin. | CO₂ entering the atmosphere from respiration or combustion. |
| Outputs | Energy or matter leaving the system. | River discharge to the sea. | Carbon locked into sedimentary rock over geological timescales. |
| Stores (components) | Places where energy or matter is held. | Ice, soil moisture, vegetation, groundwater. | Atmosphere, oceans, fossil fuels, biomass. |
| Flows (transfers) | Movement of energy or matter between stores. | Evaporation, infiltration, percolation, runoff. | Photosynthesis, respiration, decomposition, combustion. |
| Processes | Physical or biological actions that drive transfers or transformations. | Condensation, transpiration. | Diffusion, sequestration. |
Energy in the System
Both cycles are driven primarily by two sources of energy:
- Solar energy – drives evaporation, photosynthesis, and atmospheric circulation.
- Geothermal energy – drives volcanic and tectonic activity, releasing water vapour and carbon dioxide from the lithosphere.
- Energy flows allow constant movement and transformation within and between stores, maintaining the operation of both cycles.
Feedback and Equilibrium
Dynamic Equilibrium
A dynamic equilibrium exists when inputs and outputs are balanced, and stores remain stable over time despite continuous change. For example, although water is constantly moving between stores through precipitation and evaporation, the total amount of water on Earth remains roughly constant.
Feedback Loops
Feedback occurs when a change in one part of a system leads to a response that affects other parts of the system.
Positive and Negative Feedback
- Positive feedback amplifies change, moving the system away from equilibrium.
- Water cycle example: Rising temperatures increase evaporation → more water vapour → enhanced greenhouse effect → further warming.
- Carbon cycle example: Melting permafrost releases CO₂ and methane → increases global temperatures → more permafrost melt.
- Negative feedback counteracts change, helping maintain balance.
- Water cycle example: Higher rainfall promotes vegetation growth → increased interception and infiltration → reduced runoff and flood risk.
- Carbon cycle example: Higher atmospheric CO₂ increases plant growth → more carbon absorbed through photosynthesis → reduced CO₂ levels.
Exam Tip
Positive and negative feedback are not “good” or “bad” — they describe whether change is reinforced or reduced.
Applying the Systems Approach to the Water and Carbon Cycles
The water and carbon cycles are vast global systems that move energy and matter through different parts of the Earth. Studying them with a systems approach allows geographers to see how individual processes connect, how stores and flows change over time, and how human or natural disturbances can shift the balance or dynamic equilibrium of these cycles.
The Water Cycle System
The diagram above illustrates a simplified version of the water cycle system. At a global scale, the water cycle can be viewed as a closed system. Water is constantly exchanged between the atmosphere, land, and oceans, but the total amount of water on Earth remains relatively constant. Energy enters and leaves the system, but no new water is gained or lost to space.
However, at a smaller scale, for example, a drainage basin, the water cycle operates as an open system. Precipitation is an input, and water can leave the system through river discharge, evapotranspiration or groundwater flow.
The water cycle contains many stores of water, including oceans, soil moisture, ice, vegetation storage, groundwater and the atmosphere.
Water moves between these stores through flows and transfers such as evaporation, transpiration, condensation, precipitation, surface run-off, throughflow and percolation.
Changes in climate, vegetation cover or human activity can alter how much water is held in each store, how quickly it moves, and whether the system stays in dynamic equilibrium or becomes disrupted.
The Carbon Cycle System
The Global Carbon Cycle System
The diagram above shows the carbon cycle as a systems diagram. At the global scale, the carbon cycle behaves as a closed system. Carbon is stored in major reservoirs, including the atmosphere, oceans, vegetation, soils, fossil fuels and sedimentary rocks. Carbon constantly circulates between these stores, but the total amount of carbon on Earth remains stable over geological time.
At a local scale, for example, in a forest, soil profile, or ocean surface, the carbon cycle acts as an open system, gaining and losing carbon through processes such as photosynthesis, respiration, decomposition, combustion, and exchange between the ocean and the atmosphere.
Carbon moves rapidly through the fast carbon cycle (atmosphere, vegetation, soils and surface ocean) and much more slowly through the slow carbon cycle (deep ocean, rocks, fossil fuels and sediments).
Natural changes or human actions such as deforestation, burning fossil fuels, or climate warming can disturb flows between stores and shift the system away from equilibrium.
Interactions Between the Water and Carbon Cycles
The water and carbon cycles are closely interlinked:
- Photosynthesis requires water and transfers carbon from the atmosphere to plants.
- Respiration and decomposition release carbon and water vapour.
- Oceans store both water and dissolved carbon, and exchange gases with the atmosphere.
- Changes in one cycle often influence the other — for instance, deforestation reduces both carbon storage and local evapotranspiration rates.
Summary
- The Earth operates as a closed system, made up of interacting open subsystems such as drainage basins and ecosystems.
- Each system involves inputs, outputs, stores, and flows of energy and matter.
- Dynamic equilibrium helps maintain balance, though feedback mechanisms can amplify or dampen change.
- The water and carbon cycles are interdependent, regulating the planet’s climate and supporting life.
Exam Tip
When answering A Level questions on systems:
- Use specific geographical examples (e.g. “the Amazon Basin as an open subsystem”).
- Include key terms from the specification, such as flows, stores, feedback, dynamic equilibrium, and cascading systems.
- Show understanding of scale — how global closed systems contain local open subsystems.