3.1.1.1 - Water and carbon as systems

Boundaries, stores and flows

A system is a set of connected components studied within a chosen boundary.

In a water system, a lake is a component and the volume of water held in it is a store. River inflow transfers water into it; evaporation transfers water from its surface to the atmosphere. In a carbon system, vegetation holds carbon in living matter, called biomass. Photosynthesis uses light to turn carbon dioxide and water into organic matter; it transfers carbon from the atmosphere into plants. Respiration releases carbon dioxide as organisms use organic matter for energy, transferring carbon back to the atmosphere. The carbon changes its chemical form rather than vanishing.

An input crosses the boundary into the system; an output crosses out. Movement within the boundary is an internal transfer. This distinction depends on the question being investigated. Rainfall is an input to a drainage basin, but an internal atmosphere-to-land transfer if the system is the Earth's water cycle. River flow between two points inside the basin is internal; river water leaving through its outlet is an output.

The global water cycle is commonly modelled as approximately closed to water over ordinary geographical timescales: transfers redistribute an almost fixed total. It remains open to energy. Individual basins are open to both water and energy. A closed-material model does not mean every store is constant or every water molecule stays in one place. The global carbon cycle also redistributes carbon between Earth stores; a forest is a smaller, open carbon system within it.

In a systems diagram, draw store boxes and labelled transfer arrows. Identify the boundary before deciding which arrows are inputs or outputs. A store might be measured in cubic metres of water or tonnes of carbon; a flow requires a time unit, such as cubic metres per day or tonnes of carbon per year. Comparing these quantities without their units confuses a stock with a rate.

Energy drives transfers

Material cycling requires energy, but energy and material follow different paths.

Solar radiation warms water surfaces and supplies energy for evaporation, the change from liquid water to water vapour. Some absorbed energy becomes latent heat associated with the change of state rather than a rise in temperature. When vapour condenses into droplets, latent heat is released to the surroundings. Water molecules can be recycled; energy eventually leaves Earth as radiation, so the cycle needs a continuing energy supply.

Gravity provides a direction for water moving downhill through channels or through permeable ground. Water raised above sea level has gravitational potential energy. The Sun helps lift water indirectly through evaporation and atmospheric transport; gravity then helps return it towards lower ground and the oceans. Wind also transports water vapour from one location to another. An energy input does not itself create additional water.

In the carbon cycle, sunlight powers photosynthesis and energy is stored chemically in organic matter. Respiration and decomposition release energy as carbon compounds are broken down, while carbon is transferred among organisms, soils and the atmosphere. Decomposition is the breakdown of dead organic matter by decomposers. Its rate depends partly on temperature, moisture and oxygen, so a change in the water system can alter a carbon transfer. For example, waterlogged soil can limit oxygen availability and slow aerobic decomposition. That is an interaction between systems, not proof that all wet soils store identical amounts of carbon.

Consider a sunny day followed by rainfall on a hillside. Solar energy promotes evaporation from a nearby sea. Atmospheric movement carries vapour; uplift and cooling can produce rain. Gravity drives downhill water flow. Plants on the hillside also use solar energy to fix carbon, provided other requirements such as water are met. Energy therefore drives both cycles, but evaporation changes water's state while photosynthesis incorporates carbon into organic compounds.

Dynamic equilibrium

A broadly stable store can have large inputs and outputs occurring continuously.

For a chosen store, its change equals the material entering minus the material leaving during the same period. If an illustrative lake gains 40 units of water and loses 40 units during a month, its net storage change is zero. It is still active: 40 units have entered and 40 have left. If a forest vegetation store gains 12 units of carbon through photosynthesis and loses 12 through respiration, mortality and other outputs, the vegetation store is similarly stable despite continuing transfers. These are original illustrative examples, not measured case data.

An annual balance can conceal seasonal movement. A lake may gain water in a wet winter and lose it in a dry summer, ending the year close to its starting volume. A stable annual total does not establish stability on every day. Equally, one wet month does not establish a long-term trend. State the timescale, then compare the size and direction of departures from the starting condition.

A disturbance changes an input, output or internal process. A persistent rise in water input can enlarge a lake until a larger outlet flow balances the new input. If that adjustment occurs, the lake may reach a new dynamic equilibrium at a higher level. It need not return to its original state. If outputs cannot increase sufficiently, storage can continue growing and a threshold such as bank overtopping may be crossed. Likewise, sustained extra carbon emissions can increase the atmospheric store when removals do not keep pace. Merely identifying transfers does not demonstrate equilibrium: the balance must be considered.

Feedback loops

A feedback exists when a change alters processes that then act back on the original variable.

Positive feedback amplifies the initial change; negative feedback counteracts it. The terms describe direction, not whether the outcome is desirable. A single causal link is not a complete feedback: follow the chain back to the variable that changed first.

For a water-related positive feedback, consider warming near a snow-covered surface. Warming increases snowmelt. The darker exposed surface reflects less incoming sunlight: its albedo, the proportion of radiation reflected, is lower. Greater absorption promotes further warming, reinforcing the initial temperature increase. This does not mean melting continues without limits; available snow, season and incoming energy constrain the response.

A carbon-related positive feedback can occur where warming speeds decomposition of suitable soils. Faster decomposition releases more carbon dioxide; the additional greenhouse gas can contribute to further warming, acting back on temperature. Moisture, oxygen and available organic matter affect the strength of this loop. These are possible mechanisms, not a claim that every soil responds identically.

A lake provides a clear negative-feedback example. A rise in lake level can increase the water pressure and gradient at an unobstructed outlet, increasing outflow. Extra loss counteracts the level rise. The response operates only where the outlet allows discharge to increase. A dam with fixed releases would not necessarily behave this way.

In the carbon system, increased atmospheric carbon dioxide can stimulate photosynthesis in some plants where light, nutrients and water are sufficient. Greater uptake removes additional carbon from the atmosphere, counteracting the initial increase. This potential negative feedback is limited by those resources and by subsequent carbon release; it does not imply that vegetation automatically cancels additional emissions. NASA's carbon-cycle explanation describes the interacting temperature and vegetation responses.

To classify a loop, name the initial variable, trace each process and decide whether the final effect reinforces or opposes the initial change. Negative feedback can support dynamic equilibrium, but equilibrium is a state of balance and feedback is a mechanism. A balanced system need not have every process controlled by negative feedback, and positive feedback may operate within a system that is stable over another scale or period.