AQA A-Level Geography

What a system is

A system is a set of components and the links between them, with a boundary that separates it from everything else. Geographers use the idea to simplify a landscape or a cycle into parts that can be counted, measured and compared. The same vocabulary is used for the water and carbon cycles, for hot deserts, coasts and glaciers, and for ecosystems, so it is worth knowing precisely.

A system has these parts:

• inputs, matter or energy entering across the boundary (rainfall into a drainage basin, sunlight into a forest, sediment arriving at a beach);
• outputs, matter or energy leaving it (evaporation, river discharge to the sea, sediment carried away);
• stores or components, where matter or energy is held for a time (soil moisture, ice in a glacier, carbon in a tree);
• flows or transfers, the movements between stores (infiltration, melting, longshore drift);
• energy, which drives the flows (solar radiation, wind, gravity, the heat of the Earth's interior).

Types of system

An isolated system exchanges neither matter nor energy with its surroundings. True examples are almost impossible to find on Earth. A closed system exchanges energy but not matter. The global water cycle and the global carbon cycle are treated as closed systems, because the Earth gains and loses almost no matter but is warmed by the Sun and radiates heat back to space. An open system exchanges both matter and energy. A drainage basin, a coastline, a glacier and an individual ecosystem are open systems: water, sediment, nutrients and energy cross their boundaries.

A drainage basin is the standard example. Precipitation enters at the top, evapotranspiration and river flow leave, and between them water sits in stores such as the soil and the groundwater.

Equilibrium and feedback

Most natural systems tend towards a balance in which inputs and outputs are roughly equal over time, so the stores stay about the same size. This is dynamic equilibrium: the system is not static, because it is constantly adjusting, but it fluctuates around an average state. A beach that loses sand in winter storms and regains it in summer, with no long-term change in its profile, is in dynamic equilibrium.

A change in an input, or in a condition at the boundary, disturbs the balance. The system then responds through feedback, in which the output of a process alters the process itself.

Negative feedback

Negative feedback reduces the original change and so restores the equilibrium. It is self-regulating. For example, a rise in atmospheric carbon dioxide stimulates faster plant growth in some conditions, and the extra photosynthesis takes carbon dioxide out of the air, which counteracts the rise. On a beach, a storm that cuts back the sand changes the profile so that waves break further out and lose energy, which slows further erosion.

Positive feedback

Positive feedback amplifies the original change and pushes the system further away from its starting state. It is self-reinforcing and, if nothing stops it, can carry the system across a threshold into a new equilibrium. For example, warming melts Arctic sea ice, which exposes dark ocean that absorbs more of the Sun's energy than bright ice reflects, so the warming increases and more ice melts. Thawing permafrost releases methane and carbon dioxide, which strengthen the greenhouse effect and cause more thawing.

A system can contain both kinds at once, and which one dominates decides whether it stays stable or shifts to a different state. Human activity often disturbs a system faster than its negative feedbacks can respond.

Landform, landscape and scale

A landform is a single feature of the surface, such as a spit, a corrie or a barchan dune. A landscape is an assemblage of related landforms that together give an area its character, such as a drowned coastline of rias or a glaciated valley with its trough, hanging valleys and moraines. Landforms combine into landscapes because the same processes, operating for long periods on the same rocks, produce a related set of features.

Systems operate at different scales. A hillslope, a drainage basin and the whole globe are all water systems, and the stores and flows that matter differ at each. A flood hydrograph is a drainage basin idea, whereas the size of the ocean store only makes sense globally. The timescales also differ: a flash flood lasts hours, a river meander migrates over decades, and the retreat of an ice sheet takes thousands of years.

Applying the model

When describing any physical system, name the inputs and outputs, then the main stores and the transfers between them, then say what drives the flows. Next, explain how a change in one input spreads through the system, using feedback to say whether the system will recover or move to a new state. Finally, state the scale and the timescale. That order is the same for a river catchment, a sediment cell on a coast or the carbon cycle, which is why systems thinking is the thread through the physical topics.