Open, Closed and Isolated Systems

Every system in this course gets sorted by one question: what crosses its boundary? Energy, matter, both, or neither. Answer that and you have classified it — and the answer explains why the Earth recycles everything it has.

The three types

Which category a system falls into depends entirely on how energy and matter flow across its boundary.

Open systems

Both energy and matter are exchanged with the surroundings. Open systems are usually organic (living) systems that interact with their environment by taking in energy and new matter, often as biomass, and expelling energy and matter as waste products or as organisms leaving.

An ecosystem or habitat is an open system. So is your body — energy and matter are exchanged with your environment as food, water, movement and waste.

Closed systems

Energy, but not matter, is exchanged. Closed systems are usually inorganic (non-living).

The Earth and its atmosphere can be viewed as a closed system:

• The main input of energy is solar radiation.
• The main output of energy is heat, re-radiated from the Earth’s surface as infrared.
• Matter is recycled completely within the system. Technically tiny amounts enter and leave as meteorites, spacecraft and satellites, but these are considered negligible.

Global geochemical cycles are approximated to closed systems, because elements and compounds are continuously recycled between Earth’s natural reservoirs — the atmosphere, hydrosphere, lithosphere and biosphere. Overall quantities stay relatively constant over geological timescales. Carbon, for example, moves between the atmosphere, oceans and terrestrial ecosystems through photosynthesis, respiration and oceanic absorption, maintaining a dynamic equilibrium.

Artificial closed systems

• A sealed terrarium with the right balance of water and organisms — mosses, ferns, bacteria, fungi, invertebrates — can survive for years as a closed system, provided light and heat can cross the glass.
• Biosphere 2, built in the late 1980s, was a larger attempt at the same thing: a sealed research facility containing a rainforest, ocean, desert, savannah and agricultural area plus living quarters. Energy entered as sunlight and heat, but inputs of matter were limited: air and water were recycled and food was grown inside to sustain the inhabitants.

Isolated systems

Neither energy nor matter is exchanged. Isolated systems do not exist naturally — they are a theoretical concept, though the entire Universe could arguably be considered one.

Systems at different scales

Environmental systems are interconnected networks of components and processes, found at every scale from a single organism to a whole ecosystem. They include interactions between living organisms, their habitats, and physical elements such as water, air and soil.

Earth as a single integrated system

Rather than a collection of independent parts, Earth can be seen as a complex integrated system of interconnected components.

The Gaia hypothesis

The Gaia hypothesis, proposed by James Lovelock in the 1970s, presents a holistic view of Earth as a single, self-regulating system. Lovelock argued that Earth’s biota (living organisms) and their environment are closely linked and act together as one integrated system, with feedback mechanisms maintaining stability on a global scale — rather like homeostasis in a living organism.

It was introduced to explain how the composition of the atmosphere affects global temperature, and how the two are connected through complex feedback. Greenhouse gases such as carbon dioxide and methane raise global temperatures; in response, feedback mechanisms such as increased evaporation producing more cloud cover, or enhanced plant growth absorbing more carbon dioxide, may act to moderate that increase.

Worked examples