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Systems & Feedback Loops

As you may recall from the Biosphere module, a system is is any set of ordered, interrelated components that are connected by flows of energy and matter. Systems can be open or closed:

  • Open Systems allow for the exchange of energy and matter across boundaries between parts of the system and other systems. For example, an estuary is an ecosystem that exchanges energy and matter with freshwater (river) systems and marine (ocean) systems.
  • Closed Systems are completely self-contained and do not exchange energy or matter across any boundaries. Earth has very few examples of fully closed systems. In terms of physical matter, Earth itself is a closed system - we are stuck with what we have on Earth! However, Earth is an open system in terms of energy - something we will discuss further soon.

 

Equilibrium state

Systems exist in an equilibrium state - the changing (dynamic) or relatively non-changing (steady-state) conditions of a system.

  • Steady-state Equilibrium: When a system is in balance over time, neither growing or contracting but existing in full operation. Steady-state systems may have small oscillations around an average level or condition, but the average value will remain steady over time.
As in this image, steady-state systems may have small changes around an average value, but the average value will not grow or decrease over time.
Steady-state systems may have small changes around an average value, but the average value will not grow or decrease over time.
  • Dynamic Equilibrium: When a system exhibits wide fluctuations around an average value, and the average value demonstrates a trend (e.g. growth or decrease) over time.
Until 19th century, the atmospheric concentration of methane experienced steady-state equilibrium (graph on left). However, methane concentrations have been rapidly increasing in Earth's atmosphere since the Industrial Revolution, and particularly since the mid-1900s (graph on right).
Until 19th century, the atmospheric concentration of methane experienced steady-state equilibrium (graph on left). However, methane concentrations have been rapidly increasing in Earth's atmosphere since the Industrial Revolution, and particularly since the mid-1900s (graph on right).

All systems will change over time, but at different rates.

Thus, some systems are seemingly in a steady state (at least according to our limited perception of time as humans), but are actually moving towards a new state of equilibrium. For example, geologic systems are constantly changing: rocks are constantly becoming new kinds of rocks through processes like weathering and erosion. However, these changes take place over thousands and millions of years, so seem relatively unchanging to us.

 

Feedback Mechanisms

A feedback mechanism is a process where the normal operations of a system cause a portion of the system's output to be returned as an input. This may cause changes which guide further system operations.

What happens in one part of a system impacts other parts of the system!

Negative Feedback Loops

Negative Feedback Loops tend to slow or reduce responses in a system. Negative feedback thus promotes self-regulation of the system. This tends to keep the system in its original condition, inhibiting change. In other words, negative feedback loops promote steady-state equilibrium.

One example of a negative feedback loop is arctic ice. A large mass of arctic ice keeps the air above it cold, which keeps the ice cold, which keeps the air cold, which keeps the ice cold...

A large mass of arctic ice. The light color of the snow and ice will help reflect incoming solar radiation, preventing absorption and keeping the air above the ice cool.
A large mass of arctic ice. The light color of the snow and ice will help reflect incoming solar radiation, preventing absorption and keeping the air above the ice cool.

Positive Feedback Loops

Positive Feedback Loops tend to amplify or encourage responses in a system. Positive feedback induces progressively greater changes in other parts of the system. Colloquially, this is often termed a "snowball effect" - picture a snowball rolling down a hill, picking up more and more snow as it rolls and therefore getting bigger and bigger.

Arctic ice is another example of how positive feedback works. In the summer, sea ice melts in the Arctic region. As arctic temperatures rise, summer sea ice and glacial melting accelerates. Light-colored snow and sea-ice surfaces, which reflect more sunlight and remain cooler, are replaced by darker-colored open ocean surfaces or the bare ground. Open ocean and bare ground surfaces are less reflective, and thus absorb more sunlight and become warmer. As a result, the ocean absorbs more solar energy, which raises its temperature, which melts more ice, exposes more ocean, etc.

In the arctic ice positive feedback loop, sea ice melts during the summer in the Arctic. As light-colored snow and ice melt, darker-colored open ocean or bare ground are exposed. These darker surfaces are less reflective and absorb more sunlight, further raising temperatures, melting more ice, exposing more ocean, and so on.
In the arctic ice positive feedback loop, sea ice melts during the summer in the Arctic. As light-colored snow and ice melt, darker-colored open ocean or bare ground are exposed. These darker surfaces are less reflective and absorb more sunlight, further raising temperatures, melting more ice, exposing more ocean, and so on.