IB Environmental systems and societies 1.2C Positive Feedback Loops Notes

Positive Feedback: How Systems Amplify Their Own Change

1. A positive feedback loop occurs when a disturbance amplifies itself, pushing a system further away from equilibrium rather than restoring balance.
2. Because the change reinforces itself, it can cause rapid and sometimes irreversible shifts in natural and human systems.

Key Characteristics

1. Self-reinforcing: outputs feed back to strengthen the original process.
2. Amplifying and destabilising: it increases deviation and drives the system away from equilibrium.
3. Works in either direction: unlike negative feedback, it accelerates change whether a value is rising or falling.
4. Nonlinear: changes accelerate over time rather than staying proportional.
5. Common in many systems: climate processes, population dynamics, and resource depletion all show it.

Ice-Albedo Feedback: Warming That Melts Its Own Brake

How the Ice-Albedo Loop Builds

1. Global temperatures rise due to greenhouse gas emissions.
2. Ice caps and glaciers melt, reducing surface reflectivity (albedo).
3. Darker ocean and land absorb more solar radiation instead of reflecting it.
4. Temperatures rise further, which melts even more ice.

The Result: A Self-Reinforcing Warming Cycle

1. A self-reinforcing cycle of warming and melting destabilises the climate system.
2. The system moves toward a new, warmer climate regime, contributing to sea-level rise, changes in ocean circulation, and more climate extremes.

Population Growth and Decline: Amplification in Both Directions

Population Growth Feedback

1. As population increases, so does the number of breeding individuals (reproductive potential).
2. More breeding means more births, which increases population size again.
3. The larger the population, the faster it grows, an exponential growth loop.

Population Decline Feedback

1. As population decreases, fewer individuals are available to reproduce.
2. The reduced reproductive potential drives the population down further.
3. This can push a species toward extinction, especially if genetic diversity drops below sustainable levels.

Permafrost Thaw and Methane: A Runaway Climate Loop

How the Permafrost-Methane Loop Builds

1. Rising temperatures thaw Arctic permafrost.
2. Thawing releases methane (CH₄), a potent greenhouse gas.
3. Methane strengthens the enhanced greenhouse effect, trapping more heat.
4. Warmer temperatures thaw more permafrost, releasing still more methane.

The Outcome: Warming That Feeds Itself

1. This forms a runaway climate feedback loop that intensifies global warming.
2. Once initiated, the loop accelerates and is difficult to reverse.

Tipping Points and Regime Shifts: When Feedback Crosses a Threshold

1. A tipping point is reached when a small change in one part of a system triggers a large-scale transformation into a new equilibrium or stable state.
2. These regime shifts are often difficult or impossible to reverse.

How a Loop Becomes a Tipping Point

1. Small change occurs: a disturbance starts to affect the system.
2. Positive feedback reinforces it: the disturbance accelerates itself.
3. A threshold is crossed: the system can no longer return to its previous equilibrium.
4. A new equilibrium is reached: the system settles into a new state, which may be less stable or less desirable.

Example

• Eutrophication in lakes and rivers

• Cause: excess nitrates and phosphates from fertilisers, sewage, or runoff enter a water body.
• Tipping point: at first the nutrients boost plant and algae growth.
• Once a critical nutrient level is reached, algal blooms explode and cover the surface.
• Regime shift: decomposing algae drive oxygen levels down, killing fish and other aquatic life.
• The water body becomes a dead zone that cannot support most life.

Systems Near Tipping Points

1. Amazon rainforest: reduced rainfall and deforestation cut transpiration, causing more drying and a possible shift to savanna.
2. Coral reefs: warmer water causes bleaching and loss of symbiotic algae, leading to coral death and an algal-dominated state.
3. Thermohaline circulation: freshwater from ice melt lowers ocean salinity, disrupting deep-water formation and altering global climate patterns.