Positive/Negative feedback

• Dynamic Equilibriums


  • Continuous inputs, throughputs, outputs and variable
  • Stores of energy
  • Stores of materials
  • Inputs and outputs
  • Long term flows will fluctuate year to year
  • Stores usually maintain balance, allowing system to retain its stability
• Negative feedback loops within system

  Restore balance
• Drainage basin has unusually heavy rainfall

  Increases water in aquifer, raises water table, increases flow from springs until balance is restored
• Burning fossil fuels


  Increases CO2 in atmosphere, but stimulates photosynthesis to remove excess CO2 and restore equilibrium
• Tipping point is a point of no return where the system can never recover
• Positive feedback


  Initial change causes further change in the same direction, amplifying the effect
• Negative feedback


  Counters system change, restoring equilibrium
• Positive feedback: Warmer temperatures

  1. Increases evaporation

  2. Atmosphere holds more vapour

  3. Increases cloud cover and precipitation

  4. Water vapour is a greenhouse gas, causing further temperature rise
• Negative feedback: More vapour creates greater cloud cover


  1. Reflects solar radiation back to space

  2. Reduces solar radiation absorbed by atmosphere, oceans, land

  3. Average global temperatures fall
• Negative feedback: Drainage basin responds to above average precipitation

  1. Increases river flow and evaporation

  2. Excess water recharges aquifers, increasing water storage

  3. During droughts, system adjusts by reducing evapotranspiration as water table falls, helping conserve groundwater stores
• Negative feedback: Stressed shallow rooted trees

  Reduce transpiration losses by shedding leaves, balancing tree survival
• Increase in atmospheric CO2

  Acidifies oceans, but carbon fertilisation stimulates photosynthesis to extract excess CO2, allowing it to be stored in biosphere and oceans, returning the system to a steady state
• Global warming


  Intensifies carbon cycle, speeding up decomposition and releasing more CO2, amplifying greenhouse effect (positive feedback)
• Arctic tundra and sea ice/snow cover shrinks

  Exposes more land, absorbing more sunlight and warming the tundra, melting permafrost (positive feedback)
• Planet warms


  Less trees can sequester CO2, cooling the planet less (positive feedback)
• Increasing temperatures


  Increases wildfires, releasing more CO2 (positive feedback)