Feedback and Resilience
1.2 Systems
Negative Feedback
Feedback that tends to damp down, neutralize or counteract any deviation from an equilibrium, and promotes stability.
Negative Feedback
Feedback that tends to damp down, neutralize or counteract any deviation from an equilibrium, and promotes stability.
Negative feedback mechanisms have a stabilizing effect as they prevent significant change.
Positive Feedback
Feedback that amplifies or increases change: it leads to exponential deviation away from an equilibrium.
Positive Feedback
Feedback that amplifies or increases change: it leads to exponential deviation away from an equilibrium.
Positive feedback mechanisms have a destabilizing effect.
TASK
For each of the following scenarios, state whether it is an example of positive or negative feedback.
As carbon dioxide levels in the atmosphere rise the
temperature of the Earth rises.
As the Earth warms the rate of photosynthesis in
plants increases, more carbon dioxide is therefore
removed from the atmosphere by plants, reducing
the greenhouse effect and reducing global
temperatures.
1
As carbon dioxide levels in the atmosphere rise the
temperature of the Earth rises.
As the Earth warms the rate of photosynthesis in
plants increases, more carbon dioxide is therefore
removed from the atmosphere by plants, reducing
the greenhouse effect and reducing global
temperatures.
1
Negative
As the Earth warms:
Ice cover melts, exposing soil or water.
Albedo decreases (albedo is the fraction of light that
is reflected by a body or surface) .
More energy is absorbed by Earth’s surface.
Global temperature rises.
More ice melts.
2
As the Earth warms:
Ice cover melts, exposing soil or water.
Albedo decreases (albedo is the fraction of light that
is reflected by a body or surface) .
More energy is absorbed by Earth’s surface.
Global temperature rises.
More ice melts.
2
Positive
As Earth warms, upper layers of permafrost melt,
producing waterlogged soil above frozen ground.
Methane gas is released in an anoxic environment.
The greenhouse effect is enhanced.
Earth warms, melting more permafrost.
3
As Earth warms, upper layers of permafrost melt,
producing waterlogged soil above frozen ground.
Methane gas is released in an anoxic environment.
The greenhouse effect is enhanced.
Earth warms, melting more permafrost.
3
Positive
As Earth warms, increased evaporation produces
more clouds.
Clouds increase albedo, reflecting more light away
from Earth.
Temperature falls.
Rates of evaporation fall.
4
As Earth warms, increased evaporation produces
more clouds.
Clouds increase albedo, reflecting more light away
from Earth.
Temperature falls.
Rates of evaporation fall.
4
Negative
As Earth warms, organic matter in soil is decomposed
faster:
More carbon dioxide is released.
Enhanced greenhouse effect occurs.
Earth warms further.
Rates of decomposition increase.
5
As Earth warms, organic matter in soil is decomposed
faster:
More carbon dioxide is released.
Enhanced greenhouse effect occurs.
Earth warms further.
Rates of decomposition increase.
5
Positive
As Earth warms, evaporation increases:
Snowfall at high latitudes increases.
Icecaps enlarge.
More energy is reflected by increased albedo of
ice cover.
Earth cools.
Rates of evaporation fall.
6
As Earth warms, evaporation increases:
Snowfall at high latitudes increases.
Icecaps enlarge.
More energy is reflected by increased albedo of
ice cover.
Earth cools.
Rates of evaporation fall.
6
Negative
As Earth warms, polar ice sheets melt releasing large
numbers of icebergs into oceans.
Warm ocean currents such as Gulf Stream are
disrupted by additional freshwater input into ocean.
Reduced transfer of energy to poles reduces
temperature at high latitudes.
Ice sheets reform and icebergs retreat.
Warm currents are re-established.
7
As Earth warms, polar ice sheets melt releasing large
numbers of icebergs into oceans.
Warm ocean currents such as Gulf Stream are
disrupted by additional freshwater input into ocean.
Reduced transfer of energy to poles reduces
temperature at high latitudes.
Ice sheets reform and icebergs retreat.
Warm currents are re-established.
7
Negative
Ecosystems and Feedback
Ecosystems generally exist in a steady state equilibrium (or one developing over time in the case of succession).
They stay in a relatively constant state because of negative feedback loops.
Ecosystems and Feedback
More birds…
…more fish eaten…
…fewer fish…
…less food for birds…
…fewer birds survive…
…fewer fish are eaten…
…fish population increases… (and so on…)
pixabay.com
Tipping Points
Positive feedback can drive a system to a tipping point, causing the system to change to a new equilibrium.
Image: Creative commons Rauter et. al.
Tipping Points
Some systems can resist more change than others (they are more resilient).
Systems that have larger storages and are more complex are more resilient.
Image: Creative commons Rauter et. al.
The Impact of Humans
Human activities can reduce the resilience of a system in 2 main waysL
E.g. by overfishing, deforestation
Converting forests (complex ecosystems) into farmland (monocultures).