Habitable Earth

Earth is unique (at least within our solar system) in its ability to sustain complex life
What is needed for complex life?
- Temperatures that permit liquid water
- An oxygenated atmosphere

Forms the basis of all life forms on earch
Can dissolve many substances more than most other liquids, enables key chemical reactions in cells
Allows cells to use nutrients, minerals, and chemicals in biological processes

Key factor in the habitability of planets
Essential for the existence of life on earth
Energy source: living organisms store energy in molecules (fat, sugars, etc), release energy when combined with oxygen
21% of earths dry atmosphere
Almost completely absent in early earth
3.5 billion years ago mostly nitrogen and carbon dioxide.
- Only found in small concentrations
- Reacted with other chemicals rapidly moving form the atmosphere
- Cyanobacteria (first algae)
  - conducted photosynthesis
- $C0_2$ levels in atmosphere gradually decreased and $O_2$ increased, allowing for life

A network of relationships, parts elements and or components that interact and influence one another through the exchange of energy, matter, and/or information
Functions together to act as a whole
- Responds to inputs and outputs
Earth's system examples
- Processes that change landscape (erosion, substrate movement)
- Species interactions
- Cycle Chemical interactions
Two General Types of Systems
- Open System: some energy or material moves into or out of system
- Closed System: no such movements occur, doesn't change any matter with its surroundings
Static System
- Fixed condition
- System stays in that condition
Dynamic Condition
- Changes (over time), often continually

Stable System
- Condition that remains, unless disturbed
- If disturbed it returns to the same condition and the cause of disturbance stops
Steady state is not attained or maintained for a very long time

Dynamic Equilibrium
- The steady state that all natural systems move towards without the influence of people
- Seldom attained or maintained for long
Linear Flow
- if you add the same amount of anything to a compartment in a system, the change will always be the same
- if you add salt to a glass of water the salinity will increase linearly
Non Linear Flow
- The effect of adding a specific amount of something changes depending on how much has been added before
- If you add 10 wolves to a forest, the amount of deer that they will eat will depend on the number of wolves already present in the forest
Lag Time
- The delay between a cause and the appearance of its effect
Average Residence Time
- How long on average a quantity will remain in a reservoir of a system
- Equal to reservoir size divided by the rate of output

Occur when outputs of a system are routed back as inputs as part of a chain of cause and effect that causes a loop or circuit
Feedback
- When on part of system changes
- These affect another part of the system
- Which then affects first change
Negative Feedback
- Increase in output leads to later decrease
- Move the system in opposite direction
- Neutralizes and stabilizes the system
Positive Feedback
- Drives system towards an extreme
- an increase in output leads to a further increase in output
  - Destabilizing
- More growth, people have more children, more output, destabilizes

Negative Feedback
- Warmer temperatures cause ice to melt over the tundra
- More plants can grow
- Plants use up $CO_2$, causing earth to cool
Positive Feedback's
- Colder temperatures will lead to growth of the polar ice caps
  - Ice reflects more light back into space than land and sea
  - More light is reflected to space, earth cools further
- Warmer temperatures lead to increased water vapor in atmosphere (humidity higher)

What are the most abundant elements on earth?
- Oxygen (air)
- Hydrogen (water)
- Silicon (crust)
- Carbon (living)