The “Green House Effect”
Key Concepts: Lecture 11
• Temperature set by balancing
“energy in” and “energy out”
Earth’s Atmosphere and Greenhouse Effect
– Energy In: From sunlight: 50% reaches
the ground (the rest reflect back to
space)
– Energy Out: Infrared light from the
heated surface
Blackbody Radiation and Temperature
• Without Atmosphere
T(average)=-18C = 255 K
• With Atmosphere T(average)=+15C
= 288 K
Earth’s Oceans
– Atmosphere is transparent to optical
sunlight
– Water and Carbon Dioxide are
opaque to infrared from surface energy is trapped and thus heats the
Earth’s surface
Earth’s Magnetic Field and Aurora
Note Kelvin Temperature scale:
Absolute zero: 0K = -273 Celsius (C)
0C = +273K
So -18C = 273-18=255K
and +15C = 273+15=288K
Earth’s Atmosphere
• Atmospheric
Composition
Recall the Electro-Magnetic (EM) Spectrum
– 78% Nitrogen
– 21% Oxygen
– 1 % Argon, 0.03% Carbon
dioxide, Water
• Electro-magnetic
waves of different
wavelengths. Visible
light is only a very
small part of the
whole EM spectrum.
• Protects the surface
– Blocks ultraviolet radiation from
Sun
– Blocks cosmic rays and radiation
– Blocks small impacts
• Regulates Surface
Temperature
– Clouds reflect sunlight
– Atmosphere traps and holds heat
– Atmospheric circulation moderates
temperatures
For Kelvin (K) temperature
scale, see next slide
Light is emitted from all objects depending on
their temperature: “Blackbody Radiation”
(see Ch. 2.4 in textbook)
• All objects with a temperature above absolute
zero (-273C; -459F; 0K) emit light: the light
waves carry energy.
• Temperature is a measure of how quickly the
atoms in an object are moving and vibrating.
• Hotter objects emit more light energy because
the motions of the electrons in their atoms are
more violent: larger amplitude of waves in
electric field.
• Hotter objects emit shorter wavelength light
because the frequency of oscillation of electrons
is higher.
Question?
• Since the industrial
revolution humans have
been burning more trees
and fossil fuel. This has
caused the amount of CO2
to increase in the
atmosphere.
• What effect would you
expect this to have on
temperatures on Earth?
400 ppm was breached on May 9, 2013
Earth’s temperature is about 300K
400
• Room temperature objects (i.e. T~300K)
emit light that peaks in the infrared, with
wavelengths ~10 microns.
• This is about 20 times longer than visible
light and is not visible to our eyes.
“Astro” News
Atmospheric Escape
• Properties of a gas
– behaves like a group of hard balls moving and
colliding
– Speed depends on the temperature
• Light gas particles move faster
– When they collide with more massive particles
they recoil at higher speeds
Equal forces
• The gravity of a planet holds the gas in
– More massive planets can hold lighter gasses
Larger
accel
The Primordial Atmosphere
• The original atmosphere - we expect this
was mostly hydrogen and helium
– These are the most common elements in the
universe
– Came from formation of Earth
• Atmosphere was lost
– Light atoms move faster than heavy
– Hydrogen and Helium have low mass
– Their velocity is above escape velocity (11km/s)
Evolution of the Atmosphere
• Secondary atmosphere from volcanic
outgassing
– Volcanoes emit CO2, SO2, H2, N2, water (H2O),
methane (CH4), ammonia (NH3)
• Removing the carbon dioxide
– Dissolves in the oceans and is subducted
– Ends up in rocks
– 3 billion years ago mostly methane, H2, SO2
• Formation of N2 and CO2
– Ultraviolet sunlight breaks up methane and ammonia
– Nitrogen from ammonia - CO2 from methane and water
– Hydrogen escapes into space
Oxygen (O2) in the Atmosphere
Origin of the Ocean
• After formation
• Very little primordial oxygen
– Almost no oxygen 2 billion years ago
– 1 billion years ago 2% of atmosphere was O2
– 600 million years ago sudden increase
• Origin of oxygen
– Biological activity started ~3.5 billion years ago
– Plants convert CO2 into O2 and trap carbon
– Dead biomass is trapped in the Earth and
subducted -> oil
The Ocean
• Water covers a large fraction of the
Earth
– 71% of surface area
– Average depth 4 km
• Helps regulate and moderate
temperatures
• Important for evolution of
atmosphere
• Must have liquid water for life
• Contains much of the life on Earth
– Almost no liquid water
– Water trapped inside the Earth
• Water from inside the Earth
– Volcanoes emit gasses and steam
– Steam cools and condenses into water
– Oceans in 4.5 billion years at current rate
• Water also came from space
– Earth is continual hit by comets
– Comets contain a large amount of water ice
• Water can be removed into the Earth by
subduction
Question?
• If the Earth had no water and therefore life
had never occurred, what would the Earth’s
atmosphere be like?
• If you were trying to locate a planet with
life on it, what atmospheric constituents
would you look for?
The Magnetic Field
• The solid core rotates faster than
the surrounding liquid core.
• This Dynamo acts like a
generator.
• The field is roughly aligned with
the rotation axis.
• The field flips every 10000 years
or so.
• Rotating, electrically
conducting, i.e. metallic, liquid
core is necessary to produce a
strong magnetic field.
The Aurora
• The Sun emits a large
number of charged
particles from solar
activity
• These particles are
trapped in the Earth’s
magnetic field
• The field channels
them near the pole
• When they hit the
atmosphere it causes
the air to glow
Question
• If the Earth had no magnetic field. What
would happen to the Aurora?