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Topic 8: Greenhouse Effect


Topic 8: Greenhouse Effect & Global Warming 8.5 & 8.6 Allen High School IB Physics SL Source: Chris Hamper Physics – PowerPoint PPT presentation

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Title: Topic 8: Greenhouse Effect

Topic 8 Greenhouse Effect Global Warming8.5
  • Allen High School
  • IB Physics SL
  • Source Chris Hamper Physics

  • This section is the most political and
    opinionated for the whole year.
  • Please focus on the tested material, not your
  • There will be questions on the May test, the
    answer will not necessarily match your opinion.

Greenhouse Effect
  • This is the warming of a planet due to its
    atmosphere absorbing ultraviolet radiation from
    the Sun and trapping the infrared radiation
    emitted by the warm Earth.
  • Two foundation principles
  • How does the energy of the Sun get to Earth?
  • How does this energy interact with the atmosphere?

Solar Radiation
  • As previously discussed, the power per unit area
    that reaches Earth is a constant 1380 W/m2
  • The intensity is related to its distance from the
  • We learned in Topic 7 that the electrons of atoms
    can be excited and move to a higher energy level.
    Light can excite electrons if the energy of the
    photon is exactly the same as the required
  • The reverse is true too If an excited electron
    drops down to a lower level, then it can release
    a photon.
  • Different level jumps/drops can result in
    different frequencies of light.
  • This is what happens when the atmosphere
    scatters light.

Ionization Excitation
  • Ionization happens when the photon of high
    frequency/energy is absorbed and causes an
    electron to be ejected from an atom.
  • Molecules can become excited when it absorbs a
    photon of the same resonant frequency. This
    absorption causes the molecules to move more,
    thus a higher temperature.
  • Typically, it is the infrared region (lower
    frequencies) of the spectrum that cause this sort
    of vibration.

  • The Suns electromagnetic radiation shines on a
    surface, it is either absorbed (causing surface
    to get warmer) or it is reflected.
  • The RATIO of reflected to absorbed radiation is
    called Albedo.
  • Snow has a high albedo b/c most radiation is
    reflected than absorbed. (albedo 90)
  • Dark forests have a low albedo (10).
  • The average for Earth is 30. This means it is
    not reflecting most of the radiation.

Albedo Chart
Blackbody Radiation
  • A heated solid will radiate (emit) a wide range
    of frequencies/wavelengths (colors), however not
    every wavelength will be equally intense.
  • Blackbody radiationis the radiationemitted by a
    perfect emitter.
  • A Blackbody absorbs all wavelengths, andcan
    emit all too.

Stefan-Boltzmann Law
  • As seen in the intensity distribution graph, as
    the temperature of a black body increase, the
    intensity of the radiation also increases. Or,
    the amount of energy emitted from the surface
  • The S-B Law relates the power emitted per unit
    area to the temperature of the surface
  • Stefan-Boltzmann Constant, s 5.67 x 10 -8

  • Interesting If the Earth had no atmosphere, the
    calculated temperature of Earth would be -18C!
    However, Earth is much warmer (thank goodness)
    because Earths atmosphere absorbs some of the
    radiated radiation. This is the Greenhouse
  • Earth does not radiate as well as a true
  • Emissivity is the ratio of the energy radiated by
    a body to the energy radiated by a blackbody of
    the same temperature. (e values are usually

  • Suns rays beam down on Earth (100)
  • The outer atmosphere (ozone) absorbs high energy
    (UV X-rays) parts of the spectrum (20).
  • Lower layers absorb infrared radiation via water
    vapour and CO2 (remember resonance) (30).
  • That leaves 50 of the suns energy arriving to
    us on Earth.
  • At Earths surface, some energy is reflected,
    which goes back through the atmosphere, not
    absorbed again.
  • The remaining is absorbed (visible) and increases
    the temperature of the ground.
  • Energy is then re-emitted from the ground in the
    IR region.

Surface Heat Capacity
  • The temperature increase of the ground can be
    calculated based on the heat absorbed.
  • (Dont confuse this with Topic 3s Heat capacity
    or Specific heat capacity!)
  • Surface Heat Capacity is the amount of heat
    required to raise the temperature of 1m2 of the
    ground by 1K. For Earth, Cs4 x 108 J/km2
  • Equations

The Greenhouse Effect
  • The IR radiated from the ground travels upwards
    through the atmosphere and as it does it is
    absorbed by CO2 and H2O. As a result, the
    molecules become excited, raising the temperature
    of the atmosphere. The excited molecules then
    radiate IR radiation in all directions, some
    traveling back to Earth.
  • The Earths net temperature is based on the
    radiation leaving and arriving.
  • By reducing the amount leaving, the temperature
    at which this balance will be achieved will be

Sankey Diagrams for Greenhouse Effect
Global Warming
  • The increase of Earths temperature until
    equilibrium is restored.
  • Models are developed to help scientists predict
    future outcomes.
  • Causes
  • Radiation from the Sun is not constant.
  • Due to solar flares/sunspots
  • Due to Earths orbit
  • The amount of Sun radiation absorb by Earths
    atmosphere can vary depending on the composition
    of gases in it.
  • Greenhouse gases include H2O, CO2, Methane, and
    nitrous oxide (in order of contribution).

Ice Core Data
  • Antarctica ice has been growing (new layers atop
    old layers) for thousands of years.
  • Drilling into these layers and taking samples
    from different layers has allowed scientists to
    examine the concentration of different isotopes
    of hydrogen. The varying concentrations help to
    determine the temperature of the layers.
  • Heavier isotopes mean the temperature was colder.
  • The layers of ice also contain air bubbles that
    tell us the composition of the atmosphere at the

Ice Core Data
  • Lets compare the temperature of Earth with the
    concentration of atmospheric CO2.

The Last 50 Years Data
Volume Expansion
  • As the temperature of a liquid increases, it
  • The Coefficient of Volume Expansion is the
    fractional change in volume per degree change in
  • Applying this to the oceans (water), if the
    average temperature increases then they will
  • Over the last 100 years, sea level has risen by
    20 cm.
  • Another cause of the rise is the melting of the
    ice caps (covers land in Antarctica). This does
    not include floating ice (glaciers), they
    displace their own mass, melting has no effect.

What causes the changes in CO2?
  • The finger is pointed at Human Activity.
  • Burning of fossil fuels pollution of greenhouse
  • Deforestation less plants to absorb CO2 as
  • What can WE do?
  • Greater efficiency of power production
  • Replacing the use of coal and oil with natural
  • Use of combined heating and power systems
  • Use exhaust heat from power plants to heat homes
  • Increase use of renewable energy sources and
  • Use of hybrid vehicles
  • Carbon dioxide capture and storage

This is an International Issue
  • IPCC (Intergovernmental Panel on Climate Change),
  • Assess all available information relating to
    human induced climate change.
  • Kyoto Protocol, 1997
  • Countries committing to reduce their greenhouse
    gas emissions by given percentages. By May 1997,
    117 countries signed.
  • Eairheart thoughts We should all do what we can
    to reduce emissions, it doesnt hurt to have less
    pollution!! Why not?!