Lecture on Global Warming

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Lecture on Global Warming

• Part 1Basic physical questions concerning global
  warming
• What physical processes control the temperature
  of a planet?
• What is the greenhouse effect?
• What is a greenhouse gas?
• Is the greenhouse effect beneficial?

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Planetary temperature controlCritical assumptions

• Neglect the atmosphere
• Assume Steady State
• Neglect geothermal heat
• Neglect the pole to equator temperature gradient

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Energy received from the Sun

• Projected area of the earth
• Energy intercepted per second
• Albedo a reflected
• Energy absorbed per second

a0.33
S1380W/m2
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Full disk IR image from GOES-10showing emitted
radiation
Image is taken in emitted long wave radiation
(printed as a negative)
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Energy radiated by the Earth

• Power emitted from a surface per area
  (Stefan-Boltzmann Law)
• Surface area of a sphere
• Total power emitted from Earth

T
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Radiation BalanceSteady State Assumption

• Assume that the power absorbed and emitted are
  equal

• Observed Earth temperature T288K15C

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greenhouse effect

• With no greenhouse effect T-21C
• With greenhouse effect T15C
• This is the difference between ice and liquid
  water no-life and life

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Greenhouse effect
Sun

• Wiens Law
• The wavelength of maximum emission depends
  inversely on the objects temperature

Visible
IR
Earth
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Atmospheric composition
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Greenhouse gases

• Air
• Trace gases

N-N
O-O
These simple symmetric molecules cannot absorb
thermal radiation. They have no dipole moment.
O
O-C-O
H H
H
O
O O
H-C-H
H
The complex molecules can absorb thermal
radiation by flapping. The non-symmetric
molecules can absorb radiation by rotating.
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Atmospheric absorption
Sun
Earth
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Absorbing properties of gases
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Greenhouse on other planets
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Review of Part 1

• Planets temperature takes on a value that allows
  emission to balance absorption
• Radiation from the sun mostly penetrates the
  earths atmosphere. The earths radiation is at
  longer wavelengths. It is partly trapped by the
  atmosphere.
• Greenhouse gases have a structure that allows
  them to absorb IR radiation through vibration and
  rotational changes. Air is mostly transparent
• Greenhouse effect warms earth by about 36C,
  allowing life on earth. Other planets are also
  warmed by their greenhouse effect.

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Part 2 Observed global warming and implications

• Temperature records
• Carbon dioxide increases
• Paleo-climates
• Arguments for and against action
• Future research needs

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Characteristics of observed global warming

• Global average temperature up 0.7C since 1880
• Coincident rise in carbon dioxide concentration
  (280 to 350ppmv)
• Carbon dioxide rise is driven by fossil fuel
  burning. About half of the added carbon dioxide
  is found in the atmosphere.

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Global Warming
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Carbon dioxide at Mauna Loa
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Global Warming
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Ice Core records from the last half million years
Cause or Effect?
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A few arguments favoring actionto reduce global
warming

• Warming is largest in polar regions. Loss of
  arctic ecosystems
• Poleward spread of tropical species (insects and
  disease)
• Sea level rise (melting glaciers and sea water
  expansion)
• Loss of mountain glaciers (water reservoirs)
• Increased storminess (unproven) such as
  hurricanes
• Increasing population. Increased energy use in
  air conditioning. More fossil fuel burning.
• Positive feedbacks could trigger catastophic
  change (run-away greenhouse like Venus)

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Ursus maritimuspopulation25,000
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A few arguments favoring inaction

• Observed warming is small compared to natural ice
  age climate fluctuations
• Warming is no larger than the natural
  inter-annual fluctuation (compare winter 2001-02
  with 2002-03)
• Warming is small compared to the pole to equator
  temperature gradient (nearly 40C)
• Warming in beneficial in many areas (e.g.
  Siberia)
• Science is uncertain no theory of the ice ages
• Carbon dioxide will fertilize plants. Excess
  will be absorbed.
• Global warming is preventing a new ice age
• Mankind has always adapted to a changing climate

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Future research

• Theory of the ice ages
• Theory of Venus climate history (runaway GH)
• Theory of solar variability
• Theory of positive feedbacks
• How are greenhouse concentrations are controlled?
• How do global changes influence local climates?
• Climate impact of clouds, aerosols and albedo
• Can we sequester excess carbon dioxide?