The Greenhouse Effect (text p. 20-31)
Part 1
Radiation– visible and invisible
The infrared thermometer– effective temperature
A tale of three planets
The Earth’s energy balance
The role of greenhouse gases
Topics from the outline that we’ll be covering this week
Sensitivity
Tipping
Points
Global
Warming
Feedbacks
Greenhouse
Effect
What is a
Greenhouse Gas?
Natural
Sources
Buildup of
Greenhouse
Gases
Anthropogenic
Sources
Radiation– visible and invisible
Waves (or particles) that transmit energy in a specific direction
It’s the only way of transmitting energy through a vacuum
Matter emits and absorbs radiation
The higher the (absolute) temperature of a body
the more radiation it emits (S-B)
the shorter the wavelength of the emitted radiation
S-B: Stefan-Boltzmann law
WDL: Wien displacement law
(WDL)
As a fire dies, the peak of the emitted radiation shifts toward longer wavelengths; the yellow and
orange disappear first and the last to go is the deep red. Even after the visible radiation disappears,
the hot ashes continue to emit infrared radiation that you can feel if you are close to it.
The infrared thermometer– effective temperature
measures emitted radiation
converts it to an “effective temperature” scale
Thermal infrared imaging
measures emitted radiation
displays it on color or gray scale
Examples of infrared imagery: warm colors denote high emission and vice versa.
Thermal infrared imaging
The Earth viewed with infrared imagery. The yellow patches in the tropics are clouds with cold tops.
The deserts show up clearly as hot spots.
This figure is added to help you interpret the infrared image in the previojus slide. High clouds are
rendered in pure white; low clouds in off-white.
The infrared thermometer– effective temperature
radiation flux Watts per square meter
measures emitted radiation
converts it to an “effective temperature” scale
Temperature (K)
The infrared thermometer– effective temperature
radiation flux Watts per square meter
measures emitted radiation
converts it to an “effective temperature” scale
Temperature (K)
The infrared thermometer– effective temperature
radiation flux Watts per square meter
measures emitted radiation
converts it to an “effective temperature” scale
Temperature (K)
The infrared thermometer– effective temperature
radiation flux Watts per square meter
measures emitted radiation
converts it to an “effective temperature” scale
Temperature (K)
Earth’s effective temperature (measured from satellite)
255 K
–18°C
0°F
Just for comparison,
Earth’s surface temperature (measured with thermometers)
288 K
+15°C
60°F
A tale of three planets
Earth upper left; Venus below it; Mars to the right. The sizes are not to scale.
Inverse square law
1.56 AU F = 592 W / sq. m
1.00 AU F = 1380 W / sq. m
0.72 AU F = 2639 W / sq. m
1.00
0.72
1.52
Distance from sun
1380
2639
592
Watts per square meter
of solar radiation
Reflected / Incoming = Albedo
0.30
0.78
0.17
Albedo
Text
Emitted flux = 1/4 x (1– Albedo) x solar flux
= 1/4 x (1.00 – 0.30) 1368
= 239 watts per square meter
Emitted flux = 1/4 x (1– Albedo) x solar flux
= 1/4 x (1.00 – 0.30) 1368
= 239 watts per square meter
If we know the emitted flux,
then we know the effective temperature
Greater distance from the sun favors a lower
effective temperature
A higher albedo favors a lower effective temperature
255 K
225 K
216 K
Effective temperature
255 K
225 K
216 K
Venus has a lower effective
temperature than Earth because
it has a high albedo.
288 K
737 K
210 K
Surface temperature
Venus
Earth
Mars
Ts
737
288
210
TE
225
255
216
Ts – TE
527
33
–6
Why is Ts different from TE ?
Why is Ts – TE so different for different planets
Answer: The greenhouse effect
The Earth’s energy balance
Reconsider the Earth’s energy
balance taking the role of the
atmosphere into account.
We’ve already accounted for
the contribution of clouds to
the albedo...
... but not the effect of gas
molecules in absorbing energy
emitted from below and reemitting some of it back
downwards toward the surface
In other words, gas molecules block some of the outgoing
infrared radiation. They thus warm the Earth’s surface, much
as a blanket warms a sleeper.
If the atmosphere had no GHG, clouds, or aerosols, infrared
radiation would pass through it without being absorbed.
But if we suddenly cover the Earth with an atmosphere, most of the
radiation emitted by the Earth’s surface gets absorbed, and only a small
fraction passes through.
The atmosphere emits the radiation it absorbs. Most of it is emitted
downward, back towards the Earth’s surface.
The downward flux of infrared radiation from the atmosphere makes the
Earth’s surface warm up, and as it gets warmer it emits more radiation
Eventually the system (Earth + atmosphere) comes into equilibrium.
The Earth’s surface has been warmed by the addition of an atmosphere.
This is the essence of the greenhouse effect.
Incoming radiation alone
Balance between incoming
and outgoing energy
Incoming radiation alone
The greenhouse effect
Balance between incoming
and outgoing energy
1
0.01
92
Atmospheric mass
Comparison of the mass per unit area of the atmospheres of Venus, Earth, and Mars. Venus
has a massive atmosphere composed mainly of a strong greenhouse gas (CO2) which gives
rise to a very strong greenhouse effect.
The greenhouse
effect is
real.
Without it, Earth would
be uninhabitable
1
Atmospheric mass
but it’s possible to have
too much of a good thing
Earth Portal, Encyclopedia of Earth
The greenhouse effect would make the Earth’s surface even warmer than
288 K were it not for evaporation and heat transfer by conduction.
A more detailed rendition of the surface energy balance. This was not explained in class.