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Global Warming
Archer chapters 1 & 2
GEO 307
Dr. Garver
5/24/2017
Chapter 1: Humankind & Climate
• There is no doubt the Earth is warming.
• Is it us?
• What evidence are we seeing?
• Weather vs. Climate
• What’s the difference?
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• Human induced changes are expected to be
small compared to variability.
– T in this century expected to rise a few deg.
– Hard to calculate a change in the avg. when the
variability is so much greater than the trend.
– In addition there is long term climate change.
• Little Ice Age - 1650-1800
• Last glacial maximum (20,000 ybp) was only
5-6 deg C cooler than today
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Little Ice Age
• Period of cooling 1550 AD and 1850 AD - after
Medieval Climate Optimum
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Forecasting Climate Change
• T of Earth is determined by balance of energy in
and energy out.
• Sun drives earth's climate, heats the earth's
surface; earth radiates energy back into space.
• It is possible to change the T of Earth by
changing either incoming or outgoing energy.
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Climate Forcing:
• Sunspots change output of sun
• Changing reflection of Earth
• Greenhouse effect
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• Most gases in the atmosphere are not
gh gases.
• Greenhouse gases (water vapor, carbon dioxide,
methane) trap some of the outgoing energy.
– Water vapor is tricky, it amplifies the warming
effects from changes in other gh gases.
• Without "greenhouse effect," T would be much lower,
life would not be possible.
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Human Activity
• Carbon dioxide - burning fossils fuels
• Methane - landfills, livestock, rice cultivation
• Particulates - smokestacks, combustion engines.
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Asessing the Risk
• Forecast is an increase of 2-5 deg by
2100.
• Models - Used to forecast increase in T
and the results of that increase.
– many are economic
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Greenhouse Effect
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Chapter 2: Blackbody Radiation
• Electromagnetic Radiation
• Energy travels through a vacum from Sun to
Earth.
• Objects can absorb energy and re-emit it.
• Black Body - any object that is a perfect
emitter and a perfect absorber of radiation
• sun and earth's surface behave approximately
as black bodies.
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Radiant energy
• transfer of energy via electromagnetic
waves.
• Radiation
– examples:
• sun warms your face
• apparent heat of a fire
• wavelength, frequency
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Energy through a vacum
• EMR - travels as wavelengths
• c = speed of light, constant
• relates frequency to wavelength.
• fig 2.2
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Common wavelengths
• units of micrometers are often used to characterize the
wavelength of radiation
• 1 micrometer = 10-6 meters
• paper is about 100 micrometers thick
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Radiation emitted by objects
• All objects that have a T greater than 0 deg K
emit radiation
• hot objects emit more radiation that colder
objects
• Need to know much radiation is being emitted
by an object, and at what wavelengths.
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Black Body Radiation
• Black Body - any object that is a perfect
emitter and a perfect absorber of radiation
– sun and earth surfaces behave
approximately as black bodies
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Stefan-Boltzman Law
• relates the total amount of radiation emitted by
an object to its temperature:
E=sT4
where:
E = total amount of radiation emitted by an object per
square meter (Watts m-2)
s is a constant = 5.67 x 10-8 Watts m-2 K-4
T is the temperature of the object
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• Josef Stefan, (1835 – 1893) Austrian physicist - 1879
formulated a law which states that the radiant energy of a
black body is proportional to the fourth power of its
temperature.
• One first important steps toward understanding of radiation.
• Five years after he derived his law empirically, it was derived
theoretically by Ludwig Boltzmann of Austria and hence
became known as the Stefan–Boltzmann law.
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Weins Law
• Most objects emit radiation at many wavelengths
• There is one wavelength where an object emits the largest
amount of radiation
lmax = 2897
(mm K)
T
(K)
• At what wavelength does the sun emit most of its radiation?
• At what wavelength does the earth emit most of its
radiation?
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• Also called Wien’s displacement law
• Named after German physicist Wilhelm
Wien, who received the Nobel Prize for
Physics in 1911 for discovering the law.
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Temperature Scales
• Kelvin
• Celsius
• Fahrenheit
• Temperature Conversions:
ºC = 5/9(ºF-32)
K = ºC + 273
Absolute zero at 0 K is −273.15
°C (−459.67 °F)
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What are the similarities and differences
between the Sun and Earth radiation
curves?
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percentages in each wavelength band
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Radiative Equilibrium
If the T of an object is constant with time, the object is in radiative
equilibrium at Te
What happens if energy input > energy output?
What happens if energy input < energy output?
Is the earth in radiative equilibrium?
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Radiative Equilibrium for the Earth
•
•
Energy gained through absorption of short wave radiation is equal to the
emitted long wave radiation
So, what is the radiative equilibrium temperature for the earth?
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Radiative Equilibrium Temperature for the Earth
• Use Stefan-Boltzman Law
• Simplified case of no atmosphere
• Te = 255 Kelvin
• earth should be frozen!
• actual Te = 288 K
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• Earth emits 240 Watts m2
Using E = sTe4
then Te = (E/s)1/4
• So, for the simplified case of no
atmosphere Te= 255 K
• But Te = 288 K
• What is the reason for why the observed Te is
warmer than what we calculated using the
Stefan-Boltzman law???
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Interaction of Solar Radiation and the
Atmosphere
• Based on last figure, ~1/2 of incoming sw
radiation makes it to surface
• ~19% is absorbed by gasses in the atmosphere
• Therefore, the atmosphere is fairly
transparent to incoming solar radiation.
• Does the atmosphere have interaction with
lw radiation emitted by earth???
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Sun – Range of
primary
wavelengths
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Earth – Range
of primary
wavelengths
Interaction of Long Wave Radiation and the
Atmosphere
•
•
•
•
•
Some lw radiation emitted by earth escapes to space
Some lw is absorbed by gasses in atmosphere
These gasses then re-emit some =lw radiation back to the ground
The additional lw radiation reaching the ground further warms the earth
This is known as the "greenhouse effect"
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•
•
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Methane (CH4)
Carbon Dioxide (CO2)
Ozone (O3)
Water Vapor (H2O)
Nitrous Oxide (N2O)
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