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Global WarmingGlobal WarmingArcher chapters 1 & 2Archer chapters 1 & 2

GEO 307GEO 307

Dr. GarverDr. Garver

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Chapter 1: Humankind & ClimateChapter 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

Little Ice AgeLittle Ice Age

• Period of cooling 1550 AD and 1850 AD - after Medieval Climate Optimum

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Forecasting Climate ChangeForecasting 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 ActivityHuman Activity

• Carbon dioxide - burning fossils fuels• Methane - landfills, livestock, rice cultivation• Particulates - smokestacks, combustion engines.

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Asessing the RiskAsessing 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 EffectGreenhouse Effect

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Chapter 2: Blackbody RadiationChapter 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 energyRadiant 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 vacumEnergy through a vacum

• EMR - travels as wavelengths• c = speed of light, constant• relates frequency to wavelength.

• fig 2.2

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Common wavelengthsCommon 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 objectsRadiation 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 RadiationBlack 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 LawStefan-Boltzman Law

• relates the total amount of radiation emitted by an object to its temperature:

E=T4

where:E = total amount of radiation emitted by an object per

square meter (Watts m-2)

is a constant = 5.67 x 10-8 Watts m-2 K-4

T is the temperature of the object

• 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 LawWeins Law

• Most objects emit radiation at many wavelengths

• There is one wavelength where an object emits the largest amount of radiation

max = 2897 (m 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?

• 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 ScalesTemperature 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 bandpercentages in each wavelength band

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Radiative EquilibriumRadiative 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 EarthRadiative 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 EarthRadiative 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

• Earth emits 240 Watts m2

Using E =Te4

then Te = (E/)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 Interaction of Solar Radiation and the AtmosphereAtmosphere

• 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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Earth – Range of primary wavelengths

Sun – Range of primary wavelengths

Interaction of Long Wave Radiation and the Interaction of Long Wave Radiation and the AtmosphereAtmosphere

• 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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• Methane (CH4)

• Carbon Dioxide (CO2)

• Ozone (O3)

• Water Vapor (H2O)

• Nitrous Oxide (N2O)

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