Global Warming: or Thermal Management on the Big Spaceship

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Global Warming: or Thermal Management on the Big Spaceship Lecture #22 Space concepts The Big Spaceship Agenda Planetary temperatures Venus Earth Is Global Warming Real? What is driving it? A Simple Model Solutions Blackbody Radiation Curve Black Body Radiation Curve

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### Global Warming: or Thermal Management on the Big Spaceship

Lecture #22

Space concepts

Agenda
• Planetary temperatures
• Venus
• Earth
• Is Global Warming Real?
• What is driving it?
• A Simple Model
• Solutions

• If a body absorbs and radiates EM radiation perfectly, it is called a “blackbody”
• It then radiates power by the Stefan Boltzman Law W/m2= T4
• =emissivity, 1 for a black body ,  >1 for real surfaces
• =5.76x 10 –8 Wm –2 K-4
• ( Stefan-Boltzmann constant)

• ~ .5-micron because the sun is hotter than they are ( 10,000K)
• They radiate at much lower temperature ~300K peak radiation ~ 10microns
Kirkhoff’s Law
• Energy balance Poynting’s theorem

Sunlight

Infrared

Energy arrives at visible frequencies and is radiated at infrared frequencies

Gaussian surface

Heat Transport

Thermal Conduction:

Heat transport from hot to cold

Heat flows downhill

Thermal Conduction

Heat flows downhill

Molecular Mean Free Path

Mean free path =

A mean free path is the scale size on which molecules mix and transport energy, momentum in a gas.

Molecular Thermal Transport

Downward flux of enthalpy h= cpT

Net flux

Upward flux of enthalpy h= cpT

Photon Mean Free Path

photon

molecule

Photon Mean free path =p

A photon mean free path is the scale size on which photons of a given wavelength are absorbed and reradiated and transport energy in a gas.

Downward flux of energy E= cp (T’)T’

p

Net flux

Upward flux of enthalpy E= cp(T)T

Thermal Photon Diffusivity

Diffusivity is lowered by increased CO2!!

about 5×10^-22 m^2 per molecule =5 x 10-18cm2

3 x1019 cm3 molecles x 300 x10-6 co2 = 9x10 15 cm3

1/(sig n) = 1/(5 x 10 -18 cm2 x 9 x10 15 cm3 ) =1 /(.45 x 10-1) = 20 cm mean free path at 15 microns

Reflected Sunlight

Emitted Thermal

Thermal Emission

CO2 emission and absorption band

Global Warming and Causes
• Global warming appears to be a fact, temperatures are rising –especially since 1980