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
Blackbody radiation • 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)
Space Thermal Radiation problems • Spacecraft receive and radiate heat by blackbody radiation • They receive radiation in the visible • ~ .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
Radiant equilbrium Planetary radiance 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.
Radiation Thermal Transport 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
25% increase CO2 levels in the atmosphere
Global Warming and Causes • Global warming appears to be a fact, temperatures are rising –especially since 1980 • Solar radiance for the last decade appears flat or decreasing • Volcanic activity , indicating geothermal heating, appears flat or decreasing • Green House gases are rising –appearing to correlate with warming
Conclusions • Global warming appears to be a reality • Global warming appears principally driven by decreased infrared diffusivity in the atmosphere due to increased greenhouse gases-other explanations do not appear viable • Global warming, if unchecked, could have very negative consequences for the human race
Solutions • Humanity must shift from fossil fuels to nuclear power and other non-greenhouse producing forms of energy. • France and Japan have converted to 80% nuclear Fission – it is both possible and practical • Fusion power , cleaner and safer than Fission , can replace all fossil fuels