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Goals for Today

Radiation Balance II [http://www.elearning.ubc.ca/vista]. Goals for Today. PREDICT the consequences of varying the factors that determine the (a) effective radiating temperature and (b) mean surface temperature of a planet

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Goals for Today

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  1. Radiation Balance II [http://www.elearning.ubc.ca/vista] Goals for Today • PREDICT the consequences of varying the factors that determine the (a) effective radiating temperature and (b) mean surface temperature of a planet • DESCRIBE how incoming and outgoing electromagnetic radiation interacts with Earth’s surface and its atmosphere • PREDICT how changes in solar constant, greenhouse gases, and albedo will affect a planet’s mean surface temperature • BALANCE a radiation budget by accounting for reflection, absorption, and transmission of radiation throughout a system • PREDICT the consequences for Earth’s surface temperature of latent heat and sensible heat transfer from the Earth’s surface to the atmosphere

  2. RELEVANCE Mars (too cold) Venus (too hot) Earth (just right) The flux of solar radiation reaching Earth is one of the main factors dictating its mean temperature, and therefore its habitability

  3. Amount of solar radiation that reaches the top of the Earth’s atmosphere (Solar Constant)/4 342 W/m2 Next…how do we use this information to figure out the mean temperature of Earth?

  4. 30% of the incoming solar radiation is directly reflected back to outer space Earth’s ALBEDO = fraction of incoming solar radiation that is reflected back to space = 0.3

  5. What’s doing the reflecting? Clouds(& dust) reflect about 26% of the total Earth’s surface reflects about 4% of the total

  6. Clicker Q: Considering ONLY the effects of ALBEDO, which of the following scenarios do you think would make Earth WARMER? • Turn desert into forest • Turn tundra into desert • Lower sea level • Produce more clouds • Expand ice caps

  7. How much energy does Earth absorb? INPUT= Fin = 342 W/m2 ALBEDO = 30% 342 W/m2 ENERGY ABSORBED = Fabs = Fin * (1-0.3) = 342 W/m2 x 0.7 = 240 W/m2 What’s doing the absorbing?

  8. Earth’s radiation balanceWhat comes in must go out (or else…) Fem Fabs Fem=Fabs=240 W/m2

  9. How hot must Earth be to emit 240 W/m2? Recall Stefan-Boltzmann’s equation… 240 W/m2 240 W/m2 F = s T4 T? 240 W/m2 240 W/m2 = Fem = s Te4 240 W/m2 240 W/m2 …rearrange…calculate… Te = 255°K (-18°C) Earth’s“EFFECTIVE RADIATING TEMPERATURE” (brrrrr…..)

  10. Earth’s effective radiating temperature Geometry ofspheres& circles 63.5 million W/m2 1370 W/m2(solar constant) (Stefan-Boltzmann) Earth is a spinning sphere 5785 K (Wien’s law) 342 W/m2 Earth’s albedois 0.3 Earth’s effectiveradiating temperature is… 255K 240 W/m2 What comes in must go out (Stefan-Boltzmann again)

  11. Clicker Q: Over its lifetime (billions of years), the Sun has been getting hotter, which should influence Earth’s effective radiating temperature(how?). Which of the following could counteract this effect (that is, influence Earth’s effective radiating temperature in the opposite sense)? • Increase the size of the Sun • Decrease the Earth-Sun distance • Increase Earth’s spinning rate • Decrease Earth’s spinning rate • Increase Earth’s albedo

  12. “Effective Radiating Temperature” VS “Mean Surface Temperature” 240 W/m2 Te = -18°C Ts = +15°C Ts > Te by about 33°Cdue to GREENHOUSE WARMING

  13. Emissions Spectra for Sun and Earth

  14. UV VIS INFRARED Most energy emitted by the Earth gets absorbed by the atmosphere. Most energy from the Sun passes through Earth’s atmosphere

  15. UV VIS IR CH4 N2O O3 CO2 H2O Total Short Long What’s doing the absorbing? INCOMING OUTGOING Mostly O3 Greenhouse Gases A bit of water vapour

  16. How do atmospheric greenhouse gases increase the temperature of Earth’s surface? Scenario 1: No greenhouse gases Fem=240 W/m2 Atmosphere Fabs=240 W/m2 IRsurf = s Tsurf4 = 240 W/m2 Stefan-BoltzmannTsurf = 255K (or -18°C)

  17. Scenario 2: With greenhouse gases Assume 100% of IRsurf absorbed by greenhouse gases Fem = 240 W/m2 IN still equals OUT: Fem = Fabs = 240 W/m2 0.5 IRsurf -18°C Since Fem = 0.5 IRsurf IRsurf = (240/0.5) W/m2 = 480 W/m2 0.5 IRsurf Fabs = 240 W/m2 IRsurf +30°C Stefan-BoltzmannTsurf = 303K (or +30°C)

  18. Clicker Q:Here’s a diagram of a planet with greenhouse gases (like Scenario 2). This planet is also SPINNING. Its solar constant is 2000 W/m2. At the top of the planet’s atmosphere, how much solar radiation does the average square meter get? • 250 W/m2 • 500 W/m2 • 1000 W/m2 • 2000 W/m2 • 4000 W/m2 Fin?

  19. Clicker Q:Same planet. Its solar constant is 2000 W/m2. Its albedo is 50%. What is the flux of solar radiation the planet ABSORBS (Fabs)? Fin= 500 W/m2 • 100 W/m2 • 250 W/m2 • 500 W/m2 • 1000 W/m2 • 2000 W/m2 Fabs?

  20. Clicker Q:Same planet. Its solar constant is 2000 W/m2. Its albedo is 50%. What is the flux of infrared radiation emitted by the surface of the planet (IRsurf)? 500 W/m2 250 W/m2 • 100 W/m2 • 250 W/m2 • 500 W/m2 • 1000 W/m2 • 2000 W/m2 IRsurf? 250 W/m2

  21. Reality Te = -18°C The model Ts = +15°C Fem = 240 W/m2 This simple model predicts a surface temperature significantly higher than measured -18°C 480 W/m2 Fabs = 240 W/m2 240 W/m2 WHY? IRsurf +30°C

  22. Clicker Q: Which of these simplifying assumptions contributed to overestimating the temperature of the surface? • None of the incoming solar radiation is absorbed by the atmosphere and 240 W/m2 reach the surface Fem = 240 W/m2 -18°C X X B. All IRsurf is absorbed by the atmosphere and none leaks out to outer space 480 W/m2 X Fabs = 240 W/m2 240 W/m2 C. IR radiation is the only means whereby energy is transferred from the surface to the atmosphere IRsurf +30°C

  23. 23% of incoming shortwave radiation are absorbed by the atmosphere Atmosphere is not 100% opaque to IR radiation Earth’s surface transfers energy to the atmosphere in the form of latent and sensible heat too Te = -18°C Ts = +15°C Fem = 240 W/m2 0.5 IRsurf 0.5 IRsurf Fabs = 240 W/m2 IRsurf

  24. Earth’s radiation balance

  25. Earth’s radiation balance: SURFACE SURFACE =114% Input: 47% from the Sun 96% from the atmosphere Total: 143% Output: 109% as IR to the atmosphere 5% as IR to outer space 29% as heat to the atmosphere Total: 143%

  26. Earth’s surface temperature SURFACE Earth’s surface radiates 114% of 342 W/m2 = 390 W/m2 SURFACE TEMPERATURE: 390 W/m2 … Stefan-Boltzmann…  288K (15°C)

  27. Summary: Radiation Balance II • Earth’s mean surface temperature is higher than its effective radiating temperature because of greenhouse gases • Earth’s atmosphere lets most of the incoming, short wavelength, solar radiation through, but absorbs much of the outgoing, long wavelength, infrared radiation emitted by Earth • Latent and sensible heat transfer from Earth’s surface to its atmosphere help keep the surface cooler than it would otherwise be • The radiation budget for Earth’s surface, its atmosphere, and the planet as a whole are typically in balance. Changes in solar constant, albedo, and greenhouse gases can all perturb the system, leading to a new equilibrium temperature. Relevance: Earth’s habitability, greenhouse warming

  28. Slide about the negative feedback that keeps Earth’s T in whack? Increased T, more radiation emitted, decreased T, less radiation emitted

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