1. Goals for Today 1.PREDICT the consequences of varying the factors that determine the (a) effective radiating temperature and (b) mean surface temperature of a planet 2.DESCRIBE how incoming and outgoing electromagnetic radiation interacts with Earth’s surface and its atmosphere 3.PREDICT how changes in solar constant, greenhouse gases, and albedo will affect a planet’s mean surface temperature 4.BALANCE a radiation budget by accounting for reflection, absorption, and transmission of radiation throughout a system 5.PREDICT the consequences for Earth’s surface temperature of latent heat and sensible heat transfer from the Earth’s surface to the atmosphere Radiation Balance II [http://www.elearning.ubc.ca/vista]

2. RELEVANCE Venus (too hot) Earth (just right) Mars (too cold) 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 Next…how do we use this information to figure out the mean temperature of Earth? (Solar Constant)/4 342 W/m 2

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

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

8. INPUT= F in = 342 W/m 2 ALBEDO = 30% What’s doing the absorbing? How much energy does Earth absorb? 342 W/m 2 ENERGY ABSORBED = F abs = F in * (1-0.3) = 342 W/m 2 x 0.7 = 240 W/m 2

9. F em = F abs = 240 W/m 2 F em Earth’s radiation balance What comes in must go out (or else…) F abs

10. …rearrange…calculate… T e = 255°K (-18°C) Earth’s “EFFECTIVE RADIATING TEMPERATURE” (brrrrr…..) How hot must Earth be to emit 240 W/m 2 ? F =  T 4 Recall Stefan-Boltzmann’s equation… T? 240 W/m 2 240 W/m 2 = F em =  T e 4

11. Earth’s effective radiating temperature 63.5 million W/m 2 (Stefan-Boltzmann) 1370 W/m 2 (solar constant) Geometry of spheres & circles Earth is a spinning sphere 342 W/m 2 Earth’s albedo is 0.3 240 W/m 2 What comes in must go out 255K Earth’s effective radiating temperature is… (Stefan-Boltzmann again) 5785 K (Wien’s law)

12. 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)? A. Increase the size of the Sun B. Decrease the Earth-Sun distance C. Increase Earth’s spinning rate D. Decrease Earth’s spinning rate E. Increase Earth’s albedo

13. “Effective Radiating Temperature” VS “Mean Surface Temperature” T e = -18°C T s = +15°C 240 W/m 2 Ts > Te by about 33°C due to GREENHOUSE WARMING

14. Emissions Spectra for Sun and Earth

15. INFRAREDVISUV Most energy from the Sun passes through Earth’s atmosphere Most energy emitted by the Earth gets absorbed by the atmosphere.

16. UVVISIR ShortLong CO 2 H2OH2O O3O3 N2ON2O CH 4 Total What’s doing the absorbing? Mostly O 3 INCOMINGOUTGOING A bit of water vapour Greenhouse Gases

17. Atmosphere F abs =240 W/m 2 IR surf =  T surf 4 = 240 W/m 2 F em =240 W/m 2 Scenario 1: No greenhouse gases How do atmospheric greenhouse gases increase the temperature of Earth’s surface? Stefan-Boltzmann  T surf = 255K (or -18°C)

18. Assume 100% of IR surf absorbed by greenhouse gases IR surf F abs = 240 W/m 2 0.5 IR surf Scenario 2: With greenhouse gases F em = 240 W/m 2 -18°C +30°C IN still equals OUT: F em = F abs = 240 W/m 2 Since F em = 0.5 IR surf IR surf = (240/0.5) W/m 2 = 480 W/m 2 Stefan-Boltzmann  T surf = 303K (or +30°C)

19. F in ? A.250 W/m 2 B.500 W/m 2 C.1000 W/m 2 D.2000 W/m 2 E.4000 W/m 2 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/m 2. At the top of the planet’s atmosphere, how much solar radiation does the average square meter get?

20. F in = 500 W/m 2 F abs ? A.100 W/m 2 B.250 W/m 2 C.500 W/m 2 D.1000 W/m 2 E.2000 W/m 2 Clicker Q: Same planet. Its solar constant is 2000 W/m 2. Its albedo is 50%. What is the flux of solar radiation the planet ABSORBS (F abs )?

21. 250 W/m 2 500 W/m 2 250 W/m 2 A.100 W/m 2 B.250 W/m 2 C.500 W/m 2 D.1000 W/m 2 E.2000 W/m 2 IR surf ? Clicker Q: Same planet. Its solar constant is 2000 W/m 2. Its albedo is 50%. What is the flux of infrared radiation emitted by the surface of the planet (IR surf )?

22. IR surf F abs = 240 W/m 2 F em = 240 W/m 2 This simple model predicts a surface temperature significantly higher than measured -18°C +30°C 240 W/m 2 480 W/m 2 WHY? T e = -18°C T s = +15°C Reality The model

23. IR surf F abs = 240 W/m 2 F em = 240 W/m 2 Clicker Q: Which of these simplifying assumptions contributed to overestimating the temperature of the surface? -18°C +30°C 240 W/m 2 480 W/m 2 A.None of the incoming solar radiation is absorbed by the atmosphere and 240 W/m 2 reach the surface X X B. All IR surf is absorbed by the atmosphere and none leaks out to outer space C. IR radiation is the only means whereby energy is transferred from the surface to the atmosphere X

24. IR surf F abs = 240 W/m 2 0.5 IR surf F em = 240 W/m 2 23% of incoming shortwave radiation are absorbed by the atmosphere Earth’s surface transfers energy to the atmosphere in the form of latent and sensible heat too Atmosphere is not 100% opaque to IR radiation T e = -18°C T s = +15°C

25. Earth’s radiation balance

26. SURFACE Earth’s radiation balance: SURFACE 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% =114%

27. SURFACE Earth’s surface radiates 114% of 342 W/m 2 = 390 W/m 2 SURFACE TEMPERATURE: 390 W/m 2 … Stefan-Boltzmann…  288K (15°C) Earth’s surface temperature

28. 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. Summary: Radiation Balance II Relevance: Earth’s habitability, greenhouse warming

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