1. This Week: The Greenhouse Effect Reading: Continue Chapter 3 Problem Set 2 Due in Discussion Fri

2. This Week: The Greenhouse Effect (GHE) Atmospheric structure, composition, and absorptivity Which gases contribute to the GHE and why are some better than others? What are the major sources of GHG to the atmosphere?

3. 1-Layer Model of the Greenhouse Effect Surface T sf S o /4 (S o /4)A F sf IN F sf OUT Atmosphere T atm (1-  ) F sf OUT  F atm OUT

4. The Greenhouse Effect 289 K – 256 K = 33 K T true – T ”bare rock” Definition: Absorption of terrestrial long- wave radiation by the atmosphere, causing the surface T to be larger than the planet’s emission T (as determined from absorbed solar radiation flux).

5. The Greenhouse Effect is a 1.Human-induced environmental problem 2.A natural phenomenon present on many planets

6. The physics of the Greenhouse Effect can best be described by analogy to 1.A greenhouse 2.A solar powered water heater 3.Eggshells and orange peels in Earth’s energy drain

7. Earth’s Atmosphere Measures of Composition Physical Characteristics Pressure Temperature Regions

8. Earth’s Atmosphere Thin collection of mainly gases and some condensed phases Extends from Earth’s surface to about 100 Km. Primary components (% by volume) N 2 (78%) O 2 (21%) Argon (0.9%) H 2 O vapor (0.00001 – 4%) CO 2 (0.038%) Many trace and ultra-trace components

9. Measuring Atmospheric Pressure vacuum AB h P atm at pt A and B is the same. Height of fluid related to balance between gravity and P atm

10. P(z 2 ) P(z 1 ) Gravity Pressure Gradient Force Are these two forces always in balance? Barometric Law—”Hydrostatic Equation” The atmosphere’s tendency to be pulled into space is balanced by gravity.

11. Announcements Office Hours Today –4-5pm in 506 ATG –5-6pm in 406 ATG Go to Focus the Nation on Thursday JISAO lectures (see course website)

12. Today Review Pressure vs Altitude Temperature vs Altitude Atmospheric Absorptivity, key players

13. Pressure Decreases Exponentially w/Altitude P altitude An exponential decay is an exponential growth in reverse Gases (air) are compressible fluids unlike liquids. “Compressible” bricks of air stacked on each other height

14. Vertical Profiles of Pressure Pressure decreases exponentially with increasing altitude. -”air gets thinner as you go up” 1 hPa = 1 mbar ~ 0.001 atm ln(P) is a straight line when plotted vs. altitude. Mean values for 30 o N, March

15. What fraction of the atmosphere’s mass is below 15 km? 1.30% 2.60% 3.90% 15 km

16. Vertical Profiles of Temperature Temperature structure of atmosphere is complex. Regions of lower atmosphere separated by behavior of T with altitude Mean values for 30 o N, March Altitude (km)

17. Atmospheric Structure and Composition The atmosphere is a collection of ideal gases  P =  RT Pressure is force/area; difference in air pressure will cause motion Air pressure and  decrease exponentially with altitude (“air gets thinner”) T decreases from 0 – 15 km (troposphere), increases from 15 – 50 km (stratosphere), decreases again from 50 – 80 km (mesosphere) Key Points

18. The Greenhouse Effect (GHE) What gases contribute to the G.H.E.? What’s special about these “greenhouse gases” (G.H.G)? How does adding a GHG to the atmosphere warms the surface? What makes one GHG “better” than another?

19. Solar and Terrestrial Emission Spectra Assuming black bodies

20. What Gases are Greenhouse Gases (GHG’s)? Greenhouse gases absorb terrestrial outgoing long- wave radiation I.e. they absorb infrared (IR) radiation Several different gases give rise to the overall Greenhouse Effect.

21. Why are only some gases GHG? The answer lies in our analogy to charges on springs interacting with EM radiation. IR radiation carries enough energy to make molecules vibrate and rotate.

22. Announcements Office Hours Today –4-5pm in 506 ATG Go to Focus the Nation on Thursday JISAO lectures (see course website)

23. Greenhouse Gases Absorb IR Radiation Kirchoff’s law: to absorb radiation, the molecules must be able to emit that radiation. For gas to absorb IR radiation: must generate oscillations in E&M fields when vibrate and rotate C OO -- -- ++ C OO -- -- ++ O HH ++ ++ -- O -- HH ++ ++ O HH ++ ++ -- O HH ++ ++ -- O HH ++ ++ --

24. Earth Atmosphere’s Absorptivity Absorption Spectrum Indicates the absorptivity we assumed in our 1-layer model

25. Emission Spectrum Taken From Space Spectrum taken over Niger valley, N Africa Emission from cold atmosphere and warm surface “Atmospheric Window”

26. Addition of a GHG Absorbing at 11  m 1. Initial state

27. Addition of a GHG Absorbing at 11  m 2. Emission at 11  m decreases (cold atmosphere)

28. Addition of a GHG Absorbing at 11  m 3. New equilibrium:  total emission must be same  emission at other ’s must increase  Earth surface must heat!

29. Because H 2 O vapor absorbs the larger fraction of OLR, reducing CO 2 concentrations will not reduce the Greenhouse Effect 1.True 2.False

30. GHG Ranking Factors 1.Amount: more there is, more radiation can be potentially absorbed 2.Ability: depends on the wavelength 3.Location: both where in the atmosphere and where ( ) in the outgoing radiation spectrum

31. Band Saturation ability to absorb  Fraction absorbed 1  maximum possible Simulated effect of increasing [GHG] on  Intrinsic to GHG, doesn’t depend on [GHG]

32. “Emission Height” Temperature Altitude (z) Tb4Tb4 TsTs a b Ta4Ta4 Emission to space from z = a carries much more energy than from z = b