Understanding the Greenhouse Effect and Climate Feedbacks

1. Chapter 3—Part 4 How the Greenhouse Effect Works/Feedback factors

2. incoming radiation Solar energy reaches the Earth’s surface

3. incoming radiation infrared radiation Earth’s surface warms, emits radiation

4. incoming radiation greenhouse gases infrared radiation Greenhouse gases absorb IR leaving the surface

5. incoming radiation greenhouse gases infrared radiation Gases are energized, then emit radiation (IR)

6. incoming radiation greenhouse gases infrared radiation Some of this IR reaches the planet surface, warming it further

7. incoming radiation greenhouse gases infrared radiation This process is what we call the GREENHOUSE EFFECT!

8. Earth’s Energy Balance and now the details…

9. 100 Incoming solar radiation atmosphere Earth Follow the fate of a 100 units of solar radiation

10. 100 incoming 30 reflected 70 30 units are reflected, and 70 units enter the atmosphere

11. 100 incoming 70 radiated from Earth 30 reflected 70 This is matched by 70 units radiated from the Earth

12. 100 incoming 70 radiated from Earth 30 reflected 70 25 directly to atmosphere 45 absorbed by Earth 25 are absorbed directly by the atmosphere, and 45 units reach the Earth’s surface

13. 100 incoming 70 radiated from Earth 30 reflected 70 133 from surface 25 directly to atmosphere 45 absorbed by Earth A large amount of energy is radiated by the Earth to the atmosphere

14. 100 incoming 70 radiated from Earth 30 reflected 70 133 from surface 25 directly to atmosphere 88 greenhouse radiation 45 absorbed by Earth The greenhouse effect captures radiation leaving the surface, warming the lower atmosphere and Earth’s surface

15. 100 incoming 70 radiated from Earth 30 reflected 70 133 from surface 25 directly to atmosphere 88 greenhouse radiation 45 absorbed by Earth A net of 45 units leave the earth, plus 25 units from the atmosphere yields 70 emitted to space

16. Climate feedbacks • The greenhouse effect itself can be calculated quite accurately Example: Doubled CO2 • The direct temperature effect of doubled CO2 (with no feedbacks) is to increase surface temperature by ~1.2oC • In the language of Daisyworld (and Earth 2) T0 = 1.2oC

17. Climate feedbacks • For doubled CO2: T0 = 1.2oC • But thepredicted equilibrium response from climate models is 2oC < Teq < 5oC • Hence, in the models at least, there are positive feedbacks that tend to amplify the forcing by CO2. What are these?

18. Climate feedbacks • Water vapor feedback • Ice/snow albedo feedback • Cloud feedback

19. Water vapor feedback Surface temperature Atmospheric H2O (+) Greenhouse effect  Positive feedback loop

20. Snow/ice albedo feedback Surface temperature Snow and ice cover (+) Planetary albedo  Another positive feedback loop

21. What about clouds? Some reflection 10 km Cirrus clouds (Thin) More reflection Altitude Cumulus/stratus clouds (Thicker)

22. What about clouds? Tc4 Cirrus clouds High and cold 10 km Tc4 Altitude Cumulus/stratus clouds Tw4 Low and warm Tw4 Ts4 Ts Tc Tw Temperature

23. What about clouds? • Cumulus and stratus clouds • Low and warm • Small greenhouse effect • Big effect on albedo • These clouds cool the climate • Cirrus clouds • High and cold • Large greenhouse effect • Smaller effect on albedo  These clouds warm the climate

24. Cloud feedback • Most models predict that cloudiness should increase as the climate warms • If low clouds increase the most, then the feedback will be negative • If high clouds increase the most, then the feedback will be positive • The balance of evidence suggests that cloud feedback is negative. However, this is highly uncertain, as clouds are sub-grid-scale in size and are therefore difficult to model.