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Chapter 3—Part 4

Chapter 3—Part 4. Climate feedbacks: Water vapor, snow/ice albedo, and clouds. Climate feedbacks. The greenhouse effect itself can be calculated quite accurately Example : Doubled CO 2

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Chapter 3—Part 4

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  1. Chapter 3—Part 4 Climate feedbacks: Water vapor, snow/ice albedo, and clouds

  2. 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

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

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

  5. Systems Notation = system component = positive coupling = negative coupling

  6. Positive Coupling Atmospheric CO2 Greenhouse effect • An increase in atmospheric CO2 causes • a corresponding increase in the greenhouse • effect, and thus in Earth’s surface temperature • Conversely, a decrease in atmospheric CO2 • causes a decrease in the greenhouse effect

  7. Negative Coupling Earth’s albedo (reflectivity) Earth’s surface temperature • An increase in Earth’s albedo causes a • corresponding decrease in the Earth’s surface • temperature by reflecting more sunlight back to • space • Or, a decrease in albedo causes an increase in • surface temperature

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

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

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

  11. 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

  12. 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

  13. 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.

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