Climate feedbacks: Water vapor, snow/ice albedo, and clouds 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 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

Climate feedbacks For doubled CO2: T0 = 1.2oC But the predicted 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?

Climate feedbacks Water vapor feedback Ice/snow albedo feedback Cloud feedback

Systems Notation = system component = positive coupling = negative coupling

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

Negative Coupling An increase in Earth’s albedo causes a (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

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

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

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

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

What about clouds? Cumulus and stratus clouds Cirrus 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

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.