How the Greenhouse Effect Works/Feedback factors Chapter 3—Parts 3&4 How the Greenhouse Effect Works/Feedback factors

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 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

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.

Short-wavelength effect of clouds

Long-wavelength effect of clouds

Net Cloud Radiative Forcing http://iridl.ldeo.columbia.edu/SOURCES/.NASA/.ERBE/ Net Cloud Radiative Forcing

Now, let’s go back and look at some fundamental properties of greenhouse gases 

Composition of the Atmosphere Air is composed of a mixture of gases: Gas concentration (%) ppm N2 78 O2 21 Ar 0.9 H2O variable CO2 0.037 370 CH4 1.7 N2O 0.3 O3 1.0 to 0.01 (stratosphere-surface)

Composition of the Atmosphere Air is composed of a mixture of gases: Gas concentration (%) N2 78 O2 21 Ar 0.9 H2O variable CO2 0.037 370 ppm CH4 1.7 N2O 0.3 O3 1.0 to 0.01 (stratosphere-surface) greenhouse gases

Greenhouse Gases

Water Methane

N2 O2 N  N O = O Non-greenhouse Gases What distinguishes these gases from greenhouse gases?

N  N O = O Non-greenhouse Gases Answer: Symmetry! (Technically speaking, greenhouse gases have a dipole moment whereas N2 and O2 don’t)

O H H Oxygen has an unfilled outer shell (−) O H H (+) Oxygen has an unfilled outer shell of electrons (6 out of 8), so it wants to attract additional electrons. It gets them from the hydrogen atoms.

Molecules with an uneven distribution of electrons are especially good absorbers and emitters. These molecules are called dipoles.

Molecules with an uneven distribution of electrons are especially good absorbers and emitters. These molecules are called dipoles. Water H O H oxygen is more electronegative than hydrogen

Molecules with an uneven distribution of electrons are especially good absorbers and emitters. These molecules are called dipoles. Water Electron-poor region H O H oxygen is more electronegative than hydrogen Electron-rich region

Molecules with an uneven distribution of electrons are especially good absorbers and emitters. These molecules are called dipoles. Water Electron-poor region: Partial positive charge H O H oxygen is more electronegative than hydrogen Electron-rich region: Partial negative charge

Vibration Molecules absorb energy from radiation. The energy increases the movement of the molecules. The molecules rotate and vibrate. stretching bending Vibration

Approximate absorption regions H2O O3 CO2 H2O Radiant energy Sun Earth 0.1 1.0 10 15 100  (m)

Thermal IR Spectrum for Earth H2O pure rotation H2O vibration/rotation CO2 (15 m) (6.3 m) O3 (9.6 m) Note that wavelength increases towards the left in this diagram.. Ref.: K.-N. Liou, Radiation and Cloud Physics Processes in the Atmosphere (1992)

Distribution of Gases in the Atmosphere Most gases are well mixed and distributed evenly throughout the lowermost 100 km of the atmosphere. Examples: O2, N2, Ar, CO2, freons Gases with short lifetimes are not well-mixed. Example: O3

Structure of the Atmosphere Pressure = force per unit area (exerted by a gas or liquid on a surface) At sea level, P = 1 atmosphere = 1.013 bar (or 1013 mbar) Pressure decreases away from the Earth’s surface. The air becomes “thin” at high elevations.

The Barometric Law Pressure declines exponentially with altitude Thus, it forms a (nearly) straight line when plotted on a log scale 

100 80 Altitude (km) 60 40 20 10-3 1 103 Pressure (mbar)

100 80 Pressure decreases away from the Earth’s surface. Altitude (km) 60 40 20 10-3 1 103 Pressure (mbar)

100 80 Altitude (km) 60 40 20 200 250 300 (-73 -23 +27 oC) Temperature (K)

100 80 Altitude (km) 60 40 20 Temperature decreases 200 250 300 Temperature (K)

100 80 Altitude (km) 60 40 20 Troposphere 0-10 km Temperature decreases 200 250 300 Temperature (K)

100 80 Altitude (km) 60 Temperature increases 40 20 Troposphere 0-10 km 200 250 300 Temperature (K)

100 80 Altitude (km) 60 Temperature increases Stratosphere 10-50 km 40 20 Troposphere 0-10 km 200 250 300 Temperature (K)

100 80 Mesosphere 50-90 km Temperature decreases 60 Stratosphere 10-50 km 40 20 Troposphere 0-10 km 200 250 300 Temperature (K)

Temperature increases + 1000 oC Thermosphere 90 + km 100 Temperature increases 80 Mesosphere 50-90 km 60 Stratosphere 10-50 km 40 20 Troposphere 0-10 km 200 250 300 Temperature (K)

Troposphere heated by convection turbulent, mixed contains all weather (wind, rain, clouds, etc.) water is important in this region Stratosphere not well mixed, or “stratified” cold at base, warmer in upper region ozone present ozone heats upper region by absorbing uv radiation

Stratosphere 10-50 km Troposphere 0-10 km + 1000 oC 100 80 60 ozone Stratosphere 10-50 km 40 20 water Troposphere 0-10 km 200 250 300 Temperature (K)