Greenhouse gases/ Atmospheric Structure Chapter 3—Part 3 Greenhouse gases/ Atmospheric Structure

Te4 = So(1-A) 4 Review from last time… We used the balance of incoming and outgoing energy to establish an equation for the effective radiating temperature of the earth: Te4 = So(1-A) 4 This is the temperature that Earth’s surface would have if it did not have an atmosphere (but if the albedo was the same as today)

Te4 = So(1-A) 4 Dividing both sides of the equation by 4 gives: Te4 = (1370 W/m2)(1-0.3) 4 (5.67 x 10-8 W/m2/K4) Te4 = 4.23 x 109 (K4) Te = 255 K (or -18oC) (which is about 0oF)

The observed temperature (Ts) is 288 K, which is 15oC, or about 59oF. The difference between observed and expected temperatures (Tg): Tg = Ts - Te Tg = 288 K – 255 K Tg = + 33 K (or 33oC) We call this warming the greenhouse effect, and is due to absorption of energy by gases in the atmosphere.

The greenhouse effect: Heat is absorbed and reemitted by gases in the atmosphere. Earth naturally has a greenhouse effect of +33oC. Often we hear that outgoing heat energy is “trapped” by greenhouse gases. This is not really true. The heat eventually does escape.

The concern is that the amount of greenhouse warming will increase with the rise of CO2 due to human activity.

Warming results from interactions of gases in the atmosphere with incoming and outgoing radiation. To evaluate how this happens, we will focus on both the composition and structure of the Earth’s atmosphere.

Composition of the Atmosphere Air is composed of a mixture of gases: Gas concentration (%) N2 78 Major O2 21 constituents Ar 0.9

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 Minor CH4 constituents N2O O3

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)