Learning goals. Good absorbers are good emitters Albedo, and energy absorbed, changes equilibrium temperature

Transcription

1 Greenhouse effect

2 Learning goals Good absorbers are good emitters Albedo, and energy absorbed, changes equilibrium temperature Wavelength (color) and temperature related: Wein s displacement law Sun/Hot: Solar radiation is visible and short wavelengths Earth/Cooler: emission is infrared and long wavelengths Gases in the atmosphere cause the earth surface to be warmer.

3 Final answer Which of the following is NOT true a) Wavelength of light is related to temperature b) Amount of energy emitted is related to temperature c) Solar constant depends on the speed of light d) Energy leaving Earth is always about equal to solar radiation coming in because of a negative feedback e) If the distance from the earth to the Sun changes, so would the equilibrium temperature

4 Energy balance Atmosphere Shortwave radiation Emission from sun Inverse square law Albedo Solid earth Biota Energy balance means: E in = E out Hydrosphere (ocean, etc) Longwave (infrared) radiation Emission from earth (Stefan Boltzman) S 1 α = σt 4 incoming equals outgoing ( ) 4

5 Albedo experiment Albedo is the fraction of the radiation which is reflected. absobed absorbed Low albedo (absorbs radiation) albedo = 0 albedo ~ 0.5 S 1 T ( α ) = σ 4 4 High albedo (reflects radiation) albedo = 1

6 Let s test Scientific method Hypothesis Experiment Observation Revise hypothesis Conclusion LAW

7 What is the equilibrium temperature of a dark (low albedo) planet relative to a light (high albedo) planet a) Both have the same equilibrium, since the solar constant is the same b) Only the dark planet has an equilibrium c) Only the light planet has an equilibrium d) Dark planet has a higher equilibrium temperature e) Light planet has a higher equilibrium planet

8 Albedo plate demo! And now we wait.

9 Black: What type of behavior do we expect? A. Increases at a constant rate B. Increases toward an constant value C. Random changes D. Decreases toward a constant value E. Decreases with a constant rate

10 White: What type of behavior do we expect? A. Increases at a constant rate B. Increases toward an constant value C. Random changes D. Decreases toward a constant value E. Decreases with a constant rate

11 Consider the thought experiment Blackbody radiation Why do we see colors? Light of a particular wave length is reflected into our eyes What happens for a white object? Light of ALL wavelengths are reflected into our eyes (white is not a color, as such) What is happening when we see black? NO light of any wavelength is reflected! So black bodies perfectly absorb all radiation. That is, radiation with a spectrum of wave lengths If they absorb all the radiation, they must also emit it all (i.e., a balance). A backbody is an object with both perfectly absorbs and emits radiation

12 Black body spectrum What color is black body emission? Mixture of a spectum As described by the Plank function (Remember Plank from the last class? E = hυ) S = 1361 W/m2 Total energy is the area under the curve (i.e., add up radiation of all wave lengths) Monday s class, see this is related to temperature

13 Wien s displacement law λ = 2897 T Higher temperature, smaller wave length (λ). e.g. (1): Sun T~6000K, so λ = 2897/6000 = 0.48 µm e.g. (2): Earth: T ~ 300K, so λ = 2897/300 = 9.7 µm (visible, short) (infrared, long) 1 million micrometers (µm) = 1 meter 1 billion nanometers (nm) = 1 meter

14 Area under curve, total energy from sun solar constant = 1368 W/m 2 7% UV 41% visible 52% near infrared Solar radiation (Shortwave) Terrestrial radiaion (Longwave)

15 Electromagnetic spectrum Sun emits visible light about 480 nm (shortwave radiation) Earth emits infrared radiation about 9700 nm (longwave radiation)

16 Boltzman feedback Atmosphere colder than equilibrium emits LESS energy than is incoming, and warms up. Atmosphere warmer than equilibrium emits MORE energy than incoming, so cools down Since the Stefan-Boltzman law is E=σT 4, even a small deviation from equilibrium will give a strong recovery. BUT this is only the temperature at the top of the atmosphere! What controls the temperature at the ground (hint, something to do with tomatoes)

17 Focus on energy balance at the top of the atmosphere. What happens to energy once it is already in the atmosphere? Each flux (arrow) changes energy of the atmosphere/climate system

18 Radiative balance Solar input of energy (note, only on sunny side) I = ( ) S 1 α 4 α A=πR 2 A=4πR 2 Longwave loss of energy (note all directions) E=σT 4 T So for balance, incoming = outgoing S 1 T ( α ) = σ 4 4

19 Example: Earth solar in = longwave out S 4 ( ) 4 1 α = σt T = 4 ( α ) 1 S σ 4 We know S = 1370 W/m 2 We know α = 0.31 Solve for T T = ( α ) 1 S σ 4 T = 254K This is the radiative equilibrium temperature

20 Each flux (arrow) changes energy of the atmosphere/climate system

21 Energy (heat) transfer Conduction E.g., metal object in a fire, heating going into frozen soil Convection E.g., Boiling water in a pot, Thunderstorms heating the atmosphere Radiation E.g., Toaster frying outside of a pop tart, Sun heating the earths surface

22 Each flux (arrow) changes energy of the atmosphere/climate system

23 Energy balance and greenhouse Radiative equilibrium temperature Radiative equilibrium temperature I = S ( 1 α ) 4 No atmosphere E=σT4 E=σT4 S I = ( 1 α ) 4 E=σT s 4 Surface temperature Atmosphere: Greenhouse effect E=σT 4 Notice at the surface there is now both shortwave radiation from the sun and downwelling longwave radiation. More energy, leads to warmer temperature

24 Green house effect Strength of greenhouse effect can be measured as the difference between the radiative equilibrium temperature and the surface temperature I = S ( 1 α ) 4 E=σT s 4 E=σT 4 E=σT 4 Greenhouse atmosphere Earth surface temperature is about 288 K Earth radiative equilibrium temperature is 254 K Greenhouse effect is ( ) = 34 K

25 Experiment Incoming solar flux depends on albedo (reflectivity) More energy in, the higher radiative equilibrium temperature (i.e., warmer the planet) Higher albedo planets will be colder Such as cloudy planet, or planets covered in ice. We know there is an equilibrium So the Earth must emit energy This emission must balance the incoming radiation. How this emission occurs, depends on the strength of the greenhouse effect!

26 Key points The average albedo of earth is 0.31 Most of the isolation reaches the ground Energetic gets into the atmosphere from below! Convection and conduction of heat! Boltsman feedback: Atmosphere colder than equilibrium emits LESS energy than is incoming, and warms up. Atmosphere warmer than equilibrium emits MORE energy than incoming, so cools down Wavelength (color) and temperature related: Wein s displacement law Sun/Hot: Solar radiation is visible and short wavelengths Earth/Cooler: emission is infrared and long wavelengths Gases in the atmosphere cause the earth surface to be warmer.