PTYS 214 Spring Announcements. Midterm 3 next Thursday!

Transcription

1 PTYS 214 Spring 2018 Announcements Midterm 3 next Thursday! 1

2 Previously Habitable Zone Energy Balance Emission Temperature Greenhouse Effect Vibration/rotation bands 2

3 Recap: Greenhouse gases In order for a molecule to absorb or emit IR radiation, a dipole moment (charge imbalance) is needed, either permanent (e.g., H 2 O) or transient (e.g., CO 2 ): O O C O H H carbon dioxide water O O oxygen Possible modes are vibrational or rotational: 3

4 Solar Spectrum at Earth s Surface Greenhouse gases absorb IR radiation at specific wavelengths 4

5 Effect of the Atmosphere (Earth and Solar spectra are NOT to scale) reaches Earth s surface escapes to space 5

6 Atmospheric Greenhouse Effect The Greenhouse Effect increases the surface temperature by returning part of the outgoing IR radiation back to the surface The outgoing IR radiation includes Earth s radiation but also the IR part of the reflected solar spectrum The magnitude of the greenhouse effect depends on the abundance of greenhouse gases (CO 2, H 2 O, O 3, CH 4, etc.) 6

7 Non-Greenhouse Gases The molecules/atoms that constitute the bulk of the atmosphere: O 2, N 2 and Ar, do not interact with infrared radiation significantly While the oxygen and nitrogen molecules can vibrate, because of their symmetry these vibrations do not create any transient charge separation (dipole) Without a dipole moment, they can neither absorb nor emit infrared radiation 7

8 Water in the Earth s Atmosphere The water content of the atmosphere varies about 100-fold between the hot and humid tropics and the cold and dry polar ice deserts Water vapor is the main absorber of radiation in the atmosphere, accounting for about 70% of all atmospheric absorption of radiation, mainly in the IR! Liquid water and ice droplets are also present in the atmosphere as clouds Clouds both reflect sunlight, which cools the Earth, and trap heat in the same way as greenhouse gases, and thus warm the Earth 8

9 Clouds and Radiation Clouds reflect sunlight Cooling Low thick clouds have a high albedo, reflecting more sunlight Clouds absorb and re-emit outgoing IR radiation Warming High, thin clouds have a low albedo, and they let most solar radiation through Clouds also reflect outgoing IR radiation Warming 9

10 Greenhouse Effect Summary Earth s atmosphere lets through most visible light Earth is heated by visible radiation, emits thermal radiation in the IR IR radiation is absorbed by greenhouse gases in the atmosphere and by clouds, re-emitted in all directions. Some IR is re-emitted toward space, some toward Earth Much of the IR emitted by the Earth does not immediately escape! 10

11 What about other solar system objects with an atmosphere? Planet Emission Temperature Surface Temperature Venus 282K 740K Earth 255K 288K Mars 210K 210K Titan 82K 94K Difference between Emission and Surface Temperatures indicates the efficiency of the greenhouse effect 11

12 Back to the Habitable Zone Consider a planet with: Earth s atmospheric greenhouse warming (33 K) and Earth s planetary albedo (~ 0.3) Where would the boundaries of the Habitable Zone be for such planet? 12

13 Remember the Energy Balance Equation: E in = E out (1 A) S=4 σt 4 em E out AE in E in 13

14 Recall that the solar constant, S, at any given distance from the Sun, R, is determined by the Inverse Square Law: S= L 4 πd 2 D is the distance of a planet from the star (the Sun for our Solar System) 14

15 We can substitute the formula for the Solar flux to the planetary energy balance equation and solve for the distance: (1 A) S=4σT [ (1 A) D= L 4 πd 2 ] 4 em =4 σt 4 em L(1 A) 4 16 πσt em For a given T em, D depends on: Stellar luminosity L, Albedo A 15

16 The Habitable Zone We want to find the region around the Sun where water could be in liquid form Assume for the surface temperature that 273K < T s < 373K How does the surface temperature relate to the emission temperature? T s = T em + T GH 16

17 The Solar System Habitable Zone D= L (1 A ) 4 16 πσt em where: T em = T s - T GH For an Earth-like planet: T GH ~ 33K A = 0.3 The range of surface temperatures is determined by: Min: T s = 273K T em =240K D outer Max: T s = 373K T em =340K D inner 17

18 Habitable Zone Region around a star where a planetary body can maintain liquid water on its surface D out D in For A = 0.3 and ΔT GH = 33 K the Habitable Zone around our Sun is about 0.56 AU to 1.1 AU 18

19 Habitable zone summary The boundaries of the habitable zone depend on three main factors: Solar luminosity (energy emission from star) Planetary albedo (on Earth it is also affected by clouds) Greenhouse Effect (CO 2, H 2 O, CH 4, O 3 etc.) this requires the presence of an atmosphere! Complication: The amount of atmospheric greenhouse warming ( T GH ) and the planetary albedo (A) can change as a function of surface temperature (T s ) through various feedbacks in the climate system 19

20 Coupling of System Components Positive Coupling gas pedal (+) speed A change in one component leads to a change of the same direction in the linked component Negative Coupling brake pedal (-) speed A change in one component leads to a change of the opposite direction in the linked component 20

21 Feedbacks In reality, component A affects component B but component B also affects component A This two-way interaction is called a feedback loop A B Loops can be stable or unstable 21

22 Unstable Loops Number of Births (+) positive coupling positive coupling (+) World Population Positive feedback loop: An unstable system that changes further following a perturbation 22

23 Stable Loops Number of Predators (-) negative coupling positive coupling (+) Number of Prey Negative feedback loop: A stable system that resists change following a perturbation 23

24 Multiple Feedback Systems Odd numbers of negative couplings: Overall negative (stable) loop Even number of negative couplings: Overall positive (unstable) loop 24

25 Climate System We can think about climate system as a number of components (atmosphere, ocean, land, ice cover, vegetation, etc.) that continually interact with one another 25

26 Climate Feedbacks: 1. The IR Flux/Temperature Feedback T s (?) (?) (?) Outgoing IR flux 26

27 Climate Feedbacks: 1. The IR Flux/Temperature Feedback T s (+) (-) (-) Outgoing IR flux (+) (-) = (-) Short-term climate stabilization 27

28 Climate Feedbacks: 2. Water Vapor Feedback T s (+) (+) (+) Greenhouse Effect Atmospheric H 2 O (+) (+) (+) (+) = (+) 28

29 Climate Feedbacks: 2. Water Vapor Feedback T s (+) (+) (+) Greenhouse Effect Atmosphaeric H 2 O (+) (+) (+) (+) = (+) Runaway greenhouse effect! 29

30 Climate Feedbacks: 3. Ice / Albedo Feedback T s (?) (?) (?) Planetary Albedo Snow and Ice Cover (?) 30

31 Climate Feedbacks: 3. Ice / Albedo Feedback T s (-) (+) Snow and Ice Cover (-) Planetary Albedo (+) (-) (+) (-) = (+) Runaway snowball! 31