1. TOPIC III THE GREENHOUSE EFFECT

2. SOLAR IRRADIANCE SPECTRA 1  m = 1000 nm = 10 -6 m Note: 1 W = 1 J s -1

3. Solar radiation received outside atmosphere per unit area of sphere = (1370) x (  r e 2 )/(4  r e 2 ) = 342 W m -2 TOTAL SOLAR RADIATION RECEIVED BY EARTH Solar constant for earth: 1368 W m -2

4. EFFECTIVE TEMPERATURE OF EARTH Effective temperature of earth (T e ) Temperature detected from space Albedo of surface+atmosphere ~ 0.3 30% of incoming solar energy is reflected by clouds, ice, etc. Energy absorbed by surface+atmosphere = 1-0.3 = 0.7 70% of 342 W m -2 = 239.4 W m -2 Balanced by energy emitted by surface+atmosphere Stefan-Boltzman law: Energy emitted =  T e 4  = 5.67 x 10 -8 W m -2 K -4 Solve  T e 4 = 239.4 T e = 255 K

5. GLOBAL TEMPERATURE Annual and global average temperature ~ 15 C, i.e. 288 K T e = 255 K --> not representative of surface temp. of earth T e is the effective temp. of the earth + atmosphere system that would be detected by an observer in space

6. ENERGY TRANSITIONS Gas molecules absorb radiation by increasing internal energy Internal energy  electronic, vibrational, & rotational states Energy requirements Electronic transitions  UV (< 0.4  m) Vibrational transitions  Near-IR (< 0.7-20  m) Rotational transitions  Far-IR (> 20  m) Little absorption in visible range (0.4-0.7  m) Gap between electronic and vibrational transitions Greenhouse gases absorb in the range 5-50  m Vibrational and rotational transitions

7. GREENHOUSE GASES Vibrational transitions must change dipole moment of molecule Important greenhouse gases H 2 O, CO 2, CH 4, N 2 O, O 3, CFCs Non-greenhouse gases N 2, O 2, H 2, Noble gases

8. ATMOSPHERIC ABSORPTION OF RADIATION ~100% absorption of UV Electronic transitions of O 2 and O 3 Weak absorption of visible Gap in electronic and vibrational transition energies Efficient absorption of terrestrial radiation Greenhouse gas absorption Important role of H 2 O Atmospheric window between 8 and 13  m

9. A SIMPLE GREENHOUSE MODEL Incoming solar radiation = 70% of 342 W m -2 = 239.4 W m -2 IR flux from surface =  T o 4 Assume atmospheric layer has an absorption efficiency = f Kirchhoff’s law: efficiency of abs. = efficiency of emission IR flux from atmospheric layer = f  T 1 4 (up and down) 239.4 W m -2 absorbed = f  T o 4  T o 4 (1-f)  T o 4 f  T 1 4

10. RADIATION BALANCE EQUATIONS 239.4 W m -2 absorbed = f  T o 4  T o 4 (1-f)  T o 4 f  T 1 4 Balance at top of atmosphere f  T 1 4 + (1-f)  T o 4 = 239.4 Balance for atmospheric layer f  T 1 4 + f  T 1 4 = f  T o 4

11. THE GREENHOUSE EFFECT 239.4 W m -2 absorbed = f  T o 4  T o 4 (1-f)  T o 4 f  T 1 4 T o = 288 K f = 0.77; T 1 = 241 K Greenhouse gases  gases that affect f As f increases, T o and T 1 increase

12. THE IPCC THIRD ASSESSMENT

13. CONCEPT OF RADIATIVE FORCING 239.4 W m -2 absorbed = f  T o 4  T o 4 (1-f)  T o 4 f  T 1 4 Consider increase in concentration of a greenhouse gases If nothing else changes  f increases  outgoing terrestrial radiation decreases Change in outgoing terrestrial radiation = radiative forcing

14. RADIATIVE FORCING AND TEMPERATURE CHANGE 239.4 W m -2 absorbed = f  T o 4  T o 4 (1-f)  T o 4 f  T 1 4 Response to imbalance T o and T 1 increase  may cause other greenhouse gases to change  f  (positive feedback) or  (negative feedback)  T o and T 1 may  or    f   T  …  Rad. balance Radiative forcing is measure of initial change in outgoing flux

15. RADIATIVE FORCING Permits assessment of potential climate effects of different gases Radiative forcing of a gas depends not only on change in concentration, but also what wavelengths it absorbs Aerosols can exert a negative radiative effect (i.e. have a cooling effect) by reflecting radiation (direct effect) and by increasing reflectivity of clouds (indirect effect)

16. GLOBAL WARMING POTENTIAL Index used to quant. compare radiative forcings of various gases Takes into account lifetimes, saturation of absorption

17. FORCINGS AND SURFACE TEMPERATURE Climate sensitvity parameter ( ):  T o =  F Global climate models  = 0.3-1.4 K m 2 W -1

18. THE TEMPERATURE RECORD

19. Trend differences due to differences in spatial av., diff. in sea-surface temps., and handling of urbanization Same basic trend over last 100 years Increase in T by 0.6-0.7 C RECENT CHANGES IN SURFACE TEMPERATURE

20. POTENTIAL CAUSES OF TEMPERATURE CHANGES Variations in solar radiation at top of atmosphere Changes in albedo (e.g. due to changes in cloud cover) Changes in greenhouse gas forcing (i.e., change in f) 239.4 W m -2 absorbed = f  T o 4

21. SOLAR VARIABILITY Changes in sunspots and surface conditions

22. CHANGES IN CLOUD COVER Incoming solar radiation = 0.7 x 342 W m -2 = 239.4 W m -2 Consider albedo change of 2.5% Albedo = 0.3 x 1.025 = 0.3075 Incoming solar radiation = 0.6925 x 342 W m -2 = 236.8 W m -2 Radiative forcing = 236.8 – 239.4 = - 2.6 W m -2  Comparable but opposite to greenhouse gas forcing Clouds are also efficient absorbers of terrestrial radiation  Positive forcing Cloud effects are larege source of uncertainty in climate projections

23. MODEL SIMULATIONS OF RECENT PAST

24. CLIMATE PROJECTIONS

25. POTENTIAL IMPACTS

26. JULY HEAT INDEX FOR S.E. U.S.