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TOPIC III THE GREENHOUSE EFFECT

TOPIC III THE GREENHOUSE EFFECT. SOLAR IRRADIANCE SPECTRA. 1  m = 1000 nm = 10 -6 m. Note: 1 W = 1 J s -1. TOTAL SOLAR RADIATION RECEIVED BY EARTH. Solar constant for earth: 1368 W m -2. Solar radiation received outside atmosphere per unit area of sphere

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TOPIC III THE GREENHOUSE EFFECT

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  1. TOPIC IIITHE GREENHOUSE EFFECT

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

  3. TOTAL SOLAR RADIATION RECEIVED BY EARTH • Solar constant for earth: 1368 W m-2 • Solar radiation received outside atmosphere • per unit area of sphere • = (1370) x ( re2)/(4  re2) = 342 W m-2

  4. EFFECTIVE TEMPERATURE OF EARTH • Effective temperature of earth (Te) • 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 =  Te4 •  = 5.67 x 10-8 W m-2 K-4 • Solve  Te4 = 239.4 • Te = 255 K

  5. GLOBAL TEMPERATURE • Annual and global average temperature ~ 15 C, i.e. 288 K • Te = 255 K --> not representative of surface temp. of earth • Te 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 • H2O, CO2, CH4, N2O, O3, CFCs • Non-greenhouse gases • N2, O2, H2, Noble gases

  8. ATMOSPHERIC ABSORPTION OF RADIATION • ~100% absorption of UV • Electronic transitions of • O2 and O3 • Weak absorption of visible • Gap in electronic and • vibrational transition energies • Efficient absorption of terrestrial radiation • Greenhouse gas absorption • Important role of H2O • Atmospheric window between 8 and 13 m

  9. A SIMPLE GREENHOUSE MODEL 239.4 W m-2 (1-f) To4 f T14 absorbed = f  To4 f T14  To4 • Incoming solar radiation = 70% of 342 W m-2 = 239.4 W m-2 • IR flux from surface =  To4 • Assume atmospheric layer has an absorption efficiency = f • Kirchhoff’s law: efficiency of abs. = efficiency of emission • IR flux from atmospheric layer = f  T14 (up and down)

  10. RADIATION BALANCE EQUATIONS 239.4 W m-2 (1-f) To4 f T14 absorbed = f  To4 f T14  To4 • Balance at top of atmosphere • f  T14 + (1-f)  To4 = 239.4 • Balance for atmospheric layer • f  T14+f  T14 = f  To4

  11. THE GREENHOUSE EFFECT 239.4 W m-2 (1-f) To4 f T14 f T14 absorbed = f  To4  To4 • To = 288 K • f = 0.77; T1 = 241 K • Greenhouse gases  gases that affect f • As f increases, To and T1 increase

  12. THE IPCC THIRD ASSESSMENT

  13. CONCEPT OF RADIATIVE FORCING 239.4 W m-2 (1-f) To4 f T14 absorbed = f  To4 f T14  To4 • 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 (1-f) To4 f T14 absorbed = f  To4 f T14  To4 • Response to imbalance • To and T1 increase  may cause other greenhouse gases to • change  f  (positive feedback) or  (negative feedback) • To and T1 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 (): To =  F • Global climate models   = 0.3-1.4 K m2 W-1

  18. THE TEMPERATURE RECORD

  19. RECENT CHANGES IN SURFACE TEMPERATURE • 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

  20. POTENTIAL CAUSES OF TEMPERATURE CHANGES 239.4 W m-2 absorbed = f  To4 • 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)

  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.

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