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CHAPTER 7: THE GREENHOUSE EFFECT

CHAPTER 7: THE GREENHOUSE EFFECT. MILLENIAL NH TEMPERATURE TREND [IPCC, 2001]. GLOBAL CLIMATE CHANGE SINCE 1850 [IPCC, 2007]. NOAA GREENHOUSE GAS RECORDS. RADIATION & FUNDAMENTAL RELATIONSHIPS. Electromagnetic energy at wavelength (  ) has associated frequency (f) and photon energy (E):

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CHAPTER 7: THE GREENHOUSE EFFECT

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  1. CHAPTER 7: THE GREENHOUSE EFFECT

  2. MILLENIAL NH TEMPERATURE TREND [IPCC, 2001]

  3. GLOBAL CLIMATE CHANGE SINCE 1850 [IPCC, 2007]

  4. NOAA GREENHOUSE GAS RECORDS

  5. RADIATION & FUNDAMENTAL RELATIONSHIPS Electromagnetic energy at wavelength () has associated frequency (f) and photon energy (E): Also often use wavenumbers notation: h=6.62x10-34 Js c=3.0x108 m/s

  6. EMISSION OF RADIATION Radiation is energy transmitted by electromagnetic waves; all objects emit radiation One can measure the radiation flux spectrum emitted by a unit surface area of object: Here DF is the radiation flux emitted in [l, l+Dl] is the flux distribution function characteristic of the object Total radiation flux emitted by object:

  7. BLACKBODY RADIATION Objects that absorb 100% of incoming radiation are called blackbodies For blackbodies,fl is given by the Planck function: Function of T only! Often denoted B(l,T) • = 2p 5k 4/15c2h3is the Stefan-Boltzmann constant lmax = hc/5kTWien’s law lmax

  8. For any object: …very useful! KIRCHHOFF’S LAW: Emissivity e(l,T) = Absorptivity Illustrative example: Kirchhoff’s law allows determination of the emission spectrum of any object solely from knowledge of its absorption spectrum and temperature

  9. SOLAR RADIATION SPECTRUM: blackbody at 5800 K

  10. GREENHOUSE EFFECT:absorption of terrestrial radiation by the atmosphere

  11. ABSORPTION OF RADIATION BY GAS MOLECULES • …requires quantum transition in internal energy of molecule. • THREE TYPES OF TRANSITION • Electronic transition: UV radiation (<0.4 mm) • Jump of electron from valence shell to higher-energy shell, sometimes results in dissociation (example: O3+hn gO2+O) • Vibrational transition: near-IR (0.7-10 mm) • Increase in vibrational frequency of a given bond requires change in dipole moment of molecule • Rotational transition: far-IR (10-100 mm) • Increase in angular momentum around rotation axis THE GREENHOUSE EFFECT INVOLVES ABSORPTION OF NEAR-IR TERRESTRIAL RADIATION BY MOLECULES UNDERGOING VIBRATIONAL AND VIBRATIONAL-ROTATIONAL TRANSITIONS

  12. NORMAL VIBRATIONAL MODES OF CO2 forbidden allowed allowed • Greenhouse gases = gases with vib-rot absorption features at 5-50 mm • Major greenhouse gases: H2O, CO2, CH4, O3, N2O, CFCs,… • Not greenhouse gases: N2, O2, Ar, …

  13. EFFICIENCY OF GREENHOUSE GASES FOR GLOBAL WARMING The efficient GGs are the ones that absorb in the “atmospheric window” (8-13 mm). Gases that absorb in the already-saturated regions of the spectrum are not efficient GGs.

  14. = 255 K RADIATIVE EQUILIBRIUM FOR THE EARTH Solar radiation flux intercepted by Earth = solar constant FS = 1370 W m-2 Radiative balance c effective temperature of the Earth: where A is the albedo (reflectivity) of the Earth

  15. SIMPLE MODEL OF GREENHOUSE EFFECT IR VISIBLE • Energy balance equations: • Earth system Incoming solar Reflected solar Transmitted surface • Atmospheric layer Solution: To=288 K e f=0.77 T1 = 241 K Atmospheric emission Atmospheric layer (T1) abs. eff. 0for solar (VIS) f for terr. (near-IR) Atmospheric emission Surface emission Earth surface (To) Absorption efficiency 1-A in VISIBLE 1 in IR

  16. EQUILIBRIUM RADIATIVE BUDGET FOR THE EARTH Kevin Trenberth, BAMS, 2009

  17. The ultimate models for climate research

  18. TERRESTRIAL RADIATION SPECTRUM FROM SPACE:composite of blackbody radiation spectra emitted from different altitudes at different temperatures

  19. Example of a GG absorbing at 11 mm HOW DOES ADDITION OF A GREENHOUSE GAS WARM THE EARTH? 1. 1. Initial state 2. Add to atmosphere a GG absorbing at 11 mm; emission at 11 mm decreases (we don’t see the surface anymore at that l, but the atmosphere) 2. 3. 3. At new steady state, total emission integrated over all l’s must be conserved e Emission at other l’s must increase e The Earth must heat!

  20. RADIATIVE FORCING OF CLIMATE DF atmospheric emission fsT14 Reflected solar FSA/4 Flux out Flux in surface emission (1-f) sTo4 solar radiation FS/4 greenhouse layer (H2O, clouds, CO2, CH4, …) Efficiency f • Radiative equilibrium: DF = (Flux in) – (Flux out) = 0 • Increase greenhouse efficiency f e Flux out decreases e DF > 0; WARMING • Increase solar reflection e Flux in decreases e DF < 0; COOLING • Radiative forcing DF predicts equilibrium surface temperature response DTo : • DTo = l DF. In our 1-layer model, l = [4(1-f/2)sT3o]-1 = 0.3 K m2 W-1; • in research climate models, lranges from 0.3 to 1.4 K m2 W-1depending on model

  21. CLIMATE CHANGE FORCINGS, FEEDBACKS, RESPONSE Positive feedback from water vapor causes rough doubling of l

  22. IPCC [2007]

  23. GLOBAL WARMING POTENTIAL (GWP):foundation for climate policy • The GWP measures the integrated radiative forcing over a time horizon Dt from the injection of 1 kg of a species X at time to, relative to CO2:

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