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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): h=6.62x10-34 Js E hf hc , Also often use wavenumbers notation: c f c=3.0x108 m/s 1 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 [, +D] is the flux distribution function characteristic of the object Total radiation flux emitted by object: F d 0 BLACKBODY RADIATION Objects that absorb 100% of incoming radiation are called blackbodies For blackbodies, is given by the Planck function: 2p hc 2 hc 5 kT e 1 F T 4 b Function of T only! Often denoted B(,T) 2p 5k 4/15c2h3 is the Stefan-Boltzmann constant max = hc/5kT Wien’s law max KIRCHHOFF’S LAW: Emissivity e(,T) = Absorptivity For any object: Illustrative example: Kirchhoff’s law allows determination of the emission spectrum of any object solely from knowledge of its absorption spectrum and temperature …very useful! SOLAR RADIATION SPECTRUM: blackbody at 5800 K GREENHOUSE EFFECT: absorption of terrestrial radiation by the atmosphere 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 NORMAL VIBRATIONAL MODES OF CO2 Δp 0 forbidden Δp 0 allowed Δp 0 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, … 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. 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: = 255 K where A is the albedo (reflectivity) of the Earth SIMPLE MODEL OF GREENHOUSE EFFECT IR VISIBLE Incoming solar FS / 4 FS / 4 Reflected solar FS A / 4 Energy balance equations: • Earth system FS (1 A) / 4 (1 f ) To4 + f T14 Transmitted surface • Atmospheric layer (1 f ) To4 f To4 2 f T14 Solution: f T14 Atmospheric emission f T14 Atmospheric emission 1 4 To=288 K F (1 A) e f=0.77 To S f 4(1 ) T1 = 241 K 2 Atmospheric layer (T1) abs. eff. 0 for solar (VIS) f for terr. (near-IR) Surface emission FS A / 4 To4 Earth surface (To) Absorption efficiency 1-A in VISIBLE 1 in IR EQUILIBRIUM RADIATIVE BUDGET FOR THE EARTH Kevin Trenberth, BAMS, 2009 The ultimate models for climate research TERRESTRIAL RADIATION SPECTRUM FROM SPACE: composite of blackbody radiation spectra emitted from different altitudes at different temperatures HOW DOES ADDITION OF A GREENHOUSE GAS WARM THE EARTH? Example of a GG absorbing at 11 mm 1. 1. Initial state 2. 2. Add to atmosphere a GG absorbing at 11 mm; emission at 11 mm decreases (we don’t see the surface anymore at that , but the atmosphere) 3. 3. At new steady state, total emission integrated over all ’s must be conserved e Emission at other ’s must increase e The Earth must heat! RADIATIVE FORCING OF CLIMATE DF Reflected solar FSA/4 atmospheric emission f T14 Flux out Flux in solar radiation FS/4 surface emission (1-f) To4 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 = DF. In our 1-layer model, [4(1f/2)T3o]-1 = 0.3 K m2 W-1; in research climate models, ranges from 0.3 to 1.4 K m2 W-1 depending on model CLIMATE CHANGE FORCINGS, FEEDBACKS, RESPONSE Positive feedback from water vapor causes rough doubling of IPCC [2007] 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, to +Dt relative to CO2: GWP to to +Dt DF1 kg X dt DF1 kg CO2 dt to Gas Lifetime (years) GWP for time horizon 20 years 100 years 500 years CO2 ~100 1 1 1 CH4 12 63 23 7 N2O 114 279 300 158 CFC-12 (CF2Cl2) 100 10340 10720 5230 HFC-134a (CH2FCF3) 14 3580 1400 4 SF6 3200 15290 22450 32780