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): Also often use wavenumbers notation: h=6.62x Js c=3.0x10 8 m/s

Here  is the radiation flux emitted in [  is the flux distribution function characteristic of the object Total radiation flux emitted by object: 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:

BLACKBODY RADIATION Objects that absorb 100% of incoming radiation are called blackbodies For blackbodies,   is given by the Planck function:   k 4 /15c 2 h 3 is the Stefan-Boltzmann constant max = hc/5kT Wien’s law Function of T only! Often denoted B(  T) max

KIRCHHOFF’S LAW: Emissivity  T) = Absorptivity For any object:…very useful! Illustrative example: Kirchhoff’s law allows determination of the emission spectrum of any object solely from knowledge of its absorption spectrum and temperature

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  m) Jump of electron from valence shell to higher-energy shell, sometimes results in dissociation (example: O 3 +h   O 2 +O) –Vibrational transition: near-IR (  m) Increase in vibrational frequency of a given bond requires change in dipole moment of molecule –Rotational transition: far-IR (  m) 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 CO 2 forbidden allowed Greenhouse gases = gases with vib-rot absorption features at 5-50  m Major greenhouse gases: H 2 O, CO 2, CH 4, O 3, N 2 O, CFCs,… Not greenhouse gases: N 2, O 2, Ar, …

EFFICIENCY OF GREENHOUSE GASES FOR GLOBAL WARMING The efficient GGs are the ones that absorb in the “atmospheric window” (8-13  m). 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 F S = 1370 W m -2 Radiative balance  effective temperature of the Earth: = 255 K where A is the albedo (reflectivity) of the Earth

SIMPLE MODEL OF GREENHOUSE EFFECT Earth surface (T o ) Absorption efficiency 1-A in VISIBLE 1 in IR Atmospheric layer (T 1 ) abs. eff. 0 for solar (VIS) f for terr. (near-IR) Incoming solar Reflected solar Surface emission Transmitted surface Atmospheric emission Atmospheric emission Energy balance equations: Earth system Atmospheric layer Solution:T o =288 K  f=0.77 T 1 = 241 K VISIBLE 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? Initial state Add to atmosphere a GG absorbing at 11  m; emission at 11  m decreases (we don’t see the surface anymore at that  but the atmosphere) 3. At new steady state, total emission integrated over all ’s must be conserved  Emission at other ’s must increase  The Earth must heat! 3. Example of a GG absorbing at 11  m

RADIATIVE FORCING OF CLIMATE RADIATIVE FORCING OF CLIMATE  F solar radiation F S /4 Reflected solar F S A/4 surface emission (1-f)  T o 4 atmospheric emission f  T 1 4 greenhouse layer (H 2 O, clouds, CO 2, CH 4, … ) Efficiency f Flux in Flux out Radiative equilibrium:  F = (Flux in) – (Flux out) = 0 Increase greenhouse efficiency f  Flux out decreases   F > 0; WARMING Increase solar reflection  Flux in decreases   F < 0; COOLING Radiative forcing  F predicts equilibrium surface temperature response  T o :  T o =  F. In our 1-layer model,  f/2  T 3 o ] -1 = 0.3 K m 2 W -1 ; in research climate models, ranges from 0.3 to 1.4 K m 2 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  t from the injection of 1 kg of a species X at time t o, relative to CO 2 : GasLifetime (years) GWP for time horizon 20 years 100 years 500 years CO 2 ~ CH N2ON2O CFC-12 (CF 2 Cl 2 ) HFC-134a (CH 2 FCF 3 ) SF