Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Mitigation of global warming in Australia wikipedia , lookup
General circulation model wikipedia , lookup
Climate engineering wikipedia , lookup
IPCC Fourth Assessment Report wikipedia , lookup
Instrumental temperature record wikipedia , lookup
Global warming wikipedia , lookup
Climate sensitivity wikipedia , lookup
Attribution of recent climate change wikipedia , lookup
Hemispherical photography wikipedia , lookup
ATMOSPHERIC RADIATION 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: F l d l 0 BLACKBODY RADIATION • • Objects that absorb 100% of incoming radiation are called blackbodies For blackbodies, l is given by the Planck function: Function of T only! Often denoted B(l,T) F sT 4 s 2p 5k 4/15c2h3 is the Stefan-Boltzmann constant lmax = hc/5kT Wien’s law lmax KIRCHHOFF’S LAW: Emissivity e(l,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 TERRESTRIAL RADIATION SPECTRUM FROM SPACE: composite of blackbody radiation spectra for different T Scene over Niger valley, N Africa 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 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 (0.7-20 mm) • Increase in vibrational frequency of a given bond requires change in dipole moment of molecule – Rotational transition: far-IR (20-100 mm) • Increase in angular momentum around rotation axis Gases that absorb radiation near the spectral maximum of terrestrial emission (10 mm) are called greenhouse gases; this requires vibrational or vibrational-rotational transitions NORMAL VIBRATIONAL MODES OF CO2 Δp 0 forbidden Δp 0 allowed Δp 0 IR spectrum of CO2 asymmetric stretch bend allowed GREENHOUSE EFFECT: absorption of terrestrial radiation by the atmosphere • Major greenhouse gases: H2O, CO2, CH4, O3, N2O, CFCs,… • Not greenhouse gases: N2, O2, Ar, … 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 )s To4 + f s T14 Transmitted surface • Atmospheric layer (1 f )s To4 f s To4 2 f s T14 Solution: f s T14 Atmospheric emission f s T14 Atmospheric emission 1 4 To=288 K F (1 A) e f=0.77 To S f 4(1 )s T1 = 241 K 2 Atmospheric layer (T1) abs. eff. 0 for solar (VIS) f for terr. (near-IR) Surface emission FS A / 4 s To4 Earth surface (To) Absorption efficiency 1-A in VISIBLE 1 in IR RADIATIVE AND CONVECTIVE INFLUENCES ON ATMOSPHERIC THERMAL STRUCTURE In a purely radiative equilibrium atmosphere T decreases exponentially with z, resulting in unstable conditions in the lower atmosphere; convection then redistributes heat vertically following the adiabatic lapse rate The ultimate models for climate research EQUILIBRIUM RADIATIVE BUDGET FOR THE EARTH 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 l, but the atmosphere) 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! 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 FORCING OF CLIMATE CHANGE Fin Incoming solar radiation Fout Reflected solar radiation (surface, air, aerosols, clouds) IR terrestrial radiation ~ T4; absorbed/reemitted by greenhouse gases, clouds, absorbing aerosols EARTH SURFACE • Stable climate is defined by radiative equilibrium: Fin = Fout • Instantaneous perturbation e Radiative forcing DF = Fin – Fout Increasing greenhouse gases g DF > 0 positive forcing • The radiative forcing changes the heat content H of the Earth system: DT dH DF o dt l eventually leading to steady state DTo lDF where To is the surface temperature and l is a climate sensitivity parameter • Different climate models give l = 0.3-1.4 K m2 W-1, insensitive to nature of forcing; differences between models reflect different treatments of feedbacks CLIMATE CHANGE FORCINGS, FEEDBACKS, RESPONSE Positive feedback from water vapor causes rough doubling of l CLIMATE FEEDBACK FROM HIGH vs. LOW CLOUDS Clouds reflect solar radiation (DA > 0) g cooling; …but also absorb IR radiation (Df > 0) g warming WHAT IS THE NET EFFECT? sTcloud4 < sTo4 sTcloud4≈ sTo4 Tcloud≈ To convection sTo4 To LOW CLOUD: COOLING sTo4 HIGH CLOUD: WARMING IPCC [2007] ORIGIN OF THE ATMOSPHERIC AEROSOL Aerosol: dispersed condensed matter suspended in a gas Size range: 0.001 mm (molecular cluster) to 100 mm (small raindrop) Soil dust Sea salt Environmental importance: health (respiration), visibility, radiative balance, cloud formation, heterogeneous reactions, delivery of nutrients… SCATTERING OF RADIATION BY AEROSOLS: “DIRECT EFFECT” Scattering efficiency is maximum when particle radius = l eparticles in 0.1-1 mm size range are efficient scatterers of solar radiation By scattering solar radiation, aerosols increase the Earth’s albedo 2 (diffraction limit) EVIDENCE OF AEROSOL EFFECTS ON CLIMATE: 0 Temperature decrease following large volcanic eruptions Observations Temperature Change (oC) -0.6 -0.4 -0.2 +0.2 NASA/GISS general circulation model 1991 1992 1993 Mt. Pinatubo eruption 1994 SCATTERING vs. ABSORBING AEROSOLS Scattering sulfate and organic aerosol over Massachusetts Partly absorbing dust aerosol downwind of Sahara Absorbing aerosols (black carbon, dust) warm the climate by absorbing solar radiation AEROSOL “INDIRECT EFFECT” FROM CLOUD CHANGES Clouds form by condensation on preexisting aerosol particles (“cloud condensation nuclei”)when RH>100% clean cloud (few particles): large cloud droplets • low albedo • efficient precipitation polluted cloud (many particles): small cloud droplets • high albedo • suppressed precipitation EVIDENCE OF INDIRECT EFFECT: SHIP TRACKS N ~ 100 cm-3 W ~ 0.75 g m-3 re ~ 10.5 µm N ~ 40 cm-3 W ~ 0.30 g m-3 re ~ 11.2 µm from D. Rosenfeld Particles emitted by ships increase concentration of cloud condensation nuclei (CCN) Increased CCN increase concentration of cloud droplets and reduce their avg. size Increased concentration and smaller particles reduce production of drizzle Liquid water content increases because loss of drizzle particles is suppressed Clouds are optically thicker and brighter along ship track SATELLITE IMAGES OF SHIP TRACKS NASA, 2002 Atlantic, France, Spain AVHRR, 27. Sept. 1987, 22:45 GMT US-west coast OTHER EVIDENCE OF CLOUD FORCING: CONTRAILS AND “AIRCRAFT CIRRUS” Aircraft condensation trails (contrails) over France, photographed from the Space Shuttle (©NASA).