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2. Millenial NH temperature trend [IPCC, 2001]

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

6. KIRCHHOFF’S LAW: Emissivity  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 For any object: … very useful!

7. SOLAR RADIATION SPECTRUM: blackbody at 5800 K

8. TERRESTRIAL RADIATION SPECTRUM FROM SPACE: composite of blackbody radiation spectra for different T Scene over Niger valley, N Africa

10. 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: where A is the albedo (reflectivity) of the Earth = 255 K

12. NORMAL VIBRATIONAL MODES OF CO 2 forbidden allowed allowed IR spectrum of CO 2 bend asymmetric stretch

16. 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

17. EQUILIBRIUM RADIATIVE BUDGET FOR THE EARTH

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

19. HOW DOES ADDITION OF A GREENHOUSE GAS WARM THE EARTH? 1. 1. Initial state 2. 2. 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

20. 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.

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

23. CLIMATE FEEDBACK FROM HIGH vs. LOW CLOUDS convection T o T cloud ≈ T o Clouds reflect solar radiation (  A > 0)  cooling; … but also absorb IR radiation (  f > 0)  warming WHAT IS THE NET EFFECT?  T o 4  T cloud 4 ≈  T o 4 LOW CLOUD: COOLING  T cloud 4 <  T o 4  T o 4 HIGH CLOUD: WARMING

24. IPCC [2007]

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

27. ORIGIN OF THE ATMOSPHERIC AEROSOL Soil dust Sea salt Aerosol: dispersed condensed matter suspended in a gas Size range: 0.001  m (molecular cluster) to 100  m (small raindrop) Environmental importance: health (respiration), visibility, radiative balance, cloud formation, heterogeneous reactions, delivery of nutrients…

29. EVIDENCE OF AEROSOL EFFECTS ON CLIMATE: Observations NASA/GISS general circulation model Temperature decrease following large volcanic eruptions Mt. Pinatubo eruption 1991 1992 1993 1994 -0.6 -0.4 -0.2 0 +0.2 Temperature Change ( o C)

30. 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

32. EVIDENCE OF INDIRECT EFFECT: SHIP TRACKS  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 N ~ 100 cm -3 W ~ 0.75 g m -3 r e ~ 10.5 µm N ~ 40 cm -3 W ~ 0.30 g m -3 r e ~ 11.2 µm from D. Rosenfeld

33. SATELLITE IMAGES OF SHIP TRACKS AVHRR, 27. Sept. 1987, 22:45 GMT US-west coast NASA, 2002 Atlantic, France, Spain

34. OTHER EVIDENCE OF CLOUD FORCING: CONTRAILS AND “AIRCRAFT CIRRUS” Aircraft condensation trails (contrails) over France, photographed from the Space Shuttle (©NASA).