The Greenhouse Effect

1. The Greenhouse Effect Lisa Goddard goddard@iri.columbia.edu EESC W4400x

2. Electromagnetic Spectrum Sensitivity of human eyes to EM radiation Definition of visible spectrum EESC W4400x

3. Absorption Profile of Liquid Water Absorption coefficient for liquid water as a function of linear frequency. The visible region of the frequency spectrum is indicated by the vertical dashed lines.Note that the scales are logarithmic in both directions. (From Classical Electrodynamics, by J. D. Jackson) EESC W4400x

4. OUTLINE • Blackbody Radiation • Planetary energy balance • Greenhouse Effect • Modelling energy balance • A view of Earth’s radiation balance from space Main Points • Energy balance: In=Out (in equilibrium) • Greenhouse Effect: Difference betweensurface temperature/radiation &Earth’s effective temperature/radiation EESC W4400x

5. Blackbody: Definition A blackbody is a hypothetical body made up of molecules that absorb and emit electromagnetic radiation in all parts of the spectrum • All incident radiation is absorbed (hence the term black), and • The maximum possible emission is realized in all wavelength bands and in all directions In other words… A blackbody is a perfect absorber and perfect emitter of radiation with 100% efficiency at all wavelengths EESC W4400x

6. Planck Function & Blackbody Radiation EESC W4400x

7. Note logarithmic scale Blackbody emission curves for the Sun and Earth. The Sun emits more energy at all wavelengths. EESC W4400x

8. Fun with BB RadiationCheck out how Planck distributions evolve with temperature • Planck Function, spectrum, and color • http://cs.clark.edu/~mac/physlets/BlackBody/blackbody.htm • BlackBody, The Game! • http://csep10.phys.utk.edu/guidry/java/blackbody/blackbody.html • Planck Law Radiation Distributions • http://csep10.phys.utk.edu/guidry/java/planck/planck.html EESC W4400x

9. Blackbody Equilibrium(Energy Conservation) Energy In EESC W4400x

10. Effect of latitude on solar flux 2 1 The solar flux of beam 1 is equal to that of beam 2. However, when beam 2 reaches the Earth it spreads over an area larger than that of beam 1. The ratio between the areas (see figure above) varies like the inverse cosine of latitude, reducing the energy per unit area from equator to pole. What happens at the pole? The effect of the tilting earth surface is equivalent to the tilting of the light source

11. Blackbody Equilibrium(Energy Conservation) Energy In = Energy Out Emitted“Earthlight” 4πR2Earth x SEarth EESC W4400x

12. Why is Earth visible from space? EESC W4400x

13. Blackbody Equilibrium(Energy Conservation) Energy In = Energy Out Consider albedo  Emitted“Earthlight” 4πR2Earth x SEarth EESC W4400x

14. Reflection of Solar Radiation: The Earth’s Albedo • The ratio between incoming and reflected radiation at the top of the atmosphere (TOA) is referred to as the planetary albedo. • The albedo varies between 0 and 1. Components of the Earth’s albedo and their value in % and the processes that affect incoming solar radiation in the Earth’s atmosphere

15. Blackbody Equilibrium • What’s missing is the atmosphere EESC W4400x

16. Incomingsolar radiation Reflection Emission from atmos. Emission from atmos. Transmission Emission from surface Greenhouse Effect EESC W4400x

17. Absorption of Infrared (Longwave) Radiation in Earth’s Atmosphere Absorption of 100% means that no radiation penetrates the atmosphere. The nearly complete absorption of radiation longer than 13 micrometers is caused by absorption by CO2 and H2O. Both of these gases also absorb solar radiation in the near infrared (wavelengths between about 0.7 μm and 5 μm). The absorption feature at 9.6 micrometers is caused by ozone. (From data originally from R. M. Goody and Y. L. Yung, Atmospheric Radiation, 2nd ed., New York: Oxford University Press, 1989, Figure 1.1.) EESC W4400x

18. 1st Law of Thermodynamics dEint = dQ – dW The internal energy Eintof a system tends to increase if energy is added as heat Q and tends to decrease if energy is lost as work W done by the system. The First Law of Thermodynamics: Four Special Cases EESC W4400x

19. 1st Law of Thermodynamics dEint = dQ – dW Earth’s atmosphere: (1) Constant volume: W=0 (in equilibrium) (2) Sun is approx. constant dQ = 0 (although Q > 0) (3) Therefore: dEint = 0 If Earth’s [effective] temperature is constant (dE = 0) then how does surface temperature increase? EESC W4400x

20. Some general properties of absorption by greenhouse gases (for λ>5μm) EESC W4400x

21. Radiative Transfer Processes Visible (incoming solar radiation) • absorption by air molecules • absorption by the earth's surface • scattering by clouds and earth's surface Infrared (outgoing terrestrial radiation) • absorption/emission by air molecules • absorption/emission by clouds EESC W4400x

22. Earth’s Globally Averaged Atmospheric Energy Budget All fluxes are normalized relative to 100 arbitrary units of incident radiation. Values are approximate. EESC W4400x

23. Modeling the Earth’s Energy Balance • Energy balance models (Global) – Figure 3-19 from Kump et al. is essentially schematic for global EBM • Radiative-convective models (1-D or 2-D) or single-column models (1-D) EESC W4400x

24. S (=net solar in) F+(z=) Atmosphere Fah Fv(z=0) Foh Ocean Example: Energy budget of column of atmosphere-ocean system S = absorbed solar radiation F+() = outgoing infrared flux (outgoing longwave radiation, OLR) Fah = horizontal energy flux in atmos. Foh = horizontal energy flux in ocean Fv(0) = atmos. to ocean energy flux EESC W4400x

25. Radiation Balance The annual mean, average around latitude circles, of the balance between the solar radiation absorbed at the ground (in blue) and the outgoing infrared radiation from Earth into space (in red). The two curves must balance completely over the entire globe, but not at every single latitude. In the tropics, there is an access of radiation (solar radiation absorbed acceeds outgoing terrastrial radiation) in middle and high latitudes all the way to the poles, there is a deficit (Earth is radiating into space more than it receives from the sun). The atmosphere and ocean systems are forced to move about by this imbalance, and bring heat by convection and advection from equator to the poles. EESC W4400x

26. Earth Radiation Budget from Space: the Spatial Pattern

27. December March June September Incoming Solar Flux (Shortwave) at TOA(TOA = Top Of Atmosphere)

28. Incoming Solar Flux (Shortwave) at TOA 320 330 340 350 360 (W/m2) January April July October December The globally-averaged, monthly values of incoming solar radiation at the top of the atmosphere showing the changes due to the change in the distance between the Earth and the Sun.

29. December March June September Reflected Solar at TOA

30. December March June September Planetary Albedo

31. Earth’s Surface Properties as seen from Space

32. Global Rainfall - a Proxy for Clouds

33. December March June September Net Shortwave (Solar) Radiation(Includes albedo)

34. December March June September Outgoing Longwave Radiation (OLR) at TOA

35. December March June September Net Incoming Radiation

36. Surface vs. TOA Longwave • From surface temperature data we can calculate the surface outgoing longwave radiation by using the Stefan-Boltzmann law and by assuming emissivity* of 0.95 • Compare this with the outgoing logwave radiation at the top of the atmosphere.... Annual mean surface outgoing IR * emissivity: Natural surfaces are not perfect black bodies. They absorb and emit only some of the amount predicted by the Stefan-Boltzman Law. The ratio between actual and predicted emission is the emissivity. Annual mean TOA outgoing IR

37. Greenhouse Effect The difference between the longwave radiation from the Earth’s surface and OLR is the greenhouse effect. Note the strong GH effect in areas which are dominated by deep tropical clouds that precipitate a lot (above). These clouds reach high into the atmosphere (more than 10 Km) where the temperature is low, thus the radiative longwave flux from their tops is relatively small. At the same time the surface underneath is warm and the surface emitted longwave radiation is almost entirely trapped in the cloudy atmosphere.

38. Websites: http://yosemite.epa.gov/oar/globalwarming.nsf/content/Emissions.html http://gaw.kishou.go.jp/wdcgg.html http://www.ncdc.noaa.gov/oa/climate/globalwarming.html http://icp.giss.nasa.gov/education/methane/intro/greenhouse.html http://www.rmi.org/sitepages/pid340.php http://www.agu.org/eos_elec/99148e.html(Vol. 80, No. 39, September 28, 1999, p. 453) EESC W4400x