The Greenhouse Effect Lisa Goddard goddardiri columbia edu

The Greenhouse Effect Lisa Goddard goddard@iri. columbia. edu Sept. 7, 2006 EESC W 4400 x 1

Electromagnetic Spectrum Sensitivity of human eyes to EM radiation Definition of visible spectrum Sept. 7, 2006 EESC W 4400 x 2

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 Sept. 7, 2006 EESC W 4400 x. (From Classical Electrodynamics, by J. D. Jackson) 3

Main Points • Energy balance: In=Out (in equilibrium) • Greenhouse Effect: Difference between surface temperature/radiation & Earth’s effective temperature/radiation OUTLINE • • • Blackbody Radiation Planetary energy balance Greenhouse Effect Modelling energy balance A view of Earth’s radiation balance from space Sept. 7, 2006 EESC W 4400 x 4

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 Sept. 7, 2006 EESC W 4400 x 5

Planck Function & Blackbody Radiation Sept. 7, 2006 EESC W 4400 x 6

Note logarithmic scale Blackbody emission curves for the Sun and Earth. The Sun emits more energy at all wavelengths. Sept. 7, 2006 EESC W 4400 x 7

Fun with BB Radiation Check out how Planck distributions evolve with temperature • Planck Function, spectrum, and color • http: //cs. clark. edu/~mac/physlets/Black. Body/blackbody. htm • Black. Body, The Game! • http: //csep 10. phys. utk. edu/guidry/java/blackbody. html • Planck Law Radiation Distributions • http: //csep 10. phys. utk. edu/guidry/java/planck. html Sept. 7, 2006 EESC W 4400 x 8

Blackbody Equilibrium (Energy Conservation) Energy In Sept. 7, 2006 EESC W 4400 x 9

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

Blackbody Equilibrium (Energy Conservation) Energy In = Energy Out Sept. 7, 2006 EESC W 4400 x Emitted “Earthlight” 11 4πR 2 Earth x SEarth

Why is Earth visible from space? Sept. 7, 2006 EESC W 4400 x 12

Blackbody Equilibrium (Energy Conservation) Energy In = Energy Out Consider albedo Sept. 7, 2006 EESC W 4400 x Emitted “Earthlight” 13 4πR 2 Earth x SEarth

Reflection of Solar Radiation: The Earth’s Albedo Components of the Earth’s albedo and their value in % and the processes that affect incoming solar radiation in the Earth’s atmosphere • 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.

Blackbody Equilibrium • What’s missing is the atmosphere Sept. 7, 2006 EESC W 4400 x 15

Greenhouse Effect Incoming solar radiation Reflection Emission from atmos. Transmission Emission from surface Sept. 7, 2006 EESC W 4400 x 16

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 CO 2 and H 2 O. 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, 2 nd ed. , New York: Oxford University Press, 1989, Figure 1. 1. ) Sept. 7, 2006 EESC W 4400 x 17

st 1 Law of Thermodynamics d. Eint = d. Q – d. W The internal energy Eint of 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 Sept. 7, 2006 EESC W 4400 x 18

st 1 Law of Thermodynamics d. Eint = d. Q – d. W Earth’s atmosphere: (1) Constant volume: W=0 (in equilibrium) (2) Sun is approx. constant d. Q = 0 (although Q > 0) (3) Therefore: d. Eint = 0 If Earth’s [effective] temperature is constant (d. E = 0) then how does surface temperature increase? Sept. 7, 2006 EESC W 4400 x 19

Some general properties of absorption by greenhouse gases (for λ>5μm) Molecule Lifetime (years) Concentration Spectral Range (ppbv) (μm) Relative Forcing* CO 2 2 3. 39 x 103 13. 5 -16. 5 (center @ 15) also 5. 2, 9. 4, 10. 4 O 3 0. 1 -0. 3 variable 9. 0 & 9. 6 also 5. 75, 14. 1 N 2 O 120 300 7. 8 & 17. 0 206 CH 4 5 -10 1700 7. 7 21 CFCl 3 65 0. 26 8 - 12 12, 400 CF 2 Cl 2 110 0. 54 10. 5 – 11. 4 15, 800 CF 3 Cl 400 0. 007 8. 9 - 9. 3 (Carbon Dioxide) (Ozone) (Nitrous Oxide) (Methane) (CFC 11) (CFC 12) (CFC 13) Sept. 7, 2006 EESC W 4400 x 1 20

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 Sept. 7, 2006 EESC W 4400 x 21

Earth’s Globally Averaged Atmospheric Energy Budget All fluxes are normalized relative to 100 arbitrary units of incident radiation. Values are approximate. Sept. 7, 2006 EESC W 4400 x 22

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) Sept. 7, 2006 EESC W 4400 x 23

Example: Energy budget of column of atmosphere-ocean system S F+(z= ) (=net solar in) S = absorbed solar radiation Atmosphere F+( ) = outgoing infrared flux (outgoing longwave radiation, OLR) Fah = horizontal energy flux in atmos. Foh = horizontal energy flux in ocean Ocean Fv(z=0) Sept. 7, 2006 Fv(0) = atmos. to ocean energy flux Foh EESC W 4400 x 24

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. Sept. 7, 2006 EESC W 4400 x 25

Earth Radiation Budget from Space: the Spatial Pattern

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

Incoming Solar Flux (Shortwave) at TOA 320 330 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. 340 350 360 (W/m 2)

Reflected Solar at TOA December March June September

Planetary Albedo December March June September

Earth’s Surface Properties as seen from Space

Global Rainfall - a Proxy for Clouds

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

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

Net Incoming Radiation December March June September

Surface vs. TOA Longwave Annual mean surface outgoing IR Annual mean TOA outgoing IR • 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. . * 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.

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.

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/pid 340. php http: //www. agu. org/eos_elec/99148 e. html (Vol. 80, No. 39, September 28, 1999, p. 453) Sept. 7, 2006 EESC W 4400 x 38