Sept. 7, 2006 EESC W4400x 1

The Greenhouse EffectThe Greenhouse Effect

Lisa GoddardLisa Goddardgoddard@iri.columbia.edugoddard@iri.columbia.edu

Sept. 7, 2006 EESC W4400x 2

Electromagnetic Spectrum

Sensitivity of human eyes to EM radiation Definition of visible spectrum

Sept. 7, 2006 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)

Sept. 7, 2006 EESC W4400x 4

Main Points• Energy balance: In=Out (in equilibrium)

• Greenhouse Effect: Difference betweensurface 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 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

Sept. 7, 2006 EESC W4400x 6

Planck Function & Blackbody Radiation

Sept. 7, 2006 EESC W4400x 7

Blackbody emission curves for the Sun and Earth. The Sun emits more energy at all wavelengths.

Note logarithmicscale

Sept. 7, 2006 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

Sept. 7, 2006 EESC W4400x 9

Blackbody Equilibrium(Energy Conservation)

Energy In

Effect of latitude on solar flux

1

2

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

Sept. 7, 2006 EESC W4400x 11

Blackbody Equilibrium(Energy Conservation)

Energy In = Energy Out

Emitted“Earthlight”

4πR2Earth x SEarth

Sept. 7, 2006 EESC W4400x 12

Why is Earth visible from space?

Sept. 7, 2006 EESC W4400x 13

Blackbody Equilibrium(Energy Conservation)

Energy In = Energy Out

Emitted“Earthlight”

4πR2Earth x SEarth

Consider albedo

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

Sept. 7, 2006 EESC W4400x 15

Blackbody Equilibrium

• What’s missing is the atmosphere

Sept. 7, 2006 EESC W4400x 16

Greenhouse Effect

Incomingsolar radiation

Reflection

Transmission

Emission from surface

Emission from atmos.

Emission from atmos.

Sept. 7, 2006 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.)

Sept. 7, 2006 EESC W4400x 18

1st Law of Thermodynamics

dEint = dQ – dW

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 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?

Sept. 7, 2006 EESC W4400x 20

Some general properties of absorption by greenhouse gases (for λ>5μm)

Molecule Lifetime(years)

Concentration(ppbv)

Spectral Range(μm)

Relative Forcing*

CO2

(Carbon Dioxide)

2 3.39 x 103 13.5-16.5 (center @ 15)also 5.2, 9.4, 10.4

1

O3

(Ozone)

0.1-0.3 variable 9.0 & 9.6also 5.75, 14.1

N2O(Nitrous Oxide)

120 300 7.8 & 17.0 206

CH4

(Methane)

5-10 1700 7.7 21

CFCl3 (CFC11)

65 0.26 8 - 12 12,400

CF2Cl2 (CFC12)

110 0.54 10.5 – 11.4 15,800

CF3Cl (CFC13)

400 0.007 8.9 - 9.3

Sept. 7, 2006 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

Sept. 7, 2006 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.

Sept. 7, 2006 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)

Sept. 7, 2006 EESC W4400x 24

Example: Energy budget of column of atmosphere-ocean system

Atm

osph

ere

Oce

an

Fah

Foh

Fv(z=0)

F+(z=)S

(=net solar in)

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

Sept. 7, 2006 EESC W4400x 25

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.

Radiation Balance

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

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.

320 330 340 350 360 (W/m2)January

April

July

October

December

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

•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.

Annual mean surface outgoing IR

Annual mean TOA outgoing IR

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.

Sept. 7, 2006 EESC W4400x 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)