earth’s energy balance & the greenhouse effect€¦ · earth’s energy balance ......

Earth’s Energy Balance Earth’s Energy Balance & the Greenhouse Effect& the Greenhouse Effect

• The Earth’s Energy Balance:• Electromagnetic Spectrum:

– Ultraviolet (UV)– Visible– Infrared (IR)

• Blackbody Radiation• Albedo (reflectivity)• Greenhouse Gases• Atmospheric layers

Outline:Outline:

Planetary AtmospheresPlanetary Atmospheres

• Question: Why does the earth support life but, say Venus and Mars do not?

The Goldilocks ProblemLife as we know it requires liquid water

Just Right!Ave Temp 15C(mmmmm..)

H20 in all 3 phases

Too HotAve Temp 460C

No liquid H20

Too ColdAve Temp -55C(BRRRRRR..)

No present signs of H20

Venus Earth Mars

Planet Energy BalancePlanet Energy Balance

* How do you heat a planet?

• Or: what determines that final average temperature?

Heat transferHeat transfer

• The atmosphere and ocean are heated in three important ways:

(i) Electromagnetic Radiation(ii) Convection(iii)Conduction

Electromagnetic EnergyElectromagnetic Energy

• Short wavelength = high energy• Long wavelength = low energy

The Electromagnetic SpectrumHighEnergy

LowEnergy

10-9m

10-6m

Electromagnetic SpectrumElectromagnetic Spectrum

UV: HighEnergy

IR: LowEnergy

Energy FluxEnergy Flux

• Energy Flux= radiation per unit area per second

• Units: Joules per second per square meter– Written: J s-1 m-2 or J/s m2

– J/second = ? • = Watts ( POWER = energy/ time)

• W/m2 = most common expression of energy flux

Concept of TemperatureConcept of Temperature

• In terms of atmosphere (or water) What is “Heat”?

Concept of TemperatureConcept of Temperature

• Temperature = average kinetic energy of molecules

• Kinetic energy is energy possessed by virtue of motion

• Measures of temperature is…

Temperature of air: Temperature of air: total total motion of moleculesmotion of molecules

Concept of TemperatureConcept of Temperature

• Measures of temperature are…

Fahrenheit: freezing point of a mixture of snow and salt (0F) and body temp (100F).Celcius: based on freezing point (0C) and boiling point (100C) of pure water.Kelvin: absolute temperature based on molecular motion. All atoms at rest at 0K

(absolute zero).

Radiation vs. temperatureRadiation vs. temperature

• All objects with a temperature above 0 deg K(absolute zero) emit radiation.

• The intensity and wavelength of the radiation emitted is a strong function of temperature.

• All objects also absorb radiation.• A so-called “blackbody” emits and

absorbs radiation at all wavelengths. None is reflected.

As we continuously heat the ball, its color changes. Why?

Radiation flux vs temperature

Wien’s Law: High energy = LOW wavelength (wave-length of maximum intensity as a function ofTemp)

max 2898 T (T in Kelvin)

Constant temp (T)

4E TStefan-Boltzmann Law: High Temp = High Total Energy output (Total radiation flux emitted as a function of temperature)

(T in Kelvin)8 -2 45.67 10 W m K

The Sun and EarthThe Sun and Earth

• What are the consequences of Wien’s Law and the Stefan-Boltzmann Law for the Sun and Earth?

• Sun average T=3000K• Earth average T=288K

Sun emits primarily in visible(short wave radiation, SWR)

Earth emits primarily in IR(long wave radiation, LWR)

Note: Sun emissionsMUCH LARGER thanEarth emissions (Stefan-Boltzmann Law)

Solar & Terrestrial (Radiation)Solar & Terrestrial (Radiation)

Sun:“Solar

Radiation”is source of

UV and Visibleenergy

Earth:“TerrestrialRadiation”is infraredenergy

Recall “Recall “AlbedoAlbedo””(from daisy(from daisy--world!)world!)

• Albedo = “Reflectivity” of surface

• ~ how much energy actually makes it into system.

AlbedoAlbedo

86

Clean dense snow

14%

Solar Radiation is NOT completely absorbed: Solar Radiation is NOT completely absorbed: Albedo Albedo (reflectivity)(reflectivity)

Albedo is expressed on a scale from 0 to 1. 0 = none of sun’s energy reflected1 = all of sun’s energy reflected

At 90˚ sun angleSurface type % reflected albedoSea water 2 0.02

Moist black soil 8 0.08

High grass 19 0.19

White sand40 0.40

Dirty snow 60 0.60

Clean dense snow 86 0.86

Average albedo of Earth: 0.31

Albedo is expressed on a scale from 0 to 1. 0 = none of sun’s energy reflected1 = all of sun’s energy reflected

At 90˚ sun angleSurface type % reflected albedoSea water 2 0.02

Moist black soil 8 0.08

High grass 19 0.19

White sand40 0.40

Dirty snow 60 0.60

Clean dense snow 86 0.86

Average albedo of Earth: 0.31

2

Sea water

98%

But it REALLY matters what the surface is..But it REALLY matters what the surface is..

AlbedoAlbedo

Sea water at different angles

Angle % reflected90 ˚ 260 ˚ 230 ˚ 615 ˚ 216 ˚ 53

Angle (of “incidence”) is also very important..Angle (of “incidence”) is also very important..

Albedo is expressed on a scale from 0 to 1. 0 = none of sun’s energy reflected1 = all of sun’s energy reflected

At 90˚ sun angleSurface type % reflected albedoSea water 2 0.02

Moist black soil 8 0.08

High grass 19 0.19

White sand40 0.40

Dirty snow 60 0.60

Clean dense snow 86 0.86

Average albedo of Earth: 0.31

Contributions to Contributions to AlbedoAlbedo

Now we can calculate a basic Now we can calculate a basic Earth energy balance (and Earth energy balance (and

predicted Temp!)predicted Temp!)

• Pieces?• 1) Incoming radiation• 2) Albedo = “Reflectivity” (out) • 3) Blackbody radiation (out)

Simplified radiation budgetSimplified radiation budget

Earth

Incoming visible

• Energy arrives as visible (shortwave) sunlight

Reflected solar

• About 30% is reflected “albedo”

• Higher reflection “albedo” at higher latitudes

Outgoing infrared• The remainder leaves

as outgoing infrared (longwave) radiation

• 70% is absorbed and re-radiated as IR (long wavelenghs)

Radiation Balance equationRadiation Balance equation

Incoming solar energy heats theearth (blackbody)

Outgoing terrestrial energy leaves earth

4(1-A)S

S = solar constant= 1370 W/m2

A = albedo

= Stefan-Boltzman constant= 5.67 X 10-8 W/m2/K4

Tb = blackbody temperature

= Tb4

So if you do this calculation, So if you do this calculation, what do you think the current what do you think the current

average average temp of the earth’s temp of the earth’s surface turns out to be? surface turns out to be?

Answer: ~ Answer: ~ --20 C ( 20 C ( --4 F ) avg. global T4 F ) avg. global T

Amazon Basin landscapeAmazon Basin landscape(Predicted based on radiation balance alone(Predicted based on radiation balance alone))

Solar SystemSolar System

Interesting exercise: Can compare predicted vs. actual temp of three

closest “earth-like” planets

predictedActual

Whats going on ?

Mars: About rightEarth & Venus : way, way wrong.. too

hot! • What’s going on?

Whats Whats going on?going on?

Greenhouse Gasses!Greenhouse Gasses!

Four most common Four most common GHGsGHGs

0.3Nitrous Oxide (N2O)

1.7Methane (CH4 )

365Carbon Dioxide (CO2)

~ 3000Water Vapor (H2O)

Amount (ppm)

Greenhouse Gas

The “greenhouse effect”The “greenhouse effect”

1. Incoming Energy Budget : The Black Body Effect warm earth radiates energy

2. Radiation Adsorption by certain molecules Heat

The Rest of the Planet table.The Rest of the Planet table...

HeliHeli--touring in Tahiti:touring in Tahiti:Earth with No Greenhouse effectEarth with No Greenhouse effect

Ignore the Hype!Ignore the Hype!

Earth’s global temperatureEarth’s global temperature

• Earth’s global temperature is determined by THREE things:– SOLAR HEATING– GLOBAL ALBEDO– ATMOSPHERIC CONCENTRATION OF

GREENHOUSE GASES

Blackbody T, Blackbody T, Albedo Albedo & greenhouse& greenhouse

Tb = blackbody temperature

0˚C +20˚C-20˚C

Ta= -18˚C Tactual= +15˚C

+33˚C = greenhouse effect

Tb= +5˚C-23˚C = albedo effect

TA = temperature it take into account albedo of 0.3

Finally: Atmospheric layersFinally: Atmospheric layers

• Recall Atmosphere ( and ocean) is heated in three ways:

(i) Direct Electromagnetic Radiation(ii)Convection(iii)Conduction

Convection vs. conductionConvection vs. conduction

• Convection is the transfer of heat by mass movement of fluid.

• Conduction is the transfer of heat from molecule to molecule as a result of molecular collisions.

AtmAtm: Pressure vs. Temp: Pressure vs. Temp

Whats going on here?

The Stratosphere is heated from above!The Stratosphere is heated from above!

• The stratosphere contains abundant ozone.

• Ozone is an excellent absorber of UV radiation.

• stratosphere is directly heated from above by incident solar UV (via greenhouse effect).

• (A good thing for life on earth)

BUT BUT --Troposphere heated from below!Troposphere heated from below!

• Most of the atmosphere is largely transparent to incident solar radiation.

• (why?)• Incident solar radiation heats the earth

surface.• Troposphere is heated from below by

conduction, convection and radiation.

AtmAtm. Temp structure. Temp structure

Direct Heat from above-incoming UV + ozone greenhouse

Indirect Heating from below-radiated blackbody + greenhouse

EndEnd