Earth’sEarth’s EnergyEnergy BalanceBalance && thethe GreenhouseGreenhouse EffectEffect Outline:Outline:

• The Earth’s Energy Balance: • Electromagnetic Spectrum: – Ultraviolet (UV) – Visible – Infrared (IR) • Blackbody Radiation • Albedo (reflectivity) • Greenhouse Gases • Atmospheric layers PlanetaryPlanetary AtmospheresAtmospheres

• Question: Why does the earth support life but, say Venus and Mars do not? The Goldilocks Problem Life as we know it requires liquid water

Venus Earth Mars

Too Hot Just Right! Too Cold Ave Temp 460C Ave Temp 15C Ave Temp -55C No liquid H20 (mmmmm..) (BRRRRRR..) H20 in all 3 phases No present signs of H20 PlanetPlanet EnergyEnergy BalanceBalance

* How do you heat a planet?

• Or: what determines that final average temperature? HeatHeat transfertransfer

• The atmosphere and ocean are heated in three important ways: (i) Electromagnetic Radiation (ii) Convection (iii)Conduction ElectromagneticElectromagnetic EnergyEnergy

• Short wavelength = high energy • Long wavelength = low energy High The Electromagnetic Spectrum Low Energy Energy 10-9m

10-6m ElectromagneticElectromagnetic SpectrumSpectrum

UV: High IR: Low Energy Energy EnergyEnergy FluxFlux

• 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 ConceptConcept ofof TemperatureTemperature

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

• Temperature = average kinetic energy of molecules • Kinetic energy is energy possessed by virtue of motion • Measures of temperature is… TemperatureTemperature ofof air:air: totaltotal motionmotion ofof moleculesmolecules ConceptConcept ofof TemperatureTemperature

• 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). RadiationRadiation vs.vs. temperaturetemperature

• 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 Constant temp (T)

Stefan-Boltzmann Law: High Temp = High Total Energy output (Total radiation flux emitted as a function of temperature) 4 (T in Kelvin) ET   5.67 108-24 W m K

Wien’s Law: High energy = LOW wavelength (wave-length of maximum intensity as a function of Temp) (T in Kelvin) max  2898 T TheThe SunSun andand EarthEarth

• 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)

Note: Sun emissions MUCH LARGER than Earth emissions (Stefan-Boltzmann Law)

Earth emits primarily in IR (long wave radiation, LWR) SolarSolar && TerrestrialTerrestrial (Radiation)(Radiation)

Sun: Earth: “Solar “Terrestrial Radiation” Radiation” is source of is infrared UV and energy Visible energy RecallRecall ““AlbedoAlbedo””(from daisy-world!)

• Albedo = “Reflectivity” of surface

• ~ how much energy actually makes it into system. SSolarolar RadiationRadiation isis NOTNOT completelycompletely absorbed:absorbed: AlbedoAlbedoAlbedoAlbedo (reflectivity)(reflectivity)

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

At 90˚ sun angle Surface type % reflected albedo

1

Sea water 2 0.02 4

Moist black soil8 0.08 86% High grass 19 0.19 W hite sand 40 0.40 Dirty snow 60 0.60 Clea n dense sno w 8 6 0.86

Average albedo of Earth: 0.31 Clean dense snow ButBut itit REALLYREALLY mattersmatters whatwhat thethe surfacesurface is..is..

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

At 90˚ sun angle

9

Surface type % reflected albedo 8

% Sea water 2 0.02 2 Moist black soil8 0.08 High grass 19 0.19 W hite sand 40 0.40 Dirty snow 60 0.60 Clea n dense sno w 8 6 0.86

Average albedo of Earth: 0.31 Sea water AngleAngleAlbedoAlbedo (of(of “incidence”)“incidence”) isis alsoalso veryvery important..important..

Albedo is expressed on a scale Sea water at from 0 to 1. different angles 0 = none of sun’s energy reflected 1 = all of sun’s energy reflected Angle % reflected At 90˚ sun angle 90 ˚ 2 Surface type % reflected albedo 60 ˚ 2 Sea water 2 0.02 30 ˚ 6 Moist black soil8 0.08 15 ˚ 21 High grass 19 0.19 W hite sand 40 0.40 6 ˚ 53 Dirty snow 60 0.60 Clea n dense sno w 8 6 0.86

Average albedo of Earth: 0.31 ContributionsContributions toto AlbedoAlbedo NowNow wewe cancan calculatecalculate aa basicbasic EarthEarth energyenergy balancebalance (and(and predictedpredicted Temp!)Temp!)

• Pieces? • 1) Incoming radiation • 2) Albedo = “Reflectivity” (out) • 3) Blackbody radiation (out) SimplifiedSimplified radiationradiation budgetbudget • 70% is absorbed and re-radiated as IR (long

wavelenghs) Reflected solar

• Energy arrives as visible (shortwave) sunlight

• About 30% is reflected “albedo” Earth • Higher reflection “albedo” at higher latitudes Incoming visible

• The remainder leaves as outgoing infrared (longwave) radiation Outgoing infrared RadiationRadiation BalanceBalance equationequation S 4 S = solar constant (1-A)=  Tb = 1370 W/m2  = Stefan-Boltzman constant 4 = 5.67 X 10-8 W/m2/K4 A = albedo Tb = blackbody temperature

Incoming Outgoing terrestrial solar energy energy heats the leaves earth earth (blackbody) SoSo ifif youyou dodo thisthis calculation,calculation, whatwhat dodo youyou thinkthink thethe currentcurrent averageaverage temptemp ofof thethe earth’searth’s surfacesurface turnsturns outout toto be?be? Answer:Answer: ~~ --2020 CC (( --44 FF )) avg.avg. globalglobal TT

AmazonAmazon BasinBasin landscapelandscape (Predicted(Predicted basedbased onon radiationradiation balancebalance alonealone)) SolarSolar SystemSystem

Interesting exercise: Can compare predicted vs. actual temp of three closest “earth-like” planets predicted Actual

Whats going on ? Mars: About right Earth & Venus : way, way wrong.. too hot! • What’s going on? WhatsWhats goinggoing on?on?

GreenhouseGreenhouse Gasses!Gasses! FourFour mostmost commoncommon GHGsGHGs

Greenhouse Gas Amount (ppm)

Water Vapor (H2O) ~3000

Carbon Dioxide (CO2) 365

Methane (CH4 ) 1.7

Nitrous Oxide (N2O) 0.3 TheThe “greenhouse“greenhouse effect”effect”

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

2. Radiation Adsorption by certain molecules Heat TheThe RestRest ofof thethe PlanetPlanet table.table... HeliHeli--touringtouring inin Tahiti:Tahiti: EarthEarth withwith NoNo GreenhouseGreenhouse effecteffect IgnoreIgnore thethe Hype!Hype! Earth’sEarth’s globalglobal temperaturetemperature

• Earth’s global temperature is determined by THREE things: –SOLAR HEATING – GLOBAL ALBEDO – ATMOSPHERIC CONCENTRATION OF GREENHOUSE GASES BlackbodyBlackbody T,T, AlbedoAlbedo && greenhousegreenhouse

T = temperature it take into account T = blackbody temperature A b albedo of 0.3

-23˚C = albedo effect T = +15˚C Ta= -18˚C Tb= +5˚C actual

-20˚C 0˚C +20˚C

+33˚C = greenhouse effect Finally:Finally: AtmosphericAtmospheric layerslayers

• Recall Atmosphere ( and ocean) is heated in three ways: (i) Direct Electromagnetic Radiation (ii)Convection (iii)Conduction ConvectionConvection vs.vs. conductionconduction

• 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:: PressurePressure vs.vs. TempTemp

Whats going on here? TheThe StratosphereStratosphere isis heatedheated fromfrom above!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)

BUTBUT --TroposphereTroposphere heatedheated fromfrom below!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.. TempTemp structurestructure

Direct Heat from above- incoming UV + ozone greenhouse

Indirect Heating from below- radiated blackbody + greenhouse EndEnd