global energy balance: the greenhouse effect
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Global Energy Balance: The Greenhouse Effect. Geos 110 Lectures: Earth System Science Chapter 3: Kump et al 3 rd ed. Dr. Tark Hamilton, Camosun College. 3 Inner Rocky Planets with Atmospheres. Venus -------------Earth----------------Mars - PowerPoint PPT PresentationTRANSCRIPT
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Global Energy Balance:The Greenhouse Effect
Geos 110 Lectures: Earth System ScienceChapter 3: Kump et al 3rd ed.
Dr. Tark Hamilton, Camosun College
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3 Inner Rocky Planets with Atmospheres
Venus -------------Earth----------------Mars
The Goldilocks Zone
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Then Goldilocks tried Baby Bear’s
porridge and it was just right, so she ate it all up!
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Venus: South Pole > 460°C, CO2 SO2
UV Image: Pioneer Venus Orbiter, Feb, 5, 1979
Greenschist Facies Metamorphism, No Clays
Supercritical Fluids, No Liquid Water
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Earth: The Blue Planet (Ice, Water, Steam)
Earth ~ 15°C average, Seasons, abundant liquid water
Transparent N2 – O2 – Ar Atmosphere, minor GHS’s
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Mars: -55°C, CO2 millibar atmosphere
Colder than a Polar winter, hydrated minerals, no H2O(l)
• Less atmosphere than a Bar in Nanaimo, Dry Ice Caps
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Electromagnetic Radiation - Waves
• E and B vary as wave passes at speed of light
• E-field interacts with matter through its electrons
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Energy, Frequency & Wavelength
• E = h ν , Higher Frequency Higher Energy
• E = h c/λ , Lower energy Longer Wave
• Whats nu? ……. ν = c/ λ
• Or
• λ ν = c , c = 3x108 m/s
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400 nm < Visible Light < 700 nm
• Longer wave infra-red and microwaves are “heat” for greenhouse• Shorter wavelength hard UV & X-rays are ionizing radiation
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Flux: Energy per unit area per unit timeNormal Incidence Minimizes Area
• Heat or Light per unit area decreases w/ Sun Angle• The Sun heats less at Dawn, Dusk & Winter than 12pm
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Normal Incidence = Circular FootprintMaximum Flux!
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Inclined Incidence Increases Area but Decreases Heating Decreased Flux
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Inverse Square Law
Intensity of light/heat decreases w/ square of distance
e.g. 2X distance = ¼ power, 1/3 distance = 9 x power
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Temperatures of Water Phase Changes
Celsius based on freezing & boiling or H2O
Kelvins Absolute (no offset), same size as Celsius
Farenheit Freezing & Coagulation of Human Blood…eeew!
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Temperature Scales
• Celsius: 0° Freezing, 100° Boiling
• T°F = [T°C + 1.8] + 32° or
• T°C = [T°F – 32] / 1.8 where 1.8 = 9/5
• T K = T°C + 273.15 (Kelvins, not degrees K)
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A Cold Black Body absorbs at all wavelengths
Cold = Black Hot = emits Red Hotter = emits White
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The Planck Function
• The Planck Function: variation of blackbody radiation & λ• Wein’s Law: λmax ~ 2898/T (Kelvins)• Stefan-Boltzmann Law: Sum of All Flux ~ σ T4
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The Planck Function: variation of blackbody radiation & λ (wavelength)
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Wein’s Law: λ max ~ 2898/T (Kelvins)
• The Sun’s Photosphere is ~ 5780 Kelvins
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Stefan-Boltzmann Law: Sum of All Flux ~ σ T4
Emission goes up as temperature to the 4th power!
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Blackbody Emission Spectra for Sun & Earth
• Ultraviolet.…Visible………………………..Infrared• The Sun emits more at all wavelengths λ (energies)• The Earth absorbs in visible light (0.4-0.7) μm & emits in infrared ( λ > 1μm)
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Solar Energy FluxStefan Boltzmann Law
• Fsun = σ (5780 K)4 ~ 6.3 x 107 W/m2
• If some other star were twice as hot:
• Fstar = σ (2 x 5780 K)4 • = (2)4 x σ (5780 K)4 = 16 Fsun !
• Sooo… this must be a real rock star?
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Earth’s Global Energy Balance
• For Earth’s Energy Budget to Balance
• Flux in must = Flux out
• if true T°C = Constant, One climate, No weather
• but Flux in > Flux out so Earth is Warming
• 3 Factors Control Earth’s Energy Budget & Climate:– Solar Flux at any particular distance– Earth’s reflectivity (albedo)– Greenhouse Gas Effects
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A Closer Look at Global Energy Balance
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Earth’s Energy Balance
Energy emitted = Energy absorbed :
• Energy emitted = 4π REarth2 x σTEarth
4
– This follows from Stefan-Boltzmann & Spherical Shape
E absorbed = E intercepted – E reflected:
• E absorbed = πREarth2 S - πREarth
2 SA = πREarth2 S(1-A)
– Where : S = Solar Flux & SA = Earth’s Projected Area
Therefore: 4π REarth2 x σTEarth
4 = πREarth2 S(1-A)
or: σTEarth4 = S(1-A)/4
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The Greenhouse EffectOne-Layer Atmosphere
• ~33°C net surface warming = Tmean sT - Tradiating
• Atmosphere radiates IR down & absorbs IR up
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Flux up from ground = Net Solar input + Flux down from air
For Earth’s Surface: solar input + atmospheric heat
• σTSurface4 = S(1-A)/4 + σTEarth’s Air
4
For Earth’s Air: atmosphere radiates 2 ways
• σTSurface4 = 2σTEarth’s Air
4
Equate, subtract σTEarth’s Air4 & divide by σ to obtain:
• TS = 2 ¼ TEA this is hotter with Air by 1.19
• or ΔTg = TS – TEA = 303 – 255 = 48K, Really ~15 K
The Greenhouse EffectOne-Layer Atmosphere
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Was 387, CO2 now = 390.02 ppm August 2011
Increasing ~ 2 ppm/yr, N2, O2 & Ar are “inert”
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Trace Greenhouse Gases
CFC’s from blowing gas, refrigerants & burned plastic H2O 4% = 40,000 ppm, 1.7 ppm CH4 ~ 63 ppm CO2
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Thermal Layers in Earth’s AtmosphereDominate the Atmospheric Structure
• The Pressure gradient is log-linear, decreasing 6 orders of magnitude over the 1st 100 km
• Earth’s surface & Stratopause are warmest
• The Tropopause and Mesopause are coldest
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The Log-Linear Pressure Gradient Decreases by 6 orders in 100 km
• Barometric Law: Pressure decreases with altitude by a factor of 10 for each 16 km altitude -0.625 bar/km
• Deviation from Log-Linearity is due to temperature gradients within layers
• At Jet airplane heights ~11 km the pressure 618 mb
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Atmospheric Thermal LayeringTroposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere
• Earth’s surface & Stratopause are warmest
• The Tropopause and Mesopause are coldest
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Atmospheric Thermal Layering• Exosphere: gas rarely collides, can escape to space
• Thermosphere: (85 to 120 - 500 km) > Δ~1.3°/km– Mesopause = minimum in thermal profile ~ -95°C
• Mesosphere: (50 to 60 – 85 to 120 km) Δ-2.3°/km– Stratopause = maximum in thermal profile ~ 0°C
• Stratosphere: (8 to 15 – 50 to 60 km), Δ~1.4°/km– Tropopause = minimum in thermal profile ~ -65°C
• Troposphere: (0- 8 or 15 km), densest, warmest, lowest layer, thick in Tropics, thin at Poles, Δ-6°/km– Clouds, Rain, Snow; well mixed by convection– Earth & Ocean surface is base of Troposphere
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Modes of Heat Transport & Storage
• How is each one of these important in the Atmosphere and at Earth’s Surface?
• Where and when is each of these important?
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Heat Storage and Transfer
• Sensible Heat cal/g°C is proportional to density– You can stand hot or cold air better than water of same T
• Latent Heat depends on condensable H2O
• Radiation = emission of photons by excited electons
• Convection = Heat, Mass & Momentum transfer in a fluid, via fluid motion w/ density currents/gradients
• Conduction = Heat transfer by direct contact of molecules (significant only in solids, not fluid or gas). Hot rocks, sand, hot asphalt, hot tin roof
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Heat Storage and Transfer
• Sensible Heat You can stand hot or cold air better than water of same T, more mass or density, more heat capacity
• Latent Heat Evaporated H2O carries heat to atmosphere, condensed/crystallized H2O leaves heat
• Radiation = The hotter the atmosphere, the more radiation to the air, ground and space
• Convection = Heating unevenly or from below in gravity field drives convection
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Heat Storage & Transfer: Troposphere
• Earth & Ocean are heated ~ equally by sun’s radiation
• The Earth’s surface re-radiates in IR
• This IR and that of the Sun, heats GHG’s in the Troposphere or is reflected downwards by clouds, especially near the Earth’s surface unstable lower density air rises & convects, thus we get weather
• Troposphere re-radiates IR up into less dense atmosphere layers where it can be lost to space
• There is also sensible, latent and convected heat
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Most of the O3 Ozone is in the Stratosphere
• < 5ppm H2O vapour, usually no clouds, stratifiedException is Antarctic Winter, thin Stratospheric Clouds
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Why is there such a wavy T° Profile
Earth’s surface heats lower Troposphere which convects
O3 in Stratosphere is heated above by UV, stable stratification
O2 absorbs short wave UV in Thermosphere for uppermost atmospheric heating
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Water’s Big Dipole MomentMakes it rotate when it absorbs IR
• IR λ > 12 μm is virtually all absorbed by water’s rotation band
• CO2 has 2 perpendicular π bonds which also absorb
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Molecular Absorption Spectrum: GHG’s
• Molecules can: rotate, or vibrate atoms changing bond lengths and bend changing dipole moments
• CO2 at λ > 15 μm is a bending mode for O=C=O
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CO2’s bending mode of vibration
• Alternating planes of π bonds C=O and lone pairs on end oxygens experience polarizations & bending
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Other Greenhouse GasesReduce Outgoing IR
• N2O Nitrous Oxide - several bands between 530-760/cm & between 1585-4000/cm
• O3 Ozone – 9.6 μm in window between H20 & CO2
• CH4 Methane = 37x the value of 1 CO2 for GHG, many absorption bands in 1.16 μm region
• Freons – CHClF2 , CCl2F2 , substituted lopsided polar methanes absorb in 8-12 μm window! More GHC power than a CO2 molecule
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