Transcript Document

Global Energy Balance:
The Greenhouse Effect
Geos 110 Lectures: Earth System Science
Chapter 3: Kump et al 3rd ed.
Dr. Tark Hamilton, Camosun College
3 Inner Rocky Planets with
Atmospheres
Venus -------------Earth----------------Mars
The Goldilocks Zone
Then Goldilocks
tried Baby Bear’s
porridge and it
was just right, so
she ate it all up!
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
Earth: The Blue Planet (Ice, Water, Steam)
Earth ~ 15°C average, Seasons, abundant liquid water
Transparent N2 – O2 – Ar Atmosphere, minor GHS’s
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
Electromagnetic Radiation - Waves
• E and B vary as wave passes at speed of light
• E-field interacts with matter through its electrons
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
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
Flux: Energy per unit area per unit time
Normal Incidence Minimizes Area
• Heat or Light per unit area decreases w/ Sun Angle
• The Sun heats less at Dawn, Dusk & Winter than 12pm
Normal Incidence = Circular Footprint
Maximum Flux!
Inclined Incidence Increases Area but
Decreases Heating  Decreased Flux
Inverse Square Law
Intensity of light/heat decreases w/ square of distance
e.g. 2X distance = ¼ power, 1/3 distance = 9 x power
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!
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)
A Cold Black Body absorbs at all wavelengths
Cold = Black
Hot = emits Red
Hotter = emits White
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
The Planck Function: variation of blackbody
radiation & λ (wavelength)
Wein’s Law: λ max ~ 2898/T (Kelvins)
• The Sun’s Photosphere is ~ 5780 Kelvins
Stefan-Boltzmann Law: Sum of All Flux ~ σ T4
Emission goes up as temperature to the 4th power!
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)
Solar Energy Flux
Stefan 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?
Earth’s Global Energy Balance
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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
A Closer Look at Global Energy Balance
Earth’s Energy Balance
Energy emitted = Energy absorbed :
• Energy emitted = 4π REarth2 x σTEarth4
– This follows from Stefan-Boltzmann & Spherical Shape
E absorbed = E intercepted – E reflected:
• E absorbed = πREarth2 S - πREarth2 SA = πREarth2 S(1-A)
– Where : S = Solar Flux & SA = Earth’s Projected Area
Therefore: 4π REarth2 x σTEarth4 = πREarth2 S(1-A)
or:
σTEarth4 = S(1-A)/4
The Greenhouse Effect
One-Layer Atmosphere
• ~33°C net surface warming = Tmean sT - Tradiating
• Atmosphere radiates IR down & absorbs IR up
The Greenhouse Effect
One-Layer Atmosphere
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 Air4
For Earth’s Air: atmosphere radiates 2 ways
• σTSurface4 = 2σTEarth’s Air4
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
Was 387, CO2 now = 390.02 ppm August 2011
Increasing ~ 2 ppm/yr, N2, O2 & Ar are “inert”
Trace Greenhouse Gases
CFC’s from blowing gas, refrigerants & burned plastic
H2O 4% = 40,000 ppm, 1.7 ppm CH4 ~ 63 ppm CO2
Thermal Layers in Earth’s Atmosphere
Dominate 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
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
Atmospheric Thermal Layering
Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere
• Earth’s surface & Stratopause are warmest
• The Tropopause and Mesopause are coldest
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
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?
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
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
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
Most of the O3
Ozone is in the Stratosphere
• < 5ppm H2O vapour, usually no clouds, stratified
Exception is Antarctic Winter, thin Stratospheric Clouds
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
Water’s Big Dipole Moment
Makes 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
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
CO2’s bending mode of vibration
• Alternating planes of π bonds C=O and lone pairs on
end oxygens experience polarizations & bending
Other Greenhouse Gases
Reduce Outgoing IR
• N2O Nitrous Oxide - several bands between 530760/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
So Wazzup with N2 & O2 ?
• N2 & O2 are highly symmetric w/ short strong bonds
• They absorb in UV & don’t affect IR heating
Clouds Have Variable Effects on IR
• Clouds & lower concentration aerosols block heat
• Different types: Stratus, Cumulus, Cirrus
• Can raise albedo blocking Sun or hold heat in
Low Level Stratus are Water Droplets
High Level Cirrus are Ice Crystals
Tall Cumulonimbus have all 3 Phases
• Vertical Convection, Thunderstorms
• Water-Ice (sleet/hail)-Steam
Radiation Flux versus Cloud Type
• Cirrus are high thin, pass more light, lower IR flux
• Stratus-Cumulus: low dense, reflect more, high IR
General Circulation Model  Climate
• OK, so quantify this, match it to the Earth System
• Now build a Computer model-change it-see an
effect, conclude, change something else, map it out
Global Energy Balance
At the top of the Atmosphere:
• 100 Solar in = 25 Air refl + 5 Earth refl + 70 IR out
Near the Ground
• 100 Solar in = 45 Earth abs + 55 Air refl + abs
• 53%, 45 Solar in = Water evap
• 133 Earth in = 45 Solar in + 88 GHG IR
The Multiple IR paths increase flux to surface & heat
Radiative Convective 1D Model 
Climate (RCM’s)
• Ignore lateral variations of clouds, oceans, land
• Put in Atmospheric layers with average values (no
poles or tropics)
• Just deal with Radiation in and out & Convection
• Easier to compute but how relevant is it to the real
Earth System?
• You should still get the major effects of increased
GHG (compared to our 1 layer Atmosphere model)
• How far can you trust the predictions, feedbacks?
Radiative Convective 1D Model 
Climate (RCM’s)
• RCM’s correctly predict a GHG warming +33°C
• for ΔTg = Ts + Te where g = GHG, s = surface and e
= atmospheric layer.
• This match is not so trivial!
• RCM’s predict GHG effects for doubling GHG’s
• Like CO2 from 300 ppm to 600 ppm ΔTg = 1.2°C
• This doesn’t sound like so much but ignores:
– Lateral variations, deserts, polar regions get most change
– Ignores feedback or interaction effects
Water Vapour Feedback: Hothouse 1D
RCM’s
• Positive feedback loop in the short term esp. heating
• H2O (g) is close to rain or ice/snow, condensation –
• More Heat more steam, less heat way less steam
Radiative Convective 1D Model 
Climate (RCM’s) & Relative Humidity
• Relative Humidity is %H2O/Saturation% f(T°C)\
– Steam Rooms & tropics hold way more H2O and heat
• for ΔTeq = T0 + Tf where eq = equilibrium, 0 = equil
w/no feedback and f = feedback offset Daisyworld
• The feedback for more CO2 and more H20 is double!
• ΔTg = 2.4°C so f = (2.4°/1.2°) = 2
• This doesn’t sound like so much but is a really
strong positive feedback.
• In Earth History Cretaceous & Devonian Hothouse
Earth Times
Water Vapour Feedback: Ice Ages need
2-3D models, Regional variations
• Positive feedback loop in long term, ice age effects
• H2O (g) is close to rain or ice/snow, condensation –
• Less heat way less steam, way more ice ages!
Earth’s Climate Tends to be Stable
despite changes and oscillations
• Negative feedback due to outgoing IR’s strong
dependence on surface temperature: short time scale
• Tropospheric heating from below
• Runaway GHG like Venus can break this stablity
Uncertain effects of Cloud Types
• Cirrus causes net Warming!
• Low Stratus-Cumulus can cause net coolingThe real
uncertainty here is does increased Albedo outweigh
GHG IR effects or not, Aerosols are knotty buggers!
Uncertainties in Climate Models
• How good is a climate modeller’s prediction?
• How meaningful is average temperature to how you
dress yourself hour to hour or day to day?