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KR # 6.1 opens with this observation:
In spite of the fact that the core of our Earth is over 4,000oC, Earth’s surface temperature
does not derive from the pressure and radioactive decay that heat that core.
Geophysicists estimate that the Earth’s surface would be almost ‘absolute’ zero (0oK or 273oC) without the sun.
But the sun isn’t the whole story . . .
Our Sun’s energy output combined with Earth’s distance from the Sun would generate a
temperature of -18o C:
Yet, Earth’s average temperature is about + 15o C
What factors account for ‘extra’ 33oC (the so-called ‘natural’ greenhouse effect)?
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Our current understanding suggests that those ‘extra’ 33oC:
Derive from the interactions among:
1. Electromagnetic radiation (EMR)
2. Its various behaviours (absorption, reflection, re-radiation)
3. And the way those behaviours are influenced by the atmosphere, lithosphere and
hydrosphere
Ok, so we’ll start with EMR
(Electromagnetic Radiation)
EMR is a form of energy
From KR #4 + Chp. 3 Draper: matter, energy & thermodynamics
• Energy: ability to do work (move a mass through a distance)
• Matter: occupies space and has mass
• Thermodynamics: rules governing energy (mass) transformations (1st and 2nd laws)
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Every object warmer than absolute 0 (-273oC) emits EMR
The original source for the EMR we are exposed to on Earth is the fusion of hydrogen
atoms in the 15 million Co of the Sun.
This fusion releases alternating/interacting electric and magnetic fields – hence
electromagnetic radiation
• Often when we use the term “radiation” we think of “ionizing radiation” (energy that can
produce charged particles call “ions”), but “radiation” is simply energy that travels
through either material or space. Light, heat and sound are types of radiation.
EMR comes in a number of “forms” - not all of which are created equal!
• all forms of EMR move at the same speed
• all made up of streams of tiny particles (quanta)
• but each ‘form’ is characterized by a specific wavelength (λ) and frequency (f)
The “dual” nature of EMR
(wave and particle)
EMR travels in a wave conformation
Waves:
• have wavelengths (λ) e.g. the distance from peak to peak
• B has a shorter λ than A
• have a frequency (f) e.g. the number of waves that pass a given point in a second
• B has a higher frequency than A
• the shorter the λ, the higher the f
What controls the λ of any particular type of EMR?
The temperature of the emittor
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The surface of the sun (at a temperature of + 5000oC) emits 95% of its EMR as infrared,
visible and ultraviolet radiation
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The Earth (at an average temperature of 15o C) emits most of its EMR in the infrared
Numerous substances can absorb EMR
land masses
water (primarily the oceans)
various gases in the atmosphere (O2, H2O, CO2)
The physicochemical properties of each of these substances determines which λ are
absorbed.
This is particularly true of different gases in the atmosphere which means the surface of
the Earth is exposed to a substantially modified spectrum of EMR when compared to what
reached the top of the atmosphere from the sun.
One of the most important modifications of incoming EMR comes from creation of
ozone (O3)
(1) O2 + λUV-C= 2O* + heat
(2) O* + O2 = O3 (ozone)
(3) O3 + λUV-B = O2 + O* +heat
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(We sort ultraviolet light into three categories based on wavelength: UV-A=400-320 nm, UVB=320-280 nm, UV-C=280-200 nm)
It’s the absorption of UV-C and UV-B by O2 and O3 that releases the heat the
characterizes the increasing temperatures in the stratosphere!
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Highlights of incoming (shortwave) radiation:
1. 30% is reflected off clouds/gas molecules (even ice and snow) back into space
2. 19% is absorbed by gases/clouds in atmosphere (much of it in the stratosphere in the
formation of ozone). This process warms the stratosphere.
3. 51% is absorbed by oceans and land. This process warms the oceans and land, drives
substantial evaporation of water (and hence the hydrologic cycle) as well as maintaining
the world’s wind system.
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Highlights of outgoing
(longwave) radiation:
The air, land and water are heated by adsorption of incoming short wave radiation
These heated surfaces then re-radiate longer wavelength infrared EMR back into
atmosphere.
Some fraction of this outgoing radiation (~15%) is absorbed by CO2 and H2O in the
atmosphere leading to warming (the natural greenhouse effect).
As CO2 levels increase (along with other “greenhouse” gases), ever more outgoing radiation
will be trapped and the atmosphere will warm further (anthropogenic warming).
However incoming EMR (and heating) also drives evaporation of water and convective lift
of hot air
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A significant fraction of Earth’s extra 33o comes from latent & sensible heat (23 +
7% of outgoing, longwave radiation)
Water evaporated by absorption of incoming short-wave radiation, is carried up towards
the top of the troposphere (the tropopause) by convection.
As it cools, it condenses (rain), a process that releases heat (latent heat).
Some of this heat escapes into space and some is re-captured by CO2 and H2O which also
contributes to atmospheric warming. (Problematic source of frustrating variability!!)
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sensible heat
(or what goes up must come down!)
Trapped against the tropopause and propelled by air rising behind it, cool heavy air is
forced north and south and falls back to Earth.
The condensation of the moisture in the air means the air is also dry
This column of falling air compresses as it reaches the ground, a process that releases heat
(sensible heat)
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Variability = Uncertainty
Evaporation and precipitation are highly variable.
Consequently, there is substantial uncertainty about exactly how much latent heat is
moving into the atmosphere, how much escapes into space, how much is recaptured by
greenhouse gases and how these proportions may change as the climate warms.
+ or - feedback(?):
As the climate warms, there may be increased rates of evaporation and hence increased
inputs of latent heat into the atmosphere leading to more warming (positive feedback).
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Increased evaporation could generate more clouds, which could reflect more incoming EMR
leading to cooling (negative feedback)
Increased evaporation could lead to more snow, increased reflection of incoming EMR also
leading to cooling (negative feedback)
Reflectivity is called albedo
Sigh! Clouds are not simple
Reviewing: The concept map ☺
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