The Radiation Environment and the Greenhouse Effect

1
The Radiation Environment and the Greenhouse
Effect

• The sun's energy makes life possible.

• The CO2 and H2O in the atmosphere create a
  natural greenhouse effect. This increases keeps
  the Earths surface by about 33K.

• The atmospheric ozone layer provides a filter
  removing harmful Ultra-Violet radiation that can
  kill living cells.

An understanding of these effects requires an
understanding of what is called the radiation
environment.
2
The Suns spectrum
Hot dense objects emit electro-magnetic radiation
with a continuous spectrum. This approximates to
a universal form called the black-body spectrum
which depends only on the object's temperature.
A black body is an idealised absorber that
absorbs all the radiation falling upon it.
The sun has a black-body spectrum.
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The Black Body Spectrum
The emitted intensity is c -speed of
light. Alternatively
The peak position, obtained from setting dI/d?
0, is giving by which agrees with the long
experimentally established Wiens Law. By
integration we obtain the Stefan-Boltzmann Law
for the total intensity. where s 5.667 ? 10-8
W m-2 K-4 is Stefans constant.
4
Measured Solar Spectrum

• The Suns' spectrum, measured in space, is
  reasonably close to that of a black body with a
  temperature of 5777K.

The peak in the spectrum is close to 0.5µm, which
is the blue/green boundary in the visible.
It is not a coincidence that this peak coincides
with the visible spectrum eyes sensitive to
other parts of the electro-magnetic spectrum
would be of little use.
5
The Solar constant
The total intensity of the solar radiation at the
distance of the earth is termed the Solar
constant, S, or Solar irradiance.
It is defined as the solar power per square meter
incident on a surface normal to the Suns' rays at
the Earth-Sun distance in the absence of an
atmosphere.
The solar constant can be measured by satellite.
The average measured value is 1,367 Wm-2.
6
Measurements of The Solar Constant
7
Calculating the Solar Constant

• We can calculate S for a Black-Body Sun of
  temperature Ts as follows. The total power
  emitted from the sun in Watts is

(power emitted per m2)?(surface area of the sun)
? sTs4?4pRs2   where Rs is the Suns'
radius.
To find the power per unit area at the Earth-Sun
distance, dE-S, we must then divide this by
4pdE-S2, which is the area of the sphere over
which this energy is distributed. Thus
Rs 6.960 ? 108m and dE-S 1.496 ? 1011m so for
Ts 5.777K we obtain
S 1, 367 Wm-2.
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Solar intensity at the Earths Surface
The area presented to the sun by the earth is
pRE2, where RE is its radius. The total surface
area of the Earth is 4pRE2. The Earth plus
its' atmosphere has an average reflection
coefficient (also called the Albedo). The current
value is r 0.30. So the average Solar energy
absorbed per unit area of the earths' surface is
So the average solar energy per unit area of the
earths surface in the absence of an atmosphere
would be a quarter of the solar constant.
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Radiation BalanceThe equilibrium temperature
without an atmosphere

• The Earth and its' atmosphere absorb an
  average solar power of 239W per m2 of the Earths
  surface. At equilibrium the Earths surface and
  atmosphere must radiate the same power back into
  space.
• Treating the Earth as a black body sTE4 ? 239
  Wm-2
• This gives an equilibrium temperature TE 255K
  or -18oC.
• Without an atmosphere the surface temperature
  would have this value.
• Measured average temperature at the earth's
  surface 288K or 15oC.
• The Natural Greenhouse increases the surface
  by 33oC and makes life on Earth possible.

10
Mars and Venus

• Mars Atmospheric pressure 0.7 of that on
  Earth, 80 CO2. Greenhouse effect raises surface
  temperature by 24K above the equilibrium value of
  216K.
• Venus Atmospheric pressures 90 times that on
  Earth, 90 CO2. Greenhouse effect raises surface
  temperature from the equilibrium value of 227K to
  750K.
• Venus has suffered a runaway greenhouse effect.
  Was once much cooler with seas and oceans as
  Solar constant 30 smaller in early Solar
  system.
• Increasing Solar power increased the surface
  temperature and the evaporation of water. Water
  vapour in the atmosphere created a strong
  greenhouse effect. Eventually all water was
  removed from surface. Dissociation of water
  molecules by UV radiation then led to CO2
  formation.

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Structure of atmosphere
12
Atmosphere pressure, temperature, composition.
About 80 of the mass of atmosphere is in the
region of the atmosphere called the troposphere
that extends up to about 11km from the earths
surface.

• Atmospheric composition by weight,
• 78 N2 , 21 O , 0.9 Ar, 0.035 CO2
• Plus a very variable amount of water vapour.

13
Interaction of Solar radiation with atmosphere
Incoming solar radiation, has a peak intensity
close to 0.5 ?m. It interacts quite strongly with
gas molecules, water droplet and dust in the
atmosphere. On average only 25 of the incoming
solar energy reaches the Earths surface
directly. 25 is absorbed by the atmosphere and
about 25 reflected. About 5 is reflected from
the surface.
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Absorption of terrestrial radiation by atmosphere
The outgoing terrestrial radiation has an,
approximately, black body spectrum, with a peak
intensity close to 10µm. Absorption by molecules
such as CO2 and H2O is extremely strong at such
wavelengths. About 95 of the energy radiated
from the Earths surface is absorbed by the
atmosphere. This gives rise to the greenhouse
effect in which energy is trapped in the
atmosphere.
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Estimating the Magnitude of the Natural
Greenhouse Effect
An estimate of greenhouse warming can be obtained
by considering the atmosphere as a slab of
absorbing material. Consider the earth plus
atmosphere to have a reflection coefficient of
30 so about 240Wm-2 is initially absorbed by the
earth's surface. Let the atmosphere have zero
absorption for solar radiation and 100
absorption for terrestrial radiation.
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The 240 Wm-2 absorbed by the surface is
re-emitted as Infra-Red radiation. Assume that
this is totally absorbed by the atmosphere. The
atmosphere re-emits this energy in all
directions.

• 120Wm-2 is radiated downwards and 120Wm-2 is
  radiated upwards and lost to space.
• The additional energy 120Wm-2 will then be
  absorbed by the surface, and on re-emission the
  process continues.

The net result is that the total power downwards
is given by the sum of the geometrical series,
17
Result for the equilibrium temperature in the
simplest model

• In this model the surface receives twice the
  energy it would have without the atmosphere.
• At equilibrium the surface must therefore radiate
  480Wm-2.
• sTs4 480Wm-2 for a black-body earth.
• This gives a surface temperature is 303K.
• c.f. measured value of 288K.

18
Improving the simple model of greenhouse warming
A slightly better treatment (see notes) gives an
equilibrium temperature of 285K. c.f. measured
value 288K. This model is very crude as it does
not include the atmospheres vertical structure,
clouds etc.
19
Absorption by molecules

• An electromagnetic wave, frequency n and
  wavelength l consists of photons of energy E hn
  hc/l (c - speed of light)

 
Wavelength
Energy
 
 
20
Absorption UV by the ozone layer

• The highest energy photons are absorbed in the
  upper atmosphere (mostly in the stratosphere).
• Oxygen molecules (O2) have a binding energy of
  5.1eV and are strong absorbers for hv gt 5.1 eV,
  i.e. ? lt 0.24 µm
• The free oxygen atoms produced react with other
  O2 molecules to produce ozone O O2 ? O3
• O3 molecules have a binding energy of 4.3 eV and
  strongly absorbs UV with ? lt 0.29 µm.
• O3 hv ? O2 O ? O3 K.E.

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Absorption in the visible

• Electrons in atoms occupy discrete energy
  levels.
• Photons of the correct energy can promote an
  electron from a lower energy level to a higher
  level.
• This produces weak absorption in narrow ranges
  of wavelength.

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Absorption in the infra-red

• Molecules can be modelled as masses (the atoms)
  connected by springs (the inter-atomic forces).
  The natural vibrational frequency is about 3 ?
  1013 Hz.
• Light of frequency v0 can cause such a molecule
  to vibrate and absorbed energy for ? c/v0 ? 10
  µm.
• Complex molecules have more vibrational modes,
  and more polar molecules are stronger absorbers.

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Greenhousegases

• The greenhouse effect is mainly due to CO2 and
  H2O but CH4, NO2 and CFCs also contribute.
•  
• The relative importance of the same quantity of
  different gases released into the atmosphere is
  dependent on
• (a) The strength and number of absorption bands
• (b) The position of the bands relative to the
  terrestrial spectrum
• (c) The overlap of absorption bands with existing
  absorptions
• (d) The lifetime of the gas in the atmosphere.

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Atmospheric absorption
25
Radiative forcing

• The radiative forcing is the increase in the
  total downward flux of infra red, emitted by the
  atmosphere, due to the additional amount of gas
  to the atmosphere.

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Gases and radiative forcing
27
Global warming potentials

• The relative contribution to global warming of a
  gas may be expressed as the Global Warming
  Potential (GWP)
• ?F additional radiative forcing due to adding of
  1 kg of a gas to the atmosphere. c(t) is the
  fraction of the gas remaining in the atmosphere
  after time t. T is the integration period. ?FCO2
  and c(t)CO2 are the same quantities for CO2.

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The Enhanced Greenhouse Effect

• Equate the total energy incident m-2 of the
  Earth's surface to sT4 to obtain the surface
  temperature.

YEAR
1800
2000
2030
2100
?F(Wm-2)
0
2.5
5
10
Incident intensity (Wm-2)
397.5
400
402.5
407.5
TS (K)
288
288.451
288.9
289.8
?TS (K)
-
0.45
0.90
1.8
?TS,WATER (K)
-
0.73
1.44
2.9
?F change in radiative forcing. Ts Equilibrium
surface temperature. ?Ts increase in surface
temperature. ?Ts,water includes effects of
increased water vapour levels. ?Ts,IPCC the
prediction of the Intergovernmental Panel on
Climate Change (2001).
29
Predictions of climate models, IPCC 2001
30
Scattering by molecules

• Consider light scattered by a particle in the
  atmosphere.
• When the wavelength of the light, ?, is large
  compared with the diameter of the scattering
  particle, xo, the scattered Intensity
  proportional to (x0/ ?)4. This is the Rayleigh
  scattering formula.
• The molecules of the atmosphere, such as O2 and
  N2, have sizes of order 5?10-10m which is very
  much smaller than the wavelengths of solar
  radiation ( 5 x 10-7) so this equation is
  applicable.

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Blue Sky and red sunsets

• Scattered intensity proportional to (x0/ ?)4
• The ratio of the scattered intensity of blue
  light (? 0.4?m) to red light (? 0.7?m) would
  therefore be ( 7/4 )4 which is about 10.
• The sky usually looks blue because the scattered
  sunlight we are seeing when we look at the sky
  is mostly from the blue end of the spectrum.
• When we view a sunset we are seeing the light
  that has passed through a thick layer of
  atmosphere without scattering which is
  predominantly from the red end of the spectrum.

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Scattering by aerosols
In the atmosphere the important particles are
water droplets, dust and H2SO4 crystals . Typical
size is 0.1?10 microns ? of visible
light.   Cross Sections A particle of radius r0
gtgtl will simply block out an area pr02 (
geometrical area). For ? gtgt r0 the wave can 'flow
around' the particle. In this case one has an
effective 'cross-section' s which is much smaller
than pr02
Horizontal axis particle radius over wavelength
of light. Vertical axis scattering cross section
divided by the geometrical area ?/?r2
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Once in a Blue Moon

• If one had a high concentration of particles, all
  of a size just larger than 0.5µm, the red end of
  the spectrum would be scattered more strongly
  than the blue because s a (xo/?)n with nlt0.
• The last major occurrence of this type in Britain
  was in 1951 when smoke particles of a uniform
  size drifted over from huge forest fires in
  Canada.
• The sky had a pinkish tinge by day and the Moon
  appeared blue.
• Such events are very rare, hence the expression.

34
Global Cooling

• Aerosol particles injected into the atmosphere
  gradually precipitate out. Particles in the range
  0.1µm to 1µm have half-lives of about one month
  in the upper troposphere and 1 year in the lower
  stratosphere. Smaller and larger particles have a
  significantly shorter half-life.
• A typical long life particle, diameter 0.5µm,
  will scatter incoming solar radiation very
  strongly as the peak in the Solar spectrum is at
  ? 0.5µm.
• At the terrestrial radiation spectrum peak, 10µm,
  the scattered intensity will be reduced by a
  factor of order ( 10/0.5 )4 (Rayleigh
  Scattering).
• Aerosols very effectively scatter incoming solar
  radiation but not outgoing terrestrial radiation
  a reverse greenhouse effect.

35
Volcanic Eruptions

• Large Volcanic eruptions put huge amounts of
  material into the stratosphere. The aerosol level
  was about sixty times normal six months after the
  1990 Pinatubo eruption.
• Climate models predict that such eruptions should
  cause global temperature drops of 0.5oC for a
  periods of about three years. Such short-term
  temperature dips are evident in the temperature
  records.

36
Extreme cooling Nuclear Winter

• A Nuclear war could result in large amounts of
  dust and smoke in the stratosphere due to
  explosions and the resulting fires.
• In 1984 Carl Sagan and co-workers predicted
  temperature drops of between 20o and 40o degrees
  lasting many months for a plausible 5,000
  megaton exchange. They coined the term nuclear
  winter to describe this global disaster.
• Better climate models now suggest a shorted
  period with a smaller temperature drop. The
  predictions depend crucially on the assumptions
  about the war.

37
Death of the dinosaurs
About 65 million years ago the Cretaceous period
ended when about 70 of all species on earth
became extinct. These included the dinosaurs
leaving mammals to thrive.

• This happened in less that 10,000 yrs, possible
  much quicker, but it is very difficult to measure
  shorter time periods.
• One theory is that a very large asteroid hit the
  earth throwing a huge mass of material into the
  atmosphere. The mass extinction then resulted
  from the ensuing darkness and large temperature
  drop.

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Meteor impact?

• Theory supported by the presence of a rock layer,
  deposited around this time, rich in Iridium,
  which is 10,000 times more abundant in asteroids
  than in the earths crust.
• The Iridium co-exists with fused and shocked
  quartz crystals consistent with such an impact.

Evidence is also emerging that suggests that
other mass extinction may be related to meteor
impact.
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Dinosaur extinction