8.5
Greenhouse effect
Solar radiation
Sun Intensity (W m–2) on a planet
the total power of the sun divided by the surface area of a sphere with a radius
equal to the distance of the planet from the sun.
Albedo of a surface
the fraction of the light that reaching the surface that is reflected
o the ratio of the amount the light that reflects from it to by the amount reaching the surface.
Factors affecting a planets albedo include:
o cloud cover and the angle of approach of the light
o the colour, the smoothness of the surface, moisture content and chemical composition.
The Earth’s albedo varies daily and depends on season (cloud formation) and latitude.
 Oceans have a low value (0.2) but snow has a high value (0.4).
 The Earth’s global annual mean albedo is 0.3.
The Greenhouse effect
Greenhouse effect
the increase in temperature of an enclosed space due to shorter wavelength heat entering and less
long wavelength heat leaving.
Greenhouse gases
methane CH4, water H2O, carbon dioxide CO2 and nitrogen (I) oxide N2O.
Each Greenhouse gas has natural and human origins.
The closer the natural frequency of an oscillation within a molecule to the frequency of an electromagnetic wave,
the greater the absorption and the re-emission of another wave with the same frequency by the molecule.
The natural frequency of oscillation of the molecules of Greenhouse gases is in the infrared region of the spectrum.
Thermal Equilibrium no change in temperature – no net transfer of heat between objects
the amount of energy lost is equal to the amount of energy received
 The Earth receives radiant energy from the sun
 Energy is lost from earth by reflection and radiation.
 the amount of reflection depends on the albedo value
 the amount of radiation depends on the absolute temperature (Stefan-Boltzmann Law)
Global warming occurs when less energy is lost by reflection than is received.
Black Body
something that absorbs all the radiation aimed at it.
An enclosed cavity with a small hole is a good approximation to a Back Body.
If a Black Body absorbs all the radiation aimed at it, for it to maintain a temperature equal to its
surroundings, it must also be a perfect emitter of radiation.
Black body radiation is the radiation emitted by a perfect emitter.
At any temperature the radiation emitted has a range of frequencies.
Wein’s Law
The higher the temperature of a Black Body, the higher is the average frequency (lower
wavelength, λ) of the radiation it emits.
T
Stefan-Boltmann Law
max
The amount of energy radiated away from a Black Body each second equals the StefanBoltzmann constant, σ times the absolute temperature, in Kelvin to the fourth power, T4.
P  AT 4
Emissivity
2.9 10 3 mK
Stefan-Boltmann constant, σ = 5.67040×10–8 W m–2 K–4
measures how well surface emits radiation relative to a Black Body.
The emissivity of a Black body is 1 and all other objects between 0 and 1.
Surface heat capacity the energy required to raise the temperature of 1 m2 of the planets surface by 1 K
measured in J m-2 K-1.
The change of a planet’s temperature ΔT over a period of time equals the incoming radiation intensity Iin less the
outgoing radiation intensity Iout times time Δt divided by surface heat capacity, Cs.
T 
I in  I out t
C surface
This calculation is based on a simple model that assumes the Earth has a uniform surface that does not change
daily or seasonally. The model could be improved by allowing for the variations in the surface, variation in cloud
cover and seasonal changes.
Global Warming


The change of a planet’s temperature, ΔT over a period of time Δt equals the ratio of the product
of the net radiation intensity (incoming radiation intensity minus outgoing radiation intensity) and
the time interval to the surface heat capacity.
simple model assumes the Earth has a uniform surface that does not change daily or seasonally.
model could be improved by allowing for the variations in the surface, variation in cloud cover
and seasonal changes.