1. What controls climate? Energy from the Sun – Radiation
Consider the 4 inner planets of the solar system:
SUN 1
Relative
Distance from Sun
0.39
0.72
1.5
Mercury
Venus
Earth
Mars
Receives
342 W m-2 solar radiation
Scales with 1
distance2
2250 W m-2
660 W m-2
150 W m-2

2. Planetary Albedo
A fraction of the incoming solar radiation (S) is reflected back into space, the rest is absorbed by the planet. Each planet has a different reflectivity, or albedo (α):
Earth α = 0.31 (31% reflected, 69% absorbed)
Mars α = 0.15
Venus α = 0.59
Mercury α = 0.1
Net incoming solar radiation = S(1 - α)
One possible way of changing Earth’s climate is by changing its albedo.

3. Land has higher albedo than ocean
Clouds have high albedo
Ice and snow have high albedo

4. Christmas fires in Sydney 2001/2002
Smoke aerosol more reflective than ocean

5. Radiative Equilibrium
Each planet must balance net incoming solar radiation with outgoing radiation, determined by its temperature.
Stefan-Boltzmann Law:
“A body at temperature T radiates energy at a rate proportional to T4 ” (T in Kelvin)
Balance incoming and outgoing radiation: Net incoming radiation=Outgoing radiation
S(1-α) = σ T4
(σ is the Stefan-Boltzmann constant = 5.67 x 10-8 W m-2 K-4)

6. The ‘Greenhouse Effect’
Radiative equilibrium works for Mercury (no atmosphere) and just about for Mars (thin atmosphere)
The disagreement for Venus and the Earth is because these two planets have atmospheres containing certain gases which modify their surface temperatures.
This is the ‘Greenhouse Effect’ in action:
Earth’s surface is 34°C warmer than if there were no atmosphere
Venus has a ‘runaway’ Greenhouse effect, and is over 400°C warmer
Mars atmosphere slightly warms its surface, by about 10°C
The existence of the Greenhouse Effect is universally accepted (it is not controversial), and it links the composition of a planet’s atmosphere to its surface temperature.

7. Earth’s Climate System
About 31%
reflected
into space
69% absorbed at surface
Solar
radiation
Sun
Terrestrial
radiation
Atmosphere
Land
Ocean
Ice
Sub-surface Earth

8. Earth’s Energy Balance

9. Enhanced greenhouse effect
Terrestrial
radiation
To get same amount of net radiation, need higher surface temperatures
More greenhouse gases, more radiation absorbed
Extract and burn fossil fuels
add CO2 to atmosphere

10. Composition of the Atmosphere
Nitrogen N %
Oxygen O %
Argon Ar 0.934%
Carbon Dioxide CO % (360 ppmv)
Methane CH4 1.7 ppmv
Hydrogen H ppmv
Nitrous Oxide N2O 0.31 ppmv
Ozone O ppbv (troposphere) ppmv (stratosphere)
Water H2O 100 pptv – 4%
Greenhouse
Gases
A greenhouse gas is one that absorbs terrestrial (LW) radiation, i.e. emitted from the Earth’s surface/atmosphere