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. 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

9. The Enhanced Greenhouse Effect
Solar (S) and longwave (L) radiation in Wm-2 at the top of the atmosphere
S L
S L
S L
S L
T = -18°C
CO2 x 2
+ Feedbacks
H2O (+60%) Ice/Albedo (+20%)
Cloud?
Ocean?
CO2 x 2
CO2 x 2
TS = 15°C
TS = 15°C
DTS ~ 1.2K
DTS ~ 2.5K

10. Summary 2 (Greenhouse Effect…)
Radiation from the Sun drives our climate
Our distance from the Sun, and the reflectivity of the Earth determines how much radiation is absorbed
Earth’s atmosphere traps outgoing radiation (the Greenhouse Effect), warming the surface by about 34°C
On Venus, a runaway Greenhouse Effect warms its surface by over 400°C; Mars thin atmosphere warms its surface by about 10°C
So there is good evidence from the other planets that the atmospheric composition is important in determining the surface temperature
Global Warming is often called ‘The Greenhouse Effect’ – really it is the Enhanced Greenhouse Effect – the addition of more Greenhouse Gases (mainly from burning fossil fuels) to the atmosphere enhances the existing effect.
Humans have also changed the Earth’s albedo – mainly by adding aerosols to the atmosphere – these tend to cool climate, offsetting the GHG warming