Global Warming
What determines the
surface temperature of the Earth?
1. What determines the surface
temperature of the Earth?
The Simple Story
1. What determines the surface
temperature of the Earth?
Simple Story (1)
Light from the Sun heats the Earth…
The Sun
Earth
6000 °C
1.What determines the surface
temperature of the Earth?
Simple Story (2)
Drawn to Scale…
The Sun
Earth
(drawn 100 times
too big)
1. What determines the surface
temperature of the Earth?
Simple Story (3)
Light from the Sun heats the Earth…
Light from
the Sun
Earth
The Earth then radiates heat away…
1. What determines the surface
temperature of the Earth?
Simple Story (4)
heat input from
the Sun
heat lost from
the Earth by
radiation
When there is balance, the Earth’s average
temperature will be stable
1. What determines the surface
temperature of the Earth?
1.2 Some more details…
1. What determines the surface
temperature of the Earth?
More detail (1)
For a detailed answer we need to know:
• How much light reaches the Earth?
• How much light reaches the Earth’s Surface?
• How much energy is radiated from the Earth’s Surface?
• These are difficult questions to answer precisely.
1. What determines the surface
temperature of the Earth?
How much light reaches
the Earth?
Light from the Sun heats the Earth…
•At the top of the atmosphere
•Above the Equator
•At midday
•About 1360 W/m2
Earth
1. What determines the surface
temperature of the Earth?
How much light reaches
the Earth?
Light from the Sun heats the Earth…
Earth
680
North Pole
1360 W/m2
Equator
Whole Earth Average 340 W/m2
South Pole
1. What determines the surface
temperature of the Earth?
More detail (1)
Light reflected from Earth
Average Value
100 watts per square metre
Light from the Sun
Average Value
340 watts
per square metre
1. What determines the surface
temperature of the Earth?
More detail (2)
Light from the Sun heats the Earth…
Average Value
240 watts
per square metre
Average Value
240 watts per square
metre
1. What determines the surface
temperature of the Earth?
More detail (2)
How hot must the Earth be to radiate 240 W/m2 ?
Average Value
240 watts per square
metre
-18 °C
A Question
What is the average temperature of the Earth?
1 metre above the surface and averaged over:
• day and night,
• all latitudes and longitudes.
• all seasons
(a) – 15 ° C
(b) – 5 ° C
(c) + 5 ° C
(d) + 15 °C
A Question
What is the average temperature of the Earth?
1 metre above the surface and averaged over:
• day and night,
• all latitudes and longitudes.
• all seasons
(a) – 15 ° C
(b) – 5 ° C
(c) + 5 ° C
(d) + 15 °C
1. What determines the surface
temperature of the Earth?
More detail (4)
Average
Surface
Temperature
+15 °C
33 °C
Temperature
for radiation
balance
-18 °C
Greenhouse
Effect
1. What determines the surface
temperature of the Earth?
The Earth’s surface is 33 °C warmer
than it would be if had no atmosphere
So how does the atmosphere
warm the Earth’s surface?
2. The Atmosphere
2. Earth’s Atmosphere
the Greenhouse Effect
2. The Atmosphere
More detail (5)
What is the composition of the atmosphere?
Gas
%
60
Nitrogen (N2)
78.1
50
Oxygen (O2)
Nitrogen
20.1
80
70
40
30
20
Argon (Ar)
Water (H2O)
Oxygen
Argon
0.93
Water
Carbon
Dioxide
0.1 to
1
10
0
Carbon Dioxide (CO2)
Fraction
0.035
2. Illustration of atmospheric
composition
More detail (6)
O2
N2
About 100 molecules
Ar
2. Illustration of atmospheric
composition
More detail (7)
About 1200 molecules
Water(H2O)
2. Illustration of atmospheric
composition
More detail (8)
About 10000 molecules
Carbon dioxide (CO2)
2. The Atmosphere
More detail (9)
Nitrogen (N2), Oxygen (O2) & Argon (Ar)
• > 99% of the atmosphere
• Molecules have one or two atoms
• Opaque to ultra-violet light
• Transparent to infra-red and visible radiation
2. The Atmosphere
More detail (10)
With an atmosphere of Nitrogen
Oxygen & Argon, what would be
the surface temperature?
Average Value
240 watts
per square metre
-18 °C
Average Value
240 watts per square
metre
2. The Atmosphere
More detail (10)
Greenhouse warming is caused by
Water (H2O)
Carbon Dioxide (CO2)
2. The Atmosphere
More detail (11)
What is special about H2O and CO2?
• Their molecules have three atoms
• Their natural frequencies of vibration are in the
infra-red
• They make the atmosphere opaque to certain
infra-red frequencies
2. The Atmosphere
More detail (12)
• Absorbtion
frequencies
determined by exact
frequencies of
molecular jiggling
2. The Atmosphere and the
Greenhouse Effect
The Transparency of the Atmosphere
Warning!
Complicated diagram
!
2. Absorption bands
More detail (13)
from the Sun
UV
from Earth
Infra Red
Water
Absorption
CO2
Absorption
Wavelength
© Australian Bureau of
Meteorology
2. Absorption bands
Illumination by the Sun (14)
from the Sun
UV
Infra Red
Water
Absorption
CO2
Absorption
Wavelength
© Australian Bureau of Meteorology
2. The transmittance of water
vapour
Infra Red
Visible
2. The spectrum of sunlight
UV
Infra Red
2. Absorption bands
Radiation from the Earth (15)
from Earth
UV
Infra Red
(drawn on a different scale)
Water
Absorption
CO2
Absorption
Wavelength
© Australian Bureau of Meteorology
2. The Atmosphere
More detail (16)
• Out of balance…
Average
240 W/m2
Average
< 240 W/m2
Some
sent back to
Earth
Average
240 W/m2
2. What determines the surface
temperature of the Earth?
More detail (16)
• In balance at + 15 °C…
Average Value
240 W/m2
On Average
240 W/m2
150 W/m2
sent back to
Earth
On Average
390 W/m2
2. What determines the surface
temperature of the Earth?
More detail (4)
Others <1 °C
CO2 2 °C
Greenhouse
Warming
33 °C
H2O 31 °C
• Nothing so far is the subject of any dispute
– Although not all the numbers are exact
2. What’s the problem?
Greenhouse effect
• Caused by water and carbon dioxide
• Turns the Earth from a ‘snowball’ with a hot
Equator into the relatively temperate place