global warming what determines the surface temperature of the earth?
TRANSCRIPT
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Global Warming
What determines the surface temperature of the Earth?
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1. What determines the surface temperature of the Earth?
The Simple Story
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1. What determines the surface temperature of the Earth?Simple Story (1)
Light from the Sun heats the Earth…
The Sun
Earth
6000 °C
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1.What determines the surface temperature of the Earth?Simple Story (2)
Drawn to Scale…
The Sun Earth
(drawn 100 times too big)
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1. What determines the surface temperature of the Earth?Simple Story (3)
Light from the Sun heats the Earth…
The Earth then radiates heat away…
Light from the Sun
Earth
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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
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1. What determines the surface temperature of the Earth?
1.2 Some more details…
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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.
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1. What determines the surface temperature of the Earth?How much light reachesthe Earth?
Light from the Sun heats the Earth… Earth•At the top of the atmosphere•Above the Equator•At midday•About 1360 W/m2
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1. What determines the surface temperature of the Earth?How much light reachesthe Earth?
Light from the Sun heats the Earth…Earth
North Pole
South Pole
Equator
1360 W/m2
680
Whole Earth Average 340 W/m2
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1. What determines the surface temperature of the Earth?More detail (1)
Light reflected from EarthAverage Value
100 watts per square metre
Light from the SunAverage Value
340 watts per square metre
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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 Value240 watts
per square metre
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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
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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
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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
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1. What determines the surface temperature of the Earth?More detail (4)
-18 °C
Average Surface
Temperature
33 °CGreenhouse
EffectTemperature for radiation
balance
+15 °C
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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?
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2. The Atmosphere
2. Earth’s Atmosphere
the Greenhouse Effect
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2. The Atmosphere More detail (5)
What is the composition of the atmosphere?
Gas %
Nitrogen (N2) 78.1
Oxygen (O2) 20.1
Argon (Ar) 0.93
Water (H2O) 0.1 to 1
Carbon Dioxide (CO2) 0.0350
10
20
30
40
50
60
70
80
Fraction
NitrogenOxygenArgonWaterCarbon Dioxide
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2. Illustration of atmospheric compositionMore detail (6)
About 100 molecules
O2
Ar
N2
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2. Illustration of atmospheric compositionMore detail (7)
About 1200 molecules
Water(H2O)
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2. Illustration of atmospheric composition More detail (8)
About 10000 molecules
Carbon dioxide (CO2)
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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
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2. The Atmosphere More detail (10)
With an atmosphere of Nitrogen Oxygen & Argon, what would be
the surface temperature?
-18 °C
Average Value 240 watts per square
metre
Average Value240 watts
per square metre
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2. The Atmosphere More detail (10)
Greenhouse warming is caused by
Water (H2O)
Carbon Dioxide (CO2)
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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
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2. The Atmosphere More detail (12)
• Absorbtion frequencies determined by exact frequencies of molecular jiggling
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2. The Atmosphere and the Greenhouse Effect
The Transparency of the Atmosphere
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Warning!Complicated diagram
!
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2. Absorption bandsMore detail (13)
© Australian Bureau of Meteorology
Wavelength
WaterAbsorption
CO2
Absorption
Infra Red
from EarthUV
from the Sun
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2. Absorption bandsIllumination by the Sun (14)
© Australian Bureau of MeteorologyWavelength
WaterAbsorption
CO2
Absorption
Infra RedUV
from the Sun
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2. The transmittance of water vapour
VisibleInfra Red
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2. The spectrum of sunlight
Infra RedUV
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2. Absorption bandsRadiation from the Earth (15)
© Australian Bureau of MeteorologyWavelength
WaterAbsorption
CO2
Absorption
Infra RedUV
from Earth(drawn on a different scale)
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2. The AtmosphereMore detail (16)
• Out of balance…
Average < 240 W/m2
Average 240 W/m2
Somesent back to
Earth
Average240 W/m2
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2. What determines the surface temperature of the Earth?More detail (16)
• In balance at + 15 °C…
On Average 240 W/m2
On Average 390 W/m2
150 W/m2
sent back to Earth
Average Value240 W/m2
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2. What determines the surface temperature of the Earth?More detail (4)
Greenhouse Warming
33 °C
Others <1 °C
CO2 2 °C
H2O 31 °C
• Nothing so far is the subject of any dispute– Although not all the numbers are exact
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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