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…

The Earth then radiates heat away…

Light from the Sun

Earth

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 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

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

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

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

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)

-18 °C

Average Surface

Temperature

33 °CGreenhouse

EffectTemperature for radiation

balance

+15 °C

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 %

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

2. Illustration of atmospheric compositionMore detail (6)

About 100 molecules

O2

Ar

N2

2. Illustration of atmospheric compositionMore 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?

-18 °C

Average Value 240 watts per square

metre

Average Value240 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 bandsMore detail (13)

© Australian Bureau of Meteorology

Wavelength

WaterAbsorption

CO2

Absorption

Infra Red

from EarthUV

from the Sun

2. Absorption bandsIllumination by the Sun (14)

© Australian Bureau of MeteorologyWavelength

WaterAbsorption

CO2

Absorption

Infra RedUV

from the Sun

2. The transmittance of water vapour

VisibleInfra Red

2. The spectrum of sunlight

Infra RedUV

2. Absorption bandsRadiation from the Earth (15)

© Australian Bureau of MeteorologyWavelength

WaterAbsorption

CO2

Absorption

Infra RedUV

from Earth(drawn on a different scale)

2. The AtmosphereMore detail (16)

• Out of balance…

Average < 240 W/m2

Average 240 W/m2

Somesent back to

Earth

Average240 W/m2

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

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

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