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 reaches the Earth? Light from the Sun heats the Earth… Earth At the top of the atmosphere Above the Equator At midday About 1360 W/m 2

1. What determines the surface temperature of the Earth? How much light reaches the Earth? Light from the Sun heats the Earth… Earth North Pole South Pole Equator 1360 W/m Whole Earth Average 340 W/m 2

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/m 2 ? 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 °C Greenhouse Effect Temperature 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 (N 2 )78.1 Oxygen (O 2 )20.1 Argon (Ar)0.93 Water (H 2 O)0.1 to 1 Carbon Dioxide (CO 2 )0.035

2. Illustration of atmospheric composition More detail (6) About 100 molecules O2O2 Ar N2N2

2. Illustration of atmospheric composition More detail (7) About 1200 molecules Water(H 2 O)

2. Illustration of atmospheric composition More detail (8) About molecules Carbon dioxide (CO 2 )

2. The Atmosphere More detail (9) Nitrogen (N 2 ), Oxygen (O 2 ) & 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 Value 240 watts per square metre

2. The Atmosphere More detail (10) Greenhouse warming is caused by Water (H 2 O) Carbon Dioxide (CO 2 )

2. The Atmosphere More detail (11) What is special about H 2 O and CO 2 ? 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) © Australian Bureau of Meteorology Wavelength Water Absorption CO 2 Absorption Infra Red from Earth UV from the Sun

2. Absorption bands Illumination by the Sun (14) © Australian Bureau of Meteorology Wavelength Water Absorption CO 2 Absorption Infra RedUV from the Sun

2. The transmittance of water vapour Visible Infra Red

2. The spectrum of sunlight Infra RedUV

2. Absorption bands Radiation from the Earth (15) © Australian Bureau of Meteorology Wavelength Water Absorption CO 2 Absorption Infra RedUV from Earth (drawn on a different scale)

2. The Atmosphere More detail (16) Out of balance… Average < 240 W/m 2 Average 240 W/m 2 Some sent back to Earth Average 240 W/m 2

2. What determines the surface temperature of the Earth? More detail (16) In balance at + 15 °C… On Average 240 W/m 2 On Average 390 W/m W/m 2 sent back to Earth Average Value 240 W/m 2

2. What determines the surface temperature of the Earth? More detail (4) Greenhouse Warming 33 °C Others <1 °C CO 2 2 °C H 2 O 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