The Physics of our Climate

1. This presentation is designed for teachers to use in schools or with their local community. It contains reasonably ‘heavy’ science aimed at senior students or serious adults. A ‘lighter’ version is in the pipeline and will be put on vicphysics.org soon. In the meantime, for younger students some sections of this presentation could be omitted. • Other presentations in this series will include (titles may change!): • Is the climate changing? • Could the ‘climate sceptics’ be right? • What can we do about climate change? • Newer versions of this presentation and the others above can be found at: • www.vicphysics.org Follow the link from ‘teachers’ to ‘Climate Change’) • Be sure to look at the ‘Notes pages’ (below) for added comments to help in presenting and for more information and sources. Please feel free to email me with suggestions for improvements or useful comments. The Physics of our Climate Keith Burrows AIP Education Committee

2. The Physics of our Climate

3. Our place in space

4. Our place in space MARS: Atmosphere: Very thin Mean temperature: –65oC

5. Our place in space MARS: Atmosphere: Very thin CO2 Mean temperature: –65oC (but –140oC to +20oC ) No greenhouse effect

6. Our place in space VENUS: Atmosphere: Thick Mean temperature: +464oC

7. Our place in space VENUS: Atmosphere: Thick CO2! Mean temperature: +464oC Greenhouse effect gone wild!

8. Our place in space EARTH: Atmosphere: N2 , O2 , H2O and a little CO2 Mean temperature: +15oC Just right! Why?

9. Climate science • Earth’s energy balance • The average temperature of the Earth is determined by the balance between incoming solar radiation and outgoing ‘heat’ radiation

10. Climate science • ~ 1/3 reflected • ~ 2/3 absorbed then re-radiated as IR EMR. • 175,000 TW in • 175,000 TW out (But that’s if it is in equilibrium) IR EMR = Infrared Electromagnetic Radiation (just invisible ‘light’ really) TW = terawatt = 1012 watts = 1,000,000,000,000 watts

11. Climate science • Earth’s energy balance • The average temperature of the Earth is determined by the balance between incoming solar radiation and outgoing ‘heat’ radiation • Two simple laws of physics enable us to figure out the energy balance: • The Stefan-Boltzmann law... I = εσT4 • Wien’s law... λmax = 0.0029/T • S-B just tells us how much heat a hot object radiates. • Wien tells us what sort of radiation it will be. (but fortunately others have done the hard work for us!)

12. Climate science • Earth’s energy balance • Svante August Arrhenius worked it out in 1896

13. Climate science ? • Earth’s energy balance • Svante August Arrhenius worked it out in 1896 “The Earth’s average temperature should be about –18oC”

14. Climate science • Earth’s energy balance • Svante August Arrhenius worked it out in 1896 “Ah! The atmosphere must be trapping the heat”

15. Climate science ? • Earth’s energy balance • Svante August Arrhenius worked it out in 1896 “But Oxygen and Nitrogen can’t absorb the infrared radiation”

16. Climate science • Earth’s energy balance • Svante August Arrhenius worked it out in 1896 “It must be the water vapour and carbon dioxide!”

17. Climate science • Earth’s energy balance • Svante August Arrhenius worked it out in 1896 “Together they absorb heat and re-emit enough back to Earth to raise the temperature by 33 degrees!”

18. Climate science ? • Earth’s energy balance • Svante August Arrhenius worked it out in 1896 “So what will all the CO2 we are putting in the atmosphere do?”

19. Climate science • Earth’s energy balance • Svante August Arrhenius worked it out in 1896 “If we double the CO2 it could raise the temperature by about 5 degrees!” “That will make Sweden warmer – good !”

20. Climate science • Earth’s energy balance (sum up) • The average temperature of the Earth is determined by the balance between incoming solar radiation and outgoing ‘heat’ radiation • Not all the IR radiation from the surface escapes immediately... • or the average temperature would be a freezing –18ºC • No liquid water or clouds • And no life!

21. Climate science • Some of the IR from the surface is ... ? • ... trapped by the atmosphere.

22. Climate science • Some of the IR from the surface is trapped by the atmosphere – a little like a greenhouse... • The so called “Greenhouse Effect” • This keeps the Earth at a warm 15oC (average) instead of that freezing –18oC

23. Climate science • Earth’s energy balance IPCC FAQs 1.3 Fig 1

24. Climate science • The Greenhouse effect: • Natural ‘greenhouse gases’: • Water vapour • Carbon dioxide • Human produced: • Carbon dioxide • Methane etc. Human produced

25. Climate science • In order to understand the ‘greenhouse effect’ we need to know a little about ‘Electromagnetic Radiation’ (or EMR) • Here’s the whole spectrum: • This is the part we are interested in.

26. Climate science • Visible light is part of the EMR spectrum. • Its wavelength is a little less than a millionth of a metre.

27. Climate science • It turns out that ANY object emits some EMR – depending on its temperature: • Hot objects radiate infrared (which we feel as heat) and even hotter ones glow with visible EMR.

28. Colours of hot objects Kelvin is a temperature scale that starts from ‘absolute zero’ – the coldest possible temperature. 0 Kelvin is –273oC (So 0oC is 273 K) (273 has been rounded up to 300 in this chart – it’s only a guide) This is Wien’s law in action... λmax = 0.0029/T

29. Climate science • ALL objects at ANY temperature emit EMR • This polar bear is emitting just a little more than the ice!

30. Climate science • There is a simple law of physics about this: • Wien’s law: λpeak = 2900/T (λ in μm and T in K) • λpeak is the wavelength most emitted (there is a spread) • All it says is that the hotter the object (T) the shorter the wavelength (λ) of most of the radiation.

31. Climate science • Wien’s law: λpeak = 2900/T (λ in μm and T in K) • Example • At 300 K: λpeak = 2900/300 ≈ 9.7 μm (Long IR) • At 5800 K: λpeak = 2900/5800 ≈ 0.5 μm (Visible – yellow/white) (The Sun’s surface is at 5800 K)

32. Climate science • Wien’s law: λpeak = 2900/T (λ in μm and T in K) • Example • The hot metal (about 1500 K) will emit: λpeak = 2900/1500 ≈ 2 μm which is IR, but it will also emit quite a bit of visible (mostly red)

33. Climate science • Wien’s law also applies to stars • ‘Cool’ stars look red eg. Betelgeuse • ‘Hot’ stars look blue • eg. Sirius UV IR The Sun is 5800 K – UV Vis IR –

34. Climate science • Wien’s law also applies to stars • ‘Cool’ stars look red eg. Betelgeuse • ‘Hot’ stars look blue • eg. Bellatrix and Sirius UV IR The Sun is 5800 K

35. Interactions between EMR and the atmosphere: The Earth (temp ~ 300 K) radiates IR Climate science • Earth: • λpeak = 2900/300 ≈ 10 μm (Long IR) • It actually spreads from about 4 μm to 40 μm • Sun: • λpeak = 2900/5800 ≈ 0.5 μm • About 0.2μm to 2μm – UV Vis short IR – long IR

36. Climate science • Interactions between EMR and the atmosphere: • We need to know something else about EMR (light). • Quantum physics tells us that it comes as ‘photons’ • Here’s a red one • Here’s a violet one • Notice that the violet one has a shorter wavelength • But it has more energy (Violet is more ‘violent’!)

37. Climate science • Interactions between EMR and the atmosphere: • Here’s an ultraviolet (UV) one – even shorter wavelength • Here’s an infrared (IR) one • Notice that the IR one has a longer wavelength again • It also has much less energy – but it’s IR that is of most interest to us

38. Climate science • Interactions between EMR and the atmosphere: • The gases in the atmosphere absorb, and then re-radiate some types of photons but not others. • The structure of the molecule determines what sort of photon energy is absorbed. • Oxygen and Nitrogen molecules are ‘tight’ and it takes a lot of energy to ‘shake’ them (high energy UV can). • IR and visible EMR don’t have enough and go right past

39. Climate science • Interactions between EMR and the atmosphere: • H2O and CO2 molecules (and other GHGs) are more ‘floppy’ • and so take on energy more easily • IR gives them energy • which they re-radiate – in random directions. • So some goes back down to Earth • keeping us warmer • The Greenhouse effect!

40. Climate science • The effect of changes • Remember we wouldn’t be here without it! • Water vapour is the main GHG • But what if we add more CO2?

41. Climate science • The effect of changes – Feedback and Forcing • More CO2→ more warmth → more H2O (evaporation) → more warmth → more H2O → more warmth → ??? • Also, more water vapour → more clouds, which... ... reflect sunlight, and reduce the warming effect. • The actual temperature increase depends on a lot of factors. • This is why climate scientists use “computer models”

42. Climate science • The effect of changes – Feedback and Forcing • Water vapour goes in and out of the atmosphere very quickly

43. Climate science • When there is too much it rains out • This is a Feedback effect

44. Human added H2O is not a problem – it soon rains out again. Climate science

45. But CO2 is another story! Climate science

46. Climate science • Carbon dioxide molecules remain in the air for ~ 100 years • Methane for about 20 years • There is NO FEEDBACK effect that gets them out of the atmosphere • That makes a very big difference in the way they act. • CO2 and CH4 (methane) are called FORCING greenhouse gases

47. Climate science • There is another important difference between the three main greenhouse gases. • They absorb different parts of the IR spectrum...

48. Climate science Absorption spectra for greenhouse gases H2O CO2 CH4

49. Climate science • That means that even if the atmosphere is saturated with water vapour a lot of IR still gets through. • CO2 and CH4 absorb IR wavelengths that H2O doesn’t. • (Many “sceptics” don’t seem to understand that!)

50. Climate science • The BIG QUESTIONS: • If we continue to increase the greenhouse gases how much will the temperature increase? • Will that matter?