1. The Physics of our Climate Keith Burrows AIP Education Committee 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 or general public some sections of this presentation could be omitted. Other presentations in this series will include (titles may change!): -Is the climate changing? -What will be the consequences? Do they matter? -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.orgwww.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.

2. The Physics of our Climate Download from www.vicphysics.org

3. Our place in space

4. MARS: Atmosphere: Very thin Mean temperature: –65 o C

5. Our place in space MARS: Atmosphere: Very thin CO 2 Mean temperature: –65 o C (but –140 o C to +20 o C ) No greenhouse effect

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

7. Our place in space VENUS: Atmosphere: Thick CO 2 ! Mean temperature: +464 o C Greenhouse effect gone wild!

8. Our place in space EARTH: Atmosphere: N 2, O 2, H 2 O and a little CO 2 Mean temperature: +15 o C 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) TW = terawatt = 10 12 watts = 1,000,000,000,000 watts IR EMR = Infrared Electromagnetic Radiation (just invisible ‘light’ really)

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 = εσT 4  Wien’s law... λ max = 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 –18 o C” ?

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 CO 2 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 CO 2 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 15 o C (average) instead of that freezing –18 o C

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. Kelvin is a temperature scale that starts from ‘absolute zero’ – the coldest possible temperature. 0 Kelvin is –273 o C (So 0 o C 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... λ max = 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: λ peak = 2900/T (λ in μm and T in K)  λ peak 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: λ peak = 2900/T (λ in μm and T in K)  Example –At 300 K: λ peak = 2900/300 ≈ 9.7 μm (Long IR) –At 5800 K: λ peak = 2900/5800 ≈ 0.5 μm (Visible – yellow/white) (The Sun’s surface is at 5800 K)

32. Climate science  Wien’s law: λ peak = 2900/T (λ in μm and T in K)  Example –The hot metal (about 1500 K) will emit: λ peak = 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 Vis IR – UV IR The Sun is 5800 K

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

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: –H 2 O and CO 2 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 CO 2 ?

43. Climate science  The effect of changes – Feedback and Forcing –More CO 2 → more warmth → more H 2 O (evaporation) → more warmth → more H 2 O → more warmth → ??? –But 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”

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

45. Climate science  When there is too much it rains out  This is a Feedback effect

46. Climate science –Human added H 2 O is not a problem – it soon rains out again.

47. Climate science –But CO 2 is another story!

48. 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.  CO 2 and CH 4 (methane) are called FORCING greenhouse gases

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

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

51. Climate science  That means that even if the atmosphere is saturated with water vapour a lot of IR still gets through.  CO 2 and CH 4 absorb IR wavelengths that H 2 O doesn’t.  (Many “sceptics” don’t seem to understand that!)

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

53. Climate science  The BIG QUESTIONS: –If we continue to increase the greenhouse gases how much will the temperature increase? –Will that matter?  How can we find out? –We need to use our understanding of the science of climate change. –This is done mostly by putting the data into computer models and using the laws of physics.

54. Climate science  How do climate models work?  Here are some of the factors that have to be considered...

55. IPCC

56. This shows the average amount of power being absorbed by the Earth and then re- radiated. About half the incoming EMR is absorbed by the surface while almost twice that is re-absorbed from back radiation (the greenhouse effect). Overall, incoming equals outgoing (342 = 107 + 235)

57. Climate science  These show the increased number of factors the climate models now take into account since the 1970’s 1990 1995 20012007 FAR = First Assessment Report etc.

58. Climate science  The next slides show the ‘Radiative Forcing’ factors.  These are factors which alter the Earth’s heat balance and thus cause a gradual change in the Earth’s temperature.  More heat trapped – temperature rises until the heat radiated away from Earth equals that coming in.

59. IPCC SynRep

60. Even aircraft contrails are taken into account Contrails over Paris rooftops

61. IPCC 2007 From 2000 to 2005 some of the forcings had become better understood. This is the problem

62. Climate science  That extra 1 to 2 watts trapped in every square metre of the Earth means the temperature has to rise in order to get rid of it:  It changes the balance Incoming = Outgoing 342 = 107 + 235 342 = 107 + 235 becomes (say) 342 ≠ 107 + 233 342 ≠ 107 + 233

63. Climate science Repeating:

64.  How can we understand it? –Computer models are the only way of taking all this into account. –Use basic physics to calculate movement of heat, air, water, between small blocks of the atmosphere. –Here’s the basic physics: Climate science

66.  Climate models and their predictions. –These are just F = ma applied to moving fluids –This is conservation of mass –This governs the way heat flows between systems Climate science

67.  Climate models and their predictions. –The climate system is modelled as cells of air (or water) and the equations are applied to see how much air/heat flows between each pair of cells –This is repeated all around the Earth –The models have improved by making the cells smaller –They are now about 110 km square by 1 km high Climate science

68.  Climate models and their predictions. –The initial conditions have to be fed into the model and then it generates weather and climate patterns over hours, days, years or centuries! –Here is the result of one: Climate science

69. Courtesy of Graeme Pearman

70.  Climate models and their predictions. –Models are tested to see if they generate past known climate patterns. –They are becoming more and more accurate. over hours, days (7 day forecasts), years or centuries! –Anthropogenic factors can be added/removed Climate science

73.  Climate models and their predictions. –In 2007 the IPCC released the AR4 Synthesis Report which contains the most detailed and worrying predictions yet. –Unfortunately, the IPCC are very conservative in their declarations… Climate science

74. “The IPCC format …is a painstaking self- interrogation process of the pertinent scientific community. In this process, virtually every stone in the cognitive landscape is turned and the findings, however mundane or ugly, are synthesized into encyclopedic accounts. Unfortunately, such an approach is inherently tuned for burying crucial insights under heaps of facts, figures, and error bars.” Climate science Hans Joachim Schellnhuber Potsdam Institute for Climate Impact Research, Environmental Change Institute and Tyndall Centre, Oxford University

75. –But the main problem is that many of the IPCC predictions seem to be too conservative... Climate science might be underestimated due to missing carbon cycle feedbacks and do not include contributions from melting ice sheets, glaciers and ice caps

76.  For example: Climate science

77. Predicted (approx)

78. Human induced changes The Greenland summer ice melt is getting larger at a worrying rate. The Greenland ice sheets are also melting faster than expected – which may explain...

81. Climate science  It had been thought (hoped?) that the Antarctic Ice sheets are not melting. NASA

82.  There is much more ocean in the southern hemisphere – takes more heat to warm it.  More ice in Antarctic than Arctic  Warm currents don’t reach the Antarctic to the extent that they reach the Arctic  Warmer air carries more moisture which increases precipitation over Antarctica

83. Climate science  However (Jan 2008):  Colours indicate speed of ice loss: Red fast, green slower  Loss is on a par with the Greenland ice loss rate. NASA

84. Warming (red) across Antarctica, 1957-2007 NASA-GSFC

85. Climate science  We have looked at some of the basic climate science but: –Is the climate changing? –Hasn’t the climate always changed? –Could the “sceptics” be right after all? –What are the causes? –What are the consequences? Do they matter? –What can we do about it?