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The Physics of our Climate 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. Keith Burrows AIP Education Committee The Physics of our Climate Our place in space Our place in space MARS: Atmosphere: Very thin Mean temperature: –65oC Our place in space MARS: Atmosphere: Very thin CO2 Mean temperature: –65oC (but –140oC to +20oC ) No greenhouse effect Our place in space VENUS: Atmosphere: Thick Mean temperature: +464oC Our place in space VENUS: Atmosphere: Thick CO2! Mean temperature: +464oC Greenhouse effect gone wild! Our place in space EARTH: Atmosphere: N2 , O2 , H2O and a little CO2 Mean temperature: +15oC Just right! Why? 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 ~ 1/3 reflected Climate science ~ 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 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!) Climate science Earth’s energy balance Svante August Arrhenius worked it out in 1896 Climate science Earth’s energy balance Svante August Arrhenius worked it out in 1896 “The ? earTh’s average temperatu re should be about – 18oC” Climate science Earth’s energy balance Svante August Arrhenius worked it out in 1896 “ah! The atmospher e must be trapping The heaT” Climate science Earth’s energy balance Svante August Arrhenius worked it out in 1896 “BuT ? Oxygen and Nitrogen Can’T absorb the infrared radiaTion” Climate science Earth’s energy balance Svante August Arrhenius worked it out in 1896 “iT musT Be the water vapour and carbon dioxide!” 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 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?” Climate science Earth’s energy balance Svante August Arrhenius worked it out in 1896 “if we double the CO2 it could raise the temperatur e by about “ThaT will 5 degrees!” make Sweden warmer – 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! Climate science Some of the IR from the surface is ... ? ... trapped by the atmosphere. 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 Climate science Earth’s energy balance IPCC FAQs 1.3 Fig 1 Climate science The Greenhouse effect: – Natural ‘greenhouse gases’: Water vapour Carbon dioxide – Human produced: Carbon dioxide Methane etc. Human produced 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. Climate science Visible light is part of the EMR spectrum. Its wavelength is a little less than a millionth of a metre. 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. Kelvin is a temperature scale that starts from ‘absolute zero’ – the coldest possible temperature. 0 Kelvin is –273oC (So 0oC is 273 K) This is Wien’s law in action... (273 has been rounded up to 300 in λmax = 0.0029/T this chart – it’s only a guide) Climate science ALL objects at ANY temperature emit EMR – This polar bear is emitting just a little more than the ice! 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. 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) 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) Climate science Wien’s law also applies to stars UV IR – ‘Cool’ stars look red eg. Betelgeuse – ‘Hot’ stars look blue – eg. Sirius The Sun is 5800 K – UV Vis IR – Climate science Wien’s law also applies to stars UV IR – ‘Cool’ stars look red eg. Betelgeuse The Sun is 5800 K – ‘Hot’ stars look blue – eg. Bellatrix and Sirius Climate science Interactions between EMR and the atmosphere: The Earth (temp ~ 300 K) radiates IR 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 0.2 μm to 2 μm – UV Vis short IR – long IR 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’!) 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 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 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! 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? 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” Climate science The effect of changes – Feedback and Forcing – Water vapour goes in and out of the atmosphere very quickly Climate science When there is too much it rains out This is a Feedback effect Climate science – Human added H2O is not a problem – it soon rains out again. Climate science – But CO2 is another story! 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 Climate science There is another important difference between the three main greenhouse gases. They absorb different parts of the IR spectrum... Climate science Absorption spectra for greenhouse gases H2O CO2 CH4 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!) Climate science The BIG QUESTIONS: – If we continue to increase the greenhouse gases how much will the temperature increase? – Will that matter? 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. Climate science How do climate models work? Here are some of the factors that have to be considered... IPCC 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) Climate science These show the increased number of factors the climate 1990 1995 models now take into account since the 1970’s 2001 2007 FAR = First Assessment Report etc. 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. IPCC SynRep Even aircraft contrails are taken into account Contrails over Paris rooftops From 2000 to 2005 some of the forcings had become better understood. This is the problem IPCC 2007 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 becomes (say) 342 ≠ 107 + 233 Climate science Repeating: Incoming = Outgoing 342 = 107 + 235 becomes (say) 342 ≠ 107 + 233 To increase the 233 back up to 235 the 390 surface radiation needs to increase – which it does as the Climate science 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 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 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 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: Courtesy of Graeme Pearman Climate science 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 The science of climate change Climate science Climate models and their predictions. – In 2007 the IPCC released the AR4 Synthesis Report which contains the most detailed and worrying predictions yet. – The problem is that many of the predictions seem to be too conservative... Climate science For example: Human induced changes Is the climate changing? The Greenland summer ice melt is getting larger at a worrying rate. Climate science It has been thought (hoped?) that the Antarctic Ice sheets are not melting. NASA Climate science However (23 Jan 2008): Colours indicate speed of ice loss: Red fast, green slower Loss is on a par with the Greenland ice loss rate. NASA Climate science We have looked at the basic science but – What is the evidence of climate change? – Could the “sceptics” be right after all? – What are the causes? – What are the consequences? – What can we do about it?
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