Climate Change by HC12071623438

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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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