Transcript Climate Change

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:
Mean temperature:
Very thin
–65oC
Our place in space
MARS:
Atmosphere:
Mean temperature:
Very thin CO2
–65oC (but –140oC to +20oC )
No greenhouse effect
Our place in space
VENUS:
Atmosphere:
Mean temperature:
Thick
+464oC
Our place in space
VENUS:
Atmosphere:
Mean temperature:
Thick CO2!
+464oC
Greenhouse effect gone wild!
Our place in space
EARTH:
Atmosphere:
Mean temperature:
N2 , O2 , H2O and a little CO2
+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
 ~ 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
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
temperature
should be about –
18oC”
?
Climate science
 Earth’s energy balance
 Svante August Arrhenius worked it out in 1896
“Ah! The
atmosphere 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 reemit enough back to
Earth to raise the
temperature by 33
degrees!”
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
temperature by
about 5 degrees!”
“That will make
Sweden warmer –
good !”
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)
(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
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
– ‘Cool’ stars look red
eg. Betelgeuse
– ‘Hot’ stars look blue
– eg. Bellatrix and Sirius
IR
The Sun
is 5800 K
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
CH4
CO2
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 reradiated. 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
models now take into
account since the
1970’s
1990
2001
FAR = First Assessment Report etc.
1995
2007
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?