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

The Physics of our ClimateThe 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.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.

http://www.vicphysics.org/

The Physics The Physics of our Climateof our Climate

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Our place in spaceOur place in space


MARS:MARS:Atmosphere: Atmosphere: Very thinVery thinMean temperature: Mean temperature: –65–65ooCC


MARS:MARS:Atmosphere: Atmosphere: Very thin COVery thin CO22

Mean temperature: Mean temperature: –65–65ooC (but –140C (but –140ooC to +20C to +20ooC )C )No greenhouse effectNo greenhouse effect


VENUS:VENUS:Atmosphere: Atmosphere: Thick Thick Mean temperature: Mean temperature: +464+464ooCC


VENUS:VENUS:Atmosphere: Atmosphere: Thick COThick CO22! !

Mean temperature: Mean temperature: +464+464ooCCGreenhouse effect gone wild!Greenhouse effect gone wild!

Our place in spaceOur place in spaceEARTH:EARTH:Atmosphere: Atmosphere: NN22 , O , O22 , H , H22O and a little COO and a little CO22

Mean temperature: Mean temperature: +15+15ooCCJust right!Just right!

Why?

ClimateClimate science science Earth’s energy balanceEarth’s energy balance

– The average temperature of the Earth is determined by the balance The average temperature of the Earth is determined by the balance between incoming solar radiation and outgoing ‘heat’ radiationbetween incoming solar radiation and outgoing ‘heat’ radiation

ClimateClimate science science ~ 1/3 reflected~ 1/3 reflected ~ 2/3 absorbed ~ 2/3 absorbed

then re-radiated then re-radiated as IR EMR.as IR EMR.

175,000 TW in175,000 TW in 175,000 TW out175,000 TW out

(But that’s if it is (But that’s if it is in equilibrium)in equilibrium)

TW = terawatt = 1012 watts= 1,000,000,000,000 watts

IR EMR = Infrared Electromagnetic Radiation(just invisible ‘light’ really)


– The average temperature of the Earth is The average temperature of the Earth is determined by the balance between incoming determined by the balance between incoming solar radiation and outgoing ‘heat’ radiationsolar radiation and outgoing ‘heat’ radiation

– Two simple laws of physics enable us to figure Two simple laws of physics enable us to figure out the energy balance:out the energy balance: The The Stefan-Boltzmann law... I = εσTStefan-Boltzmann law... I = εσT44 Wien’s law... λWien’s law... λmaxmax = = 0.00290.0029/T/T

– S-B just tells us how much heat a hot object S-B just tells us how much heat a hot object radiates.radiates.

– Wien tells us what sort of radiation it will be.Wien tells us what sort of radiation it will be.(but fortunately others have done the hard work for us!)(but fortunately others have done the hard work for us!)

ClimateClimate science science Earth’s energy balanceEarth’s energy balance Svante August Arrhenius worked it out in 1896Svante August Arrhenius worked it out in 1896


“The Earth’s average temperature should be about –18oC”

?


“Ah! The atmosphere must be trapping the heat”


“But Oxygen and Nitrogen can’t absorb the infrared radiation”

?


“It must be the water vapour and carbon dioxide!”


“Together they absorb heat and re-emit enough back to Earth to raise the temperature by 33 degrees!”


“So what will all the CO2 we are putting in the atmosphere do?”

?


“If we double the CO2 it could raise the temperature by about 5 degrees!”

“That will make Sweden warmer – good !”

ClimateClimate science science Earth’s energy balance (sum up)Earth’s energy balance (sum up)

– The average temperature of the Earth is The average temperature of the Earth is determined by the balance between incoming determined by the balance between incoming solar radiation and outgoing ‘heat’ radiationsolar radiation and outgoing ‘heat’ radiation

– Not all the IR radiation from the surface Not all the IR radiation from the surface escapes immediately... escapes immediately...

– or the average temperature would be a or the average temperature would be a freezing –18freezing –18ºC ºC

– No liquid water or cloudsNo liquid water or clouds– And no life! And no life!

ClimateClimate science science Some of the IR from the surface is ... ?Some of the IR from the surface is ... ? ... trapped by the atmosphere.... trapped by the atmosphere.

ClimateClimate science science Some of the IR from the surface is trapped by Some of the IR from the surface is trapped by

the atmosphere – a little like a greenhouse...the atmosphere – a little like a greenhouse...

The so called “Greenhouse Effect”The so called “Greenhouse Effect” This keeps the Earth at a warm 15This keeps the Earth at a warm 15ooC C

(average) instead of that freezing –18(average) instead of that freezing –18ooCC


IPCC FAQs 1.3 Fig 1

ClimateClimate science science The Greenhouse effect:The Greenhouse effect:

– Natural ‘greenhouse gases’:Natural ‘greenhouse gases’: Water vapourWater vapour Carbon dioxideCarbon dioxide

– Human produced:Human produced: Carbon dioxideCarbon dioxide Methane etc.Methane etc.

Human produced

ClimateClimate science science In order to understand the ‘greenhouse effect’ we need to In order to understand the ‘greenhouse effect’ we need to

know a little about ‘Electromagnetic Radiation’ (or EMR)know a little about ‘Electromagnetic Radiation’ (or EMR) Here’s the whole spectrum:Here’s the whole spectrum: This is the part we are interested in.This is the part we are interested in.

ClimateClimate science science

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


It turns out that ANY object emits some EMR – depending It turns out that ANY object emits some EMR – depending on its temperature:on its temperature:

Hot objects radiate infrared (which we feel as heat) and Hot objects radiate infrared (which we feel as heat) and even hotter ones glow with visible EMReven hotter ones glow with visible EMR..

Kelvin is a temperature scale thatstarts 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 This is Wien’s law in action... action...

λλmaxmax = 0.0029/T = 0.0029/T

ClimateClimate science science ALL objects at ANY temperature emit EMRALL objects at ANY temperature emit EMR

– This polar bear is emitting just a little more than the ice!This polar bear is emitting just a little more than the ice!

ClimateClimate science science There is a simple law of physics about this:There is a simple law of physics about this: Wien’s law: λWien’s law: λpeakpeak = 2900/T (λ in = 2900/T (λ in μμm and T in K) m and T in K)

λλpeakpeak is the wavelength most emitted (there is a spread) is the wavelength most emitted (there is a spread)

All it says is that the hotter the object (T) the shorter the All it says is that the hotter the object (T) the shorter the wavelength (wavelength (λλ) of most of the radiation.) of most of the radiation.

ClimateClimate science science Wien’s law: λWien’s law: λpeakpeak = 2900/T (λ in = 2900/T (λ in μμm and T in K) m and T in K)

ExampleExample– At 300 K: At 300 K: λλpeakpeak = 2900/300 ≈ 9.7 = 2900/300 ≈ 9.7 μμm (Long IR)m (Long IR)

– At 5800 K: At 5800 K: λλpeakpeak = 2900/5800 ≈ 0.5 = 2900/5800 ≈ 0.5 μμm (Visible – yellow/white)m (Visible – yellow/white)

(The Sun’s surface is at 5800 K)(The Sun’s surface is at 5800 K)

ClimateClimate science science Wien’s law: λWien’s law: λpeakpeak = 2900/T (λ in = 2900/T (λ in μμm and T in K) m and T in K)

ExampleExample– The hot metal (about 1500 K) will emit:The hot metal (about 1500 K) will emit:

λλpeakpeak = 2900/1500 ≈ 2 = 2900/1500 ≈ 2 μμm m

which is IR, but it will also emit quite a bit of visible (mostly red)which is IR, but it will also emit quite a bit of visible (mostly red)

ClimateClimate science science Wien’s law also applies Wien’s law also applies

to starsto stars

– ‘‘Cool’ stars look red Cool’ stars look red eg. Betelgeuseeg. Betelgeuse

– ‘‘Hot’ stars look blueHot’ stars look blue– eg. Siriuseg. Sirius

– UV Vis IR –

UVIR

The Sun is 5800 K

ClimateClimate science science Wien’s law also applies Wien’s law also applies

to starsto stars

– ‘‘Cool’ stars look red Cool’ stars look red eg. Betelgeuseeg. Betelgeuse

– ‘‘Hot’ stars look blueHot’ stars look blue– eg. Bellatrix and Siriuseg. Bellatrix and Sirius

UV IR

The Sun is 5800 K

ClimateClimate science science Interactions between EMR and the atmosphere:Interactions between EMR and the atmosphere: The Earth (temp ~ 300 K) radiates IRThe Earth (temp ~ 300 K) radiates IR

– UV Vis short IR – long 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 μmAbout 0.2 μm to 2 μm

ClimateClimate science science Interactions between EMR and the atmosphere:Interactions between EMR and the atmosphere:

– We need to know something else about EMR (light).We need to know something else about EMR (light).– Quantum physics tells us that it comes as ‘photons’Quantum physics tells us that it comes as ‘photons’– Here’s a red oneHere’s a red one– Here’s a violet oneHere’s a violet one– Notice that the violet one has a shorter wavelengthNotice that the violet one has a shorter wavelength– But it has more energy (Violet is more ‘violent’!)But it has more energy (Violet is more ‘violent’!)


– Here’s an ultraviolet (UV) one – even shorter wavelengthHere’s an ultraviolet (UV) one – even shorter wavelength– Here’s an infrared (IR) oneHere’s an infrared (IR) one– Notice that the IR one has a longer wavelength againNotice that the IR one has a longer wavelength again– It also has much less energy – but it’s IR that is of most It also has much less energy – but it’s IR that is of most

interest to usinterest to us


– The gases in the atmosphere absorb, and then re-radiate some The gases in the atmosphere absorb, and then re-radiate some types of photons but not others.types of photons but not others.

– The structure of the molecule determines what sort of photon The structure of the molecule determines what sort of photon energy is absorbed.energy is absorbed.

– Oxygen and Nitrogen molecules are ‘tight’ and it takes a lot of Oxygen and Nitrogen molecules are ‘tight’ and it takes a lot of energy to ‘shake’ them (high energy UV can).energy to ‘shake’ them (high energy UV can).

– IR and visible EMR don’t have enough and go right pastIR and visible EMR don’t have enough and go right past


– HH22O and COO and CO22 molecules (and other GHGs) are more ‘floppy’ molecules (and other GHGs) are more ‘floppy’

– and so take on energy more easily and so take on energy more easily – IR gives them energyIR gives them energy– which they re-radiate – in random directions.which they re-radiate – in random directions.– So some goes back down to EarthSo some goes back down to Earth– keeping us warmerkeeping us warmer– The Greenhouse effect!The Greenhouse effect!

ClimateClimate science science The effect of changesThe effect of changes

– Remember we wouldn’t be here without it!Remember we wouldn’t be here without it!– Water vapour is the main GHGWater vapour is the main GHG

– But what if we add more COBut what if we add more CO22??

ClimateClimate science science The effect of changes – The effect of changes – FeedbackFeedback and and ForcingForcing

– More COMore CO22 → → more warmth more warmth → more H→ more H22O (evaporation) O (evaporation)

→ → more warmth more warmth → more H→ more H22O → O → more warmth more warmth → → ??????

– But also, more water vapourBut also, more water vapour → more clouds, which... → more clouds, which...

... reflect sunlight, and ... reflect sunlight, and reduce the warming effectreduce the warming effect..– The actual temperature increase depends on a lot of The actual temperature increase depends on a lot of

factors.factors.– This is why climate scientists use “computer models” This is why climate scientists use “computer models”

ClimateClimate science science The effect of changes – The effect of changes – FeedbackFeedback and and ForcingForcing

– Water vapour goes in and out of the atmosphere very Water vapour goes in and out of the atmosphere very quicklyquickly

ClimateClimate science science When there is too much it rains outWhen there is too much it rains out This is a This is a FeedbackFeedback effect effect


– Human Human added Hadded H22O O

is not a is not a problem – it problem – it soon rains soon rains out again.out again.


– But COBut CO22 is is

another story!another story!

ClimateClimate science science Carbon dioxide molecules remain in the Carbon dioxide molecules remain in the

air for ~ 100 yearsair for ~ 100 years Methane for about 20 yearsMethane for about 20 years There is There is NO FEEDBACKNO FEEDBACK effect that gets effect that gets

them out of the atmospherethem out of the atmosphere That makes a very big difference in the That makes a very big difference in the

way they act.way they act. COCO22 and CH and CH44 (methane) are called (methane) are called

FORCINGFORCING greenhouse gases greenhouse gases

ClimateClimate science science There is another important difference There is another important difference

between the three main greenhouse between the three main greenhouse gases.gases.

They absorb They absorb different parts different parts of the IR of the IR spectrum...spectrum...


H2O CO2

CH4

Absorption spectra for greenhouse gases

ClimateClimate science science That means that even if the atmosphere is That means that even if the atmosphere is

saturated with water vapour a lot of IR still saturated with water vapour a lot of IR still gets through.gets through.

COCO22 and CH and CH44 absorb IR wavelengths that absorb IR wavelengths that

HH22O doesn’t.O doesn’t.

(Many “sceptics” don’t seem to understand (Many “sceptics” don’t seem to understand that!)that!)

ClimateClimate science science The BIG QUESTIONS:The BIG QUESTIONS:

– If we continue to increase the greenhouse gases how If we continue to increase the greenhouse gases how much will the temperature increase?much will the temperature increase?

– Will that matter?Will that matter?

ClimateClimate science science The BIG QUESTIONS:The BIG QUESTIONS:

– If we continue to increase the greenhouse gases how If we continue to increase the greenhouse gases how much will the temperature increase?much will the temperature increase?

– Will that matter?Will that matter?

How can we find out?How can we find out?– We need to use our understanding of the We need to use our understanding of the

science of climate change.science of climate change.– This is done mostly by putting the data into This is done mostly by putting the data into

computer models and using the laws of physics.computer models and using the laws of physics.

ClimateClimate science science How do climate models work?How do climate models work? Here are some of the factors that have to Here are some of the factors that have to

be considered...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)


These show the These show the increased number of increased number of factors the climate factors the climate models now take into models now take into account since the account since the 1970’s 1970’s

1990 1995

2001 2007

FAR = First Assessment Report etc.


The next slides show the The next slides show the ‘Radiative Forcing’‘Radiative Forcing’ factors.factors.

These are factors which These are factors which alter the Earth’s alter the Earth’s heat balanceheat balance and thus cause a gradual and thus cause a gradual change in the Earth’s temperature.change in the Earth’s temperature.

More heat trapped – temperature rises until More heat trapped – temperature rises until the heat radiated away from Earth equals the heat radiated away from Earth equals that coming in.that coming in.

IPCC SynRep

Even aircraft contrails are taken into account

Contrails over Paris rooftops

IPCC 2007

From 2000 to 2005 some of the forcings had become better understood.

This is the problem


That extra 1 to 2 watts trapped in every square That extra 1 to 2 watts trapped in every square metre of the Earth means the temperature has metre of the Earth means the temperature has to rise in order to get rid of it:to rise in order to get rid of it:

It changes the balance It changes the balance

Incoming = OutgoingIncoming = Outgoing

342342 = = 107107 + + 235235

becomes (say)becomes (say)

342342 ≠≠ 107107 + + 233233

ClimateClimate science scienceRepeating:

How can we understand it? How can we understand it? – Computer models are the only way of taking all Computer models are the only way of taking all

this into account.this into account.– Use basic physics to calculate movement of heat, Use basic physics to calculate movement of heat,

air, water, between small blocks of the air, water, between small blocks of the atmosphere.atmosphere.

– Here’s the basic physics:Here’s the basic physics:


Climate models and their predictions.Climate models and their predictions.

– These are just F = ma These are just F = ma applied to moving fluidsapplied to moving fluids

– This is conservation of This is conservation of massmass

– This governs the way This governs the way heat flows between heat flows between systemssystems


Climate models and their predictions.Climate models and their predictions.

– The climate system is modelled The climate system is modelled as cells of air (or water) and the as cells of air (or water) and the equations are applied to see how equations are applied to see how much air/heat flows between much air/heat flows between each pair of cellseach pair of cells

– This is repeated all around the This is repeated all around the EarthEarth

– The models have improved by The models have improved by making the cells smallermaking the cells smaller

– They are now about 110 km They are now about 110 km square by 1 km highsquare by 1 km high


Climate models and their predictions.Climate models and their predictions.– The initial conditions have to be fed into the The initial conditions have to be fed into the

model and then it generates weather and climate model and then it generates weather and climate patterns over hours, days, years or centuries!patterns over hours, days, years or centuries!

– Here is the result of one:Here is the result of one:


Courtesy of Graeme Pearman

Climate models and their predictions.Climate models and their predictions.– Models are tested to see if they generate past Models are tested to see if they generate past

known climate patterns.known climate patterns.– They are becoming more and more accurate. They are becoming more and more accurate.

over hours, days (7 day forecasts), years or over hours, days (7 day forecasts), years or centuries!centuries!

– Anthropogenic factors can be added/removedAnthropogenic factors can be added/removed


Climate models and their predictions.Climate models and their predictions.– In 2007 the IPCC released the AR4 Synthesis In 2007 the IPCC released the AR4 Synthesis

Report which contains the most detailed and Report which contains the most detailed and worrying predictions yet.worrying predictions yet.

– Unfortunately, the IPCC are very conservative in Unfortunately, the IPCC are very conservative in their declarations…their declarations…


““The IPCC format …is a painstaking self-The IPCC format …is a painstaking self-interrogation process of the pertinent scientific interrogation process of the pertinent scientific community. In this process, virtually every stone community. In this process, virtually every stone in the cognitive landscape is turned and the in the cognitive landscape is turned and the findings, however mundane or ugly, are findings, however mundane or ugly, are synthesized into encyclopedic accounts. synthesized into encyclopedic accounts. Unfortunately, such an approach is inherently Unfortunately, such an approach is inherently tuned for burying crucial insights under heaps of tuned for burying crucial insights under heaps of facts, figures, and error bars.”facts, figures, and error bars.”


Hans Joachim Schellnhuber Potsdam Institute for Climate Impact Research, Environmental Change Institute and Tyndall Centre, Oxford University

– But the main problem is that many of the IPCC But the main problem is that many of the IPCC predictions seem to be too conservative...predictions seem to be too conservative...


might be underestimated due to missing carbon cycle feedbacks

and do not include contributions from melting ice sheets, glaciers and ice caps

For example:For example:


Predicted (approx)

Human induced changesHuman 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...

Climate scienceClimate science

It had been thought (hoped?) that the Antarctic Ice It had been thought (hoped?) that the Antarctic Ice sheets are not melting.sheets are not melting.

NASA

There is much more ocean in the southern There is much more ocean in the southern hemisphere – takes more heat to warm it.hemisphere – takes more heat to warm it.

More ice in Antarctic than ArcticMore ice in Antarctic than Arctic Warm currents don’t reach the Antarctic to Warm currents don’t reach the Antarctic to

the extent that they reach the Arcticthe extent that they reach the Arctic Warmer air carries more moisture which Warmer air carries more moisture which

increases precipitation over Antarcticaincreases precipitation over Antarctica


However However (Jan 2008):(Jan 2008):

Colours indicate Colours indicate speed of ice loss:speed of ice loss:

Red fast, green Red fast, green slowerslower

Loss is on a par Loss is on a par with the Greenland with the Greenland ice loss rate.ice loss rate.

NASA

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


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

Documents

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