09a.ClimateChange

7/27/2019 09a.ClimateChange

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Climate Change

and Global WarmingDebunking Common Misconceptions

Climate predictability

Climate forcingClimate models

Emission scenarios & climate of the 21st century

Responding to Climate Skeptics


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Media Myths about ClimateBe skeptical be very skeptical !

Concern about global warming is based on recenttemperature trends

9 of the 10 hottest years on record

If somebody could find some other cause for recentwarming, we could quit worrying

Global warming is a theory based on complicatedcomputer models

CO2is air pollution cutting emissions will leadto falling CO2 and therefore cooling

If we stop burning coal, well freeze in the dark!


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Global Warming isBased on Common Sense

not computer models not recent temperatures

not complicated!


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Planetary Energy Balance

Energy In = Energy Out

2 2 4(1 ) 4S R R T

o18 CT

But the observed Ts is about 15 C


5/26

Dancing Molecules and Heat Rays!

Nearly all of the air is

made of oxygen (O2)and nitrogen (N2) inwhich two atoms ofthe same elementshare electrons

Infrared (heat)

energy radiated upfrom the surface canbe absorbed by thesemolecules, but not

very well

N N

O O

Diatomic molecules can

vibrate back and forthl ike balls on a spring, but

the ends are identical


6/26

Dancing Molecules and Heat Rays!

Carbon dioxide (CO2)

and water vapor (H2O)are different!

They have many moreways to vibrate androtate, so they arevery good at absorbing

and emitting infrared(heat) radiation

Molecules that have manyways to wiggle are called

Greenhouse molecules

O OC

H H

O

Absorption spectrum of CO2 was measured by John Tyndall in 1863


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Common Sense

Doubling CO2 wouldadd 4 watts toevery square meterof the surface of

the Earth, 24/7 Doing that would

make the surfacewarmer

This was knownbefore light bulbswere invented!

4 Watts

John Tyndall , January 1863


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Common Misconception #1Expectations of future warming are based on

extrapolation of recent warming trends

WRONG! They are based on the idea that whenwe add energy to the surface, it will warm up


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19th Century Climate Physics(Svante Arrhenius, 1897)

S0(1 )r2

TS4 (4r2 )

S0(1 ) 4TS4

0 4TS

4 4(4TS

4 )

Differentiate, apply chain rule

TS

TS

4

Ar rhenius worked out a simple formula

for the change in sur face temperature

given a change in effective

atmospher ic emissivity due to CO2

Earth

Ts

aS0

esTs4

S0

h


10/26

19th Century ClimatePhysics(contd)

Plug in measured values

TS TS

4

TS

4 240 W m-2

(from satellite data)

()(

TS

4

) 4 W m-2

(for 2 x CO2

from radiative transfer)

4

240

Ts = 288 K

TS

288K

4

4

240

1.2K

For CO2 alone (no feedback), expect about 2F warming for 2 x CO2


11/26

Climate Feedback Processes

Positive Feedbacks(amplify changes) Water vapor

Ice-albedo

High clouds

De DTS Dvapor

D albedo

DLW

D hi cloud

D lo cloud

Negative feedbacks

(damp changes) Longwave cooling Low clouds


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Learning from the Past

Past climate changes reveal climate sensitivity


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Tiny Bubbles Priceless

ice age ice age ice ageice age


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CO2 and the Ice Ages

370 ppm in 2000

Vostok (400k yr) Ice Core data (Petit et al, 1999)

175

200

225

250

275

300

-400000 -300000 -200000 -100000 0

Year

iceice

ice

ice

CO2

Over the past 420,000

years atmospheric CO2has varied between 180and 280 ppm, beating intime with the last four

glacial cycles


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Review: 19th Century Physics(updated using paleo-data)

Forcing: changes in properties ofatmosphere as measured by spectroscopy(4 W m-2 per doubling of CO2)

Feedback: both positive and negative, totalresponse to forcing estimated from Ice Ageclimate data (about 0.8C per W m-2)

Response: about 3.2 C warming for 2 xCO2No climate models required just based on observations

(modern calculations agree coincidence?)


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CO2 and the Modern Age

370 ppm in 2000


180

230

280

330

380

-400000 -300000 -200000 -100000 0

year


years atmospheric CO2has varied between 180and 280 parts permillion, beating in timewith the last four glacialcycles

Since the IndustrialRevolution, CO2 has risen

very rapidly ice iceiceice

CO2

388 ppm in 2009

f rom measurements


18/26

CO2 and the Future


0

200

400

600

800

1000

-400000 -300000 -200000 -100000 0

year


years atmospheric CO2has varied between 180and 280 parts permillion, beating in timewith the last four glacial

cycles

Since the IndustrialRevolution, CO2 has risenvery rapidly

If China & India developusing 19th Centurytechnology, CO2 willreach 900 ppm in this

century

ice iceiceice

CO2

900 ppm in 2100

388 ppm in 2009

You aint seen nothing yet!


19/26

Climate vs. Weather

Weather tells you what to wear today climate tells you what clothes to buy!

Climate is an envelope of possibilitieswithin which the weather bounces around

Weather depends very sensitively on theevolution of the system from one momentto the next (initial conditions)

Climate is determined by the properties ofthe Earth system itself(the boundary conditions)


20/26

Climate Predictability

Predicting the response of the climate to achange in the radiative forcing is notanalogousto weather prediction

If the change in forcing is large and predictable,

the response can also be predictable I cant predict the weather in Fort Collins on

December 18, 2009 (nobody can!)

I can predict with 100% confidence that theaverage temperature in Fort Collins forDecember, 2009 will be warmer than the averagefor July!


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Climate Forcing

Changes in climate often reflect changes in forcing, asamplified or damped by climate feedbacks Diurnal cycle

Seasonal cycle

Ice ages

Response to volcanic aerosol

Solar variability

Greenhouse forcing

If forcing is sufficiently strong, and the forcing itself is

predictable, then the response of the climate can bepredictable too!


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Reconstructed Radiative Forcings


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Observations

Muchstrongertrend on landthan ocean

North >South

Surface >

Troposphere Acceleration

of trend


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1000 YearsofTemperatureReconstructed from TreeRings, Pollen,

etc


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Water Vapor Trends

Trends in annual mean surface water vapourpressure, 1975 to 1995, expressed as apercentage of the 1975 to 1995 mean.Areas without dots have no data. Blue

shaded areas have nominally significantincreasing trends and brown shaded areashave significant decreasing trends, both atthe 5% significance level. Biases in thesedata have been little studied so the levelof significance may be overstated. FromNew et al. (2000).


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