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The human influence on climate
A. E. DesslerDept. of Atmospheric Sciences
Texas A&M University
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NASA GISS Global-Avg. Surface T Record 1880-2009
-0.4
-0.2
0.0
0.2
0.4
Global mean tem
p. (°C)
2000198019601940192019001880Year
0.6
0.4
0.2
0.0
-0.2Tem
pera
ture
ano
mal
y (°
C)
warming of 0.74±0.2°C over the20th century
• surface thermometers• satellite temperature• glaciers• sea ice • ocean temps• sea level
unequivocal
Is the Earth warming?
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melting
warming
rising
warming
warming
consilience
How much do greenhouse gases warm
the climate?
Climate sensitivityHow much warming would occur if
carbon dioxide abundance were doubled?
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Energy in = Energy out
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Energy in
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Energy in
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Energy in = S (1-a)/4
Energy in
“Solar constant” = 1360 W/m2
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Energy in = S (1-a)/4
Energy in
“albedo” = 0.3
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Energy in = S (1-a)/4
Energy in
geometric factor
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Energy in = S (1-a)/4
Energy in
! 238 W/m2
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T = 86°F
Sun
238238
Energy out = ! Ts4 = 238 W/m2
Ts = 255 K = -18°C
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atmosphere
T = 86°F
Sun
238
238476238
Energy out = ! Ts4 = 476 W/m2
Ts = 303 K = 30°C
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Energy out = ! Ts4 Ts = 335 K = 62°C
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No greenhouse gases Some greenhouse gases
More greenhouse gases
255 K 303 K
335 K
!
Ts =n +1( )S 1" a( )
4#4
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Energy in = Energy out
Temperature increase = 1.2°C for doubled CO2
GCM
CO2 increase
Ts increase
Ice melts
More solar energy
absorbed
Calculating the effects of feedback
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climate sensitivity
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Calculating the effects of feedback
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Calculating the effects of feedback
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CO2 increase
Ts increase
Atmospheric humidity increases
Water vapor feedback
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
Ts
2009200820072006200520042003Year for Ts
-30
-20
-10
0
10
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q (g/kg)
20102009200820072006200520042003Year for q
Tropical average (30N-30S) Surface Temperature AIRS q at 350 hPa ERA-interim q
Trop
ical
Ano
mal
y (°
C)
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A simple calculation:Doubling carbon dioxide
water vapor feedback = fwv = 0.6
GCMDessler, A. E., P. Yang, and Z. Zhang (2008), The water-vapor climate feedback inferred from climate fluctuations, 2003-2008, Geophys. Res. Lett., 35, L20704, doi:10.1029/2008GL035333.
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A simple calculation:Doubling carbon dioxide
water vapor feedback = fwv = 0.6
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A simple calculation:Doubling carbon dioxide
most contentious
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A simple calculation:Doubling carbon dioxide
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A simple calculation:Doubling carbon dioxide
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A simple calculation:Doubling carbon dioxide
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A simple calculation:Doubling carbon dioxide
add some uncertainty = 2.0-4.5°C
GCM
http://www.scientificamerican.com/media/pdf/hansen.pdf
virtually identical climate sensitivity
climate sensitivity greater than 1.5°C has probablybeen a robust feature of the Earth’s climate system
over the past 420 million years
consilience
How much warming will we experience?
• Climate sensitivity is 2-4.5°C for 2xCO2
• Given carbon dioxide reaching 500-900 ppm this century (390 ppm now)
• Other GHGs are also increasing• We can therefore expect several degrees
Celsius of warming• This agrees with models, but
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GCM
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war
min
g si
nce
the
last
ice
age
http://legacy.library.ucsf.edu/tid/wjh13f00/pdf
http://legacy.library.ucsf.edu/tid/wjh13f00/pdf
Doubt is our product since it is the best way of competing with the “body of fact”
that exists in the mind of the general public. It is also the best way of establishing a
controversy.
Tobacco’s “doubt” strategy
• The facts aren’t all in
• Science can’t tell us:
• How cigarettes cause cancer
• How many cigarettes you need to smoke before you get cancer
• Why some smokers get cancer and others don’t, and why some non-smokers get cancer
• Scientists don’t all agree
Doubt is their product• Tobacco• Tobacco - second-hand smoke• Ozone depletion• Acid rain• Vaccines• Tanning beds
• and, now, climate change32
Who should you believe?• As a society, we believe in expert opinion• If you’re sick you see a doctor• In a trial, we turn to expert witnesses
–recent, relevant experience• Expertise is narrow
–specialties in medicine–narrow expertise in trials–narrow expertise of faculty
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Skeptics in Texas: Zero
• There are ~100 climate scientists in TX• No one can name one skeptic• “Debates” require flying a skeptic in from out
of state• Demand debate!
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Skeptics in Texas: Zero
• There are ~100 climate scientists in TX• No one can name one skeptic• “Debates” require flying a skeptic in from out
of state• Demand debate!
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Things to remember
• Most of what we know about climate change comes from observations and simple physics
• Climate sensitivity: 2-4.5°C• We can expect a few degrees of warming over
the next century• A few degrees of warming is quite significant
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