Why Earth is (Still) Getting Warmer
Northern Virginia Community College
Annandale Science Seminar
March 21, 2014
Basic Principles
• Temperature measures kinetic energy of
system’s molecules or atoms: thermal energy
• Energy gain ≠ energy loss: temperature
change
• Gain = loss: equilibrium temperature
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Applying the Principles to Earth
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The Beginning of an Explanation
• Earth gains/loses energy only through radiation
– Incoming solar radiation
– Outgoing reflected solar and Earth’s thermal radiation
• It is warming because incoming radiation
exceeds outgoing (positive radiative forcing)
• WHY?
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THE SAGA OF PLANET KEELING
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Long ago in a galaxy far away
• Planet Keeling ejected from home system
• No atmosphere or internal heat source
• Dull black – reflects nothing
• No atmosphere
• Cools to ~absolute zero during journey
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Keeling Arrives
• Captured by Sun-like star in near-circular orbit of
150 million km radius
• Incoming solar radiation:
– Cross-section: 1366 watts/meter2
– Average for Keeling’s surface: 342 watts/meter2
• Initial outgoing radiation: ~none
• Radiative forcing: 342 watts/meter2
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Now What?
The Path to Equilibrium
• Radiative forcing adds energy  temperature
rises
• Higher temperature  outgoing thermal
radiation  less radiative forcing
• Equilibrium temperature: equal incoming and
outgoing radiation (no radiative forcing)
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The Path to Equilibrium
500
6o C
400
Radiation
300
200
100
0
0
40
80
120
160
200
240
280
-100
-200
Keeling Surface Temperature
Incoming
Outgoing
Radiative Forcing
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For the Curious
• Stefan-Boltzmann (1870s): radiation = σT4
• Radiation is in w/m2
• Temperature is in degrees Kelvin (centigrade + 273)
• σ = 5.67 x 10-8 watts
• Task: Solve for T for σT4 = 342
• Some algebra: T4 = 342/(5.67 x 10-8)
• And more : T = (342/5.67)1/4 x (108)1/4 = 279o K = 6o C
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Equilibrium for Planet Keeling I
Incoming
Outgoing
Solar
Thermal
=
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Good (19th century) Physics
But Wrong Answer for Earth:
Not Warm Enough
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Planet Keeling Mark II: Add Albedo
• Reflects 30% of solar radiation (0.3 albedo)
• Solar radiation now balanced by:
– Keeling’s own thermal radiation plus
– Reflected solar radiation
• Equilibrium requires less thermal radiation 
reached at lower temperature
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The New Path to Equilibrium
500
-18 C
400
300
Radiation
200
100
0
0
40
80
120
160
200
240
280
-100
-200
-300
Incoming
Keeling Surface Temperature
Outgoing
Radiative Forcing
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Unreflected solar
14
The Model is More Realistic
But the Answer is More Wrong:
Earth Isn’t a Snowball
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THE GREENHOUSE EFFECT
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The Missing Piece
• Fourier (1824): Atmosphere must trap heat
• But what part of atmosphere does this
• And how does it do it?
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John Tyndall Answers the First Question
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Tyndall’s 1859 Experiment
• Determines what part of atmosphere traps
heat
• Oxygen and nitrogen (99% of atmosphere) are
transparent to infrared radiation
• Water, CO2 and methane (CH4) are not
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Tyndall’s Experiment:
Modern Version
CO2
Infrared Camera
Glass tube
Reduced infrared
component
Person
Full-spectrum
radiation
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Infrared Image of Face with Normal Atmosphere
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Add CO2 to Tube
– And More and More
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The Second Question:
How do these gases do it?
• For Tyndall: an empirical result
• Explanation requires 20th century physics
– Electromagnetic radiation
– Interaction of radiation and gas molecules
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1. Electromagnetic Radiation
• Includes X-rays, visible light, thermal radiation,
radio waves
• Behaves as particles (photons) and waves
• High/low-energy photons = short/long
wavelength
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Sun and Earth
• Source temperature determines photon energy/
wavelength
• Incoming solar radiation: high energy
photons/short wavelength
• Outgoing radiation:
– Reflected solar radiation: unchanged
– Earth’s thermal radiation: low energy photons/long
wavelength
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2. Molecules and Greenhouse Gases
• Molecules can only occupy certain energy
states
• Absorb photons if energy matches interval
between available states
• Greenhouse gases: Gases with intervals
matching energy of Earth’s thermal radiation
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Then What?
• Molecule drops back to original energy level
• Emits photon in random direction
• Further collisions repeat process
• Molecular bumper cars ends when photon
escapes into space OR strikes Earth’s surface
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Planet Keeling Mark III
(preliminary edition)
• Add CO2
• Surface warmed by
– Unreflected solar radiation plus
– Part of reemitted thermal radiation
• Surface warming continues until escaping
thermal plus reflected solar equals incoming
solar
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Planet Keeling III (preliminary)
Incoming
Escaping
Outgoing
Solar
Thermal
Reflected
=
+
Atmospheric
CO2
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With Earth’s CO2 concentration
Keeling temperature still is lower than actual
Earth temperature
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Just Add Water
• Water vapor is a greenhouse gas: roughly
doubles warming effect of CO2
• But a feedback: atmospheric water vapor
determined by temperature
• CO2 is the control knob: without it, planet’s
water vapor would be surface ice
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Planet Keeling III (final)
Incoming
Outgoing
Outgoing
Solar
Thermal
Reflected
=
+
Atmospheric
CO2 and H2O
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Equilibrium
• Equilibrium reached in all cases: Keeling I, II, III
(preliminary and final)
• Difference: temperature needed to reach it
• Lower if part of incoming solar is reflected
• Higher if only part of surface thermal radiation
escapes into space
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THE DISCOVERY OF GLOBAL WARMING
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1890s Setting
• Basic science in place for explaining Earth’s
temperature
• Implicitly also an explanation of how
temperature could change
• But someone had to ask the question
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Arrhenius asks the question
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Arrhenius 1896 Analysis
• His interest: What caused global glaciation?
• Perhaps fewer volcanic eruptions: Less CO2
 cooler  less water vapor  still cooler
• To be complete, looks at reverse: more
eruptions  more CO2  more water vapor
 warmer world
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Quantifies Impact of Doubling CO2
• Science: CO2 and water vapor feedback
• Data: atmospheric absorption of thermal
radiation from Langley experiments
• Tools: pen and paper
• Result: 5o-6o C warming
• Moral: Plausible result with 19th century science
and no computer model
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Two Reasons not to Worry
• Most emissions would be absorbed by oceans
• In any event, large emissions far in the future
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Roger Revelle Eliminates One Reason
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Revelle’s 1957 Experiment
• Grant to study ocean chemistry
– Expects to find that ocean absorbs CO2
– It does but (surprise) most pops back out again
• But still expects slow growth in emissions – no
need for dramatic actions
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Now We Need Measurements
• Is atmospheric concentration of CO2 rising?
• Are global temperatures rising?
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CO2 Concentration
• Changed by difference between flows into/out of
atmosphere
• Early 1950s: no consensus on size (or even sign)
of difference
• The problems
– Lack of continuous measurements
– Existing measurements affected by local sources and
sinks
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David Keeling Solves the Problems
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His Solution
• Monitor at isolated location: Mauna Loa
– Observatory at 3,397 meters
– No nearby industry or motorized vehicle traffic
– In barren lava field
• Monitor continuously
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The Keeling Curve
2013: 396 ppm CO2
and 478 ppm CO2eq
2005:
Keeling dies
NSF stops supporting
“routine” research –
Revelle to the rescue
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The Keeling Carbon Cycles
• Biological:
– Dominates intra-annual cycle
– Little interannual impact
• Fossil fuels dominate interannual changes
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CO2 Emissions: Man vs. Volcanoes
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Temperature Milestones: 1980-2012
• ~1980: NASA, NOAA and Hadley Center develop
estimates of global temperature from 1880
• 1998-99: Mann and co-authors estimate
temperatures from 1000 AD primarily from tree
rings
• 2011-2012: Berkeley Earth Project reanalysis
largely confirms NOAA/NASA/Hadley estimates
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Berkeley Earth Project Reanalysis
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THE CURRENT ISSUE:
GLOBAL WARMING HAS/HAS NOT
STOPPED/PAUSED/SLOWED
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Who Did It?
NOAA 2/14 Download
2010: 0.66o
0.8
0.6
1998: 0.63o
0.4
0.2
0
1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
-0.2
-0.4
-0.6
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The Deductive Path
• Assume:
– CO2 is a greenhouse gas
– Its atmospheric concentration continues to rise
– NOAA/NASA/Hadley/Berkeley properly integrate
temperature readings
• Then, CO2’s effect is being offset, or
• An increased share of thermal energy is outside
temperature readings’ spatial coverage
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Potential Offsets
• Reduced solar radiation
• Increased aerosols from small volcanoes
and/or Asian factories
• Contribution likely but probably modest
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Distribution of Earth’s Thermal Energy
• Vertical: surface and upper atmosphere
– Measurement issue but little effect
• Horizontal: distribution over surface
– The no-Mauna Loa problem
• Vertical: surface and deep ocean
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Horizontal Distribution:
The Arctic Hole
• The Arctic weather station gap
– Hadley: assumes Arctic temperature changes at
global rate
– NASA: fills gaps by interpolation
– NOAA: like Hadley?
• Cowtan and Way (2013)
– Begin with satellite data
– Develop algorithm for relating satellite data to surface
temperature
– Test algorithm for known surface temperatures
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The Oceans’ Role:
Distribution of Additional Thermal Energy
Atmosphere
Continents
Ice
Oceans
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The Current State of Play:
A Tentative Assessment
• Slowdown is less pronounced than standard
data series indicate
• Part of slowdown is due to quieter sun and
increased aerosols
• Increased warming of deep ocean is largest
contributor
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Robert C. Means
Robert.means@starpower.net
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