Why Earth is (Still) Getting Warmer
Northern Virginia Community CollegeAnnandale Science Seminar
March 21, 2014
Why Earth is still warming 2
Basic Principles
• Temperature measures kinetic energy of system’s molecules or atoms: thermal energy
• Energy gain ≠ energy loss: temperature change
• Gain = loss: equilibrium temperature
Why Earth is still warming 3
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
0 20 40 60 80100
120140
160180
200220
240260
280300
-200
-100
0
100
200
300
400
500
Incoming Outgoing Radiative Forcing
Keeling Surface Temperature
Radi
ation
6o C
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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 IIncoming
Solar
Outgoing
Thermal
=
Why Earth is still warming 12
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
0 20 40 60 80100
120140
160180
200220
240260
280300
-300
-200
-100
0
100
200
300
400
500
Incoming OutgoingRadiative Forcing Unreflected solar
Keeling Surface Temperature
Radi
ation
-18 C
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The Model is More RealisticBut 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
Infrared Camera Glass tube Person
CO2
Full-spectrum radiation
Reduced infrared component
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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
SolarEscaping
Thermal
=
Outgoing
Reflected
+Atmospheric
CO2
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With Earth’s CO2 concentrationKeeling 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
SolarOutgoing
Thermal
=
Outgoing
Reflected
+AtmosphericCO2 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
NSF stops supporting “routine” research – Revelle to the rescue
2005: Keeling dies
2013: 396 ppm CO2 and 478 ppm CO2eq
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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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1880 1889 1898 1907 1916 1925 1934 1943 1952 1961 1970 1979 1988 1997 2006
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
Who Did It?NOAA 2/14 Download
1998: 0.63o
2010: 0.66o
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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
AtmosphereContinentsIceOceans
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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