Why Earth is (Still) Getting Warmer Northern Virginia Community College Annandale Science Seminar March 21, 2014

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


Applying the Principles to Earth


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?


THE SAGA OF PLANET KEELING


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


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


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)


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


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


Equilibrium for Planet Keeling IIncoming

Solar

Outgoing

Thermal

=


Good (19th century) Physics But Wrong Answer for Earth:

Not Warm Enough


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


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


The Model is More RealisticBut the Answer is More Wrong:

Earth Isn’t a Snowball


THE GREENHOUSE EFFECT


The Missing Piece

• Fourier (1824): Atmosphere must trap heat

• But what part of atmosphere does this

• And how does it do it?


John Tyndall Answers the First Question


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


Tyndall’s Experiment:Modern Version

Infrared Camera Glass tube Person

CO2

Full-spectrum radiation

Reduced infrared component


Infrared Image of Face with Normal Atmosphere


Add CO2 to Tube– And More and More


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


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


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


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


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


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


Planet Keeling III (preliminary)Incoming

SolarEscaping

Thermal

=

Outgoing

Reflected

+Atmospheric

CO2


With Earth’s CO2 concentrationKeeling temperature still is lower than actual

Earth temperature


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


Planet Keeling III (final)Incoming

SolarOutgoing

Thermal

=

Outgoing

Reflected

+AtmosphericCO2 and H2O


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


THE DISCOVERY OF GLOBAL WARMING


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


Arrhenius asks the question


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


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


Two Reasons not to Worry

• Most emissions would be absorbed by oceans

• In any event, large emissions far in the future


Roger Revelle Eliminates One Reason


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


Now We Need Measurements

• Is atmospheric concentration of CO2 rising?

• Are global temperatures rising?


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


David Keeling Solves the Problems


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


The Keeling Curve

NSF stops supporting “routine” research – Revelle to the rescue

2005: Keeling dies

2013: 396 ppm CO2 and 478 ppm CO2eq


The Keeling Carbon Cycles

• Biological: – Dominates intra-annual cycle– Little interannual impact

• Fossil fuels dominate interannual changes


CO2 Emissions: Man vs. Volcanoes


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


Berkeley Earth Project Reanalysis


THE CURRENT ISSUE:GLOBAL WARMING HAS/HAS NOT STOPPED/PAUSED/SLOWED


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


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


Potential Offsets

• Reduced solar radiation

• Increased aerosols from small volcanoes and/or Asian factories

• Contribution likely but probably modest


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


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


http://www.realclimate.org/images/Cowtan.png


The Oceans’ Role:Distribution of Additional Thermal Energy

AtmosphereContinentsIceOceans




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


Robert C. Means

[email protected]

Documents

Why Earth is (Still) Getting Warmer Northern Virginia Community College Annandale Science Seminar March 21, 2014