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

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Presentation on theme: "Why Earth is (Still) Getting Warmer Northern Virginia Community College Annandale Science Seminar March 21, 2014."— Presentation transcript:

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

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 warming2

3 Applying the Principles to Earth Why Earth is still warming3

4 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? Why Earth is still warming4

5 THE SAGA OF PLANET KEELING Why Earth is still warming5

6 6 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

7 Why Earth is still warming7 Keeling Arrives Captured by Sun-like star in near-circular orbit of 150 million km radius Incoming solar radiation: – Cross-section: 1366 watts/meter 2 – Average for Keeling’s surface: 342 watts/meter 2 Initial outgoing radiation: ~none Radiative forcing: 342 watts/meter 2

8 Why Earth is still warming8 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)

9 The Path to Equilibrium Why Earth is still warming9 6 o C

10 For the Curious Stefan-Boltzmann (1870s): radiation = σT 4 Radiation is in w/m 2 Temperature is in degrees Kelvin (centigrade + 273) σ = 5.67 x 10 -8 watts Task: Solve for T for σT 4 = 342 Some algebra: T 4 = 342/(5.67 x 10 -8 ) And more : T = (342/5.67) 1/4 x (10 8 ) 1/4 = 279 o K = 6 o C Why Earth is still warming10

11 Why Earth is still warming11 Equilibrium for Planet Keeling I Incoming Solar Outgoing Thermal =

12 Why Earth is still warming12 Good (19 th century) Physics But Wrong Answer for Earth: Not Warm Enough

13 Why Earth is still warming13 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

14 The New Path to Equilibrium Why Earth is still warming14 -18 C

15 Why Earth is still warming15 The Model is More Realistic But the Answer is More Wrong: Earth Isn’t a Snowball

16 THE GREENHOUSE EFFECT Why Earth is still warming16

17 Why Earth is still warming17 The Missing Piece Fourier (1824): Atmosphere must trap heat But what part of atmosphere does this And how does it do it?

18 John Tyndall Answers the First Question Why Earth is still warming18

19 Tyndall’s 1859 Experiment Determines what part of atmosphere traps heat Oxygen and nitrogen (99% of atmosphere) are transparent to infrared radiation Water, CO 2 and methane (CH 4 ) are not Why Earth is still warming19

20 Tyndall’s Experiment: Modern Version Why Earth is still warming20 Infrared CameraGlass tubePerson CO 2 Full-spectrum radiation Reduced infrared component

21 Why Earth is still warming21 Infrared Image of Face with Normal Atmosphere

22 Why Earth is still warming22 Add CO 2 to Tube – And More and More

23 The Second Question: How do these gases do it? For Tyndall: an empirical result Explanation requires 20 th century physics – Electromagnetic radiation – Interaction of radiation and gas molecules Why Earth is still warming23

24 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 Why Earth is still warming24

25 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 Why Earth is still warming25

26 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 Why Earth is still warming26

27 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 Why Earth is still warming27

28 Planet Keeling Mark III (preliminary edition) Add CO 2 Surface warmed by – Unreflected solar radiation plus – Part of reemitted thermal radiation Surface warming continues until escaping thermal plus reflected solar equals incoming solar Why Earth is still warming28

29 Why Earth is still warming29 Planet Keeling III (preliminary) Incoming Solar Escaping Thermal = Outgoing Reflected + Atmospheric CO 2

30 Why Earth is still warming30 With Earth’s CO 2 concentration Keeling temperature still is lower than actual Earth temperature

31 Just Add Water Water vapor is a greenhouse gas: roughly doubles warming effect of CO 2 But a feedback: atmospheric water vapor determined by temperature CO 2 is the control knob: without it, planet’s water vapor would be surface ice Why Earth is still warming31

32 Why Earth is still warming32 Planet Keeling III (final) Incoming Solar Outgoing Thermal = Outgoing Reflected + Atmospheric CO 2 and H 2 O

33 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 Why Earth is still warming33

34 THE DISCOVERY OF GLOBAL WARMING Why Earth is still warming34

35 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 Why Earth is still warming35

36 Arrhenius asks the question Why Earth is still warming36

37 Arrhenius 1896 Analysis His interest: What caused global glaciation? Perhaps fewer volcanic eruptions: Less CO 2  cooler  less water vapor  still cooler To be complete, looks at reverse: more eruptions  more CO 2  more water vapor  warmer world Why Earth is still warming37

38 Quantifies Impact of Doubling CO 2 Science: CO 2 and water vapor feedback Data: atmospheric absorption of thermal radiation from Langley experiments Tools: pen and paper Result: 5 o -6 o C warming Moral: Plausible result with 19 th century science and no computer model Why Earth is still warming38

39 Two Reasons not to Worry Most emissions would be absorbed by oceans In any event, large emissions far in the future Why Earth is still warming39

40 Roger Revelle Eliminates One Reason Why Earth is still warming40

41 Revelle’s 1957 Experiment Grant to study ocean chemistry – Expects to find that ocean absorbs CO 2 – It does but (surprise) most pops back out again But still expects slow growth in emissions – no need for dramatic actions Why Earth is still warming41

42 Now We Need Measurements Is atmospheric concentration of CO 2 rising? Are global temperatures rising? Why Earth is still warming42

43 CO 2 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 Why Earth is still warming43

44 David Keeling Solves the Problems Why Earth is still warming44

45 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 Why Earth is still warming45

46 The Keeling Curve Why Earth is still warming46 NSF stops supporting “routine” research – Revelle to the rescue 2005: Keeling dies 2013: 396 ppm CO 2 and 478 ppm CO 2eq

47 The Keeling Carbon Cycles Biological: – Dominates intra-annual cycle – Little interannual impact Fossil fuels dominate interannual changes Why Earth is still warming47

48 CO 2 Emissions: Man vs. Volcanoes Why Earth is still warming48

49 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 Why Earth is still warming49

50 Berkeley Earth Project Reanalysis Why Earth is still warming50


52 Why Earth is still warming52 1998: 0.63 o 2010: 0.66 o

53 The Deductive Path Assume: – CO 2 is a greenhouse gas – Its atmospheric concentration continues to rise – NOAA/NASA/Hadley/Berkeley properly integrate temperature readings Then, CO 2 ’s effect is being offset, or An increased share of thermal energy is outside temperature readings’ spatial coverage Why Earth is still warming53

54 Potential Offsets Reduced solar radiation Increased aerosols from small volcanoes and/or Asian factories Contribution likely but probably modest Why Earth is still warming54

55 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 Why Earth is still warming55

56 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 Why Earth is still warming56

57 Why Earth is still warming57

58 The Oceans’ Role: Distribution of Additional Thermal Energy Why Earth is still warming58

59 Why Earth is still warming59

60 Why Earth is still warming60

61 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 Why Earth is still warming61

62 Why Earth is still warming62 Robert C. Means

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