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Why Earth is (Still) Getting Warmer

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

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Why Earth is (Still) Getting Warmer

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  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 warming

  3. Applying the Principles to Earth Why Earth is still warming

  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 warming

  5. The saga of planet keeling Why Earth is still warming

  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 Why Earth is still warming

  7. 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 Why Earth is still warming

  8. 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) Why Earth is still warming

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

  10. 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 Why Earth is still warming

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

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

  13. 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 Why Earth is still warming

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

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

  16. The Greenhouse Effect Why Earth is still warming

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

  18. John Tyndall Answers the First Question Why Earth is still warming

  19. 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 Why Earth is still warming

  20. Tyndall’s Experiment:Modern Version CO2 Infrared Camera Glass tube Person Full-spectrum radiation Reduced infrared component Why Earth is still warming

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

  22. Add CO2 to Tube– And More and More Why Earth is still warming

  23. 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 Why Earth is still warming

  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 warming

  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 warming

  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 warming

  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 warming

  28. Planet Keeling Mark III(preliminary edition) • Add CO2 • Surface warmed by • Unreflected solar radiation plus • Part of reemitted thermal radiation • Surface warmingcontinues until escaping thermal plus reflected solar equals incoming solar Why Earth is still warming

  29. Planet Keeling III (preliminary) Incoming Solar Escaping Thermal Outgoing Reflected = + Atmospheric CO2 Why Earth is still warming

  30. With Earth’s CO2concentration Keeling temperature still is lower than actual Earth temperature Why Earth is still warming

  31. 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 Why Earth is still warming

  32. Planet Keeling III (final) Incoming Solar Outgoing Thermal Outgoing Reflected = + Atmospheric CO2 and H2O Why Earth is still warming

  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 warming

  34. The discovery of global warming Why Earth is still warming

  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 warming

  36. Arrhenius asks the question Why Earth is still warming

  37. Arrhenius 1896Analysis • 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 Why Earth is still warming

  38. 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 Why Earth is still warming

  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 warming

  40. Roger Revelle Eliminates One Reason Why Earth is still warming

  41. 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 Why Earth is still warming

  42. Now We Need Measurements • Is atmospheric concentration of CO2 rising? • Are global temperatures rising? Why Earth is still warming

  43. CO2Concentration • 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 warming

  44. David Keeling Solves the Problems Why Earth is still warming

  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 warming

  46. The Keeling Curve 2013: 396 ppm CO2 and 478 ppm CO2eq 2005: Keeling dies NSF stops supporting “routine” research – Revelle to the rescue Why Earth is still warming

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

  48. CO2 Emissions: Man vs. Volcanoes Why Earth is still warming

  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 warming

  50. Berkeley Earth Project Reanalysis Why Earth is still warming

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