Earth’s Global Energy Balance Overview

1. Earth’s Global Energy Balance Overview • Electromagnetic Radiation • Radiation and temperature • Solar Radiation • Longwave radiation from the Earth • Global radiation balance • Geographic Variations in Energy Flow • Insolation over the globe • Net radiation, latitude and energy balance • Sensible and latent heat transfer

2. Overview • The global energy system • Solar energy losses in the atmosphere • Albedo • Counterradiation and the greenhouse effect • Global energy budgets of the atmosphere & surface • Climate & global change

3. What is light?

4. Light is an Electromagnetic Wave&a Particle Photons: “pieces” of light, each with precise wavelength, frequency, and energy.

5. Light as a Wave • For any wave (say, in the ocean) its speed: s = f (frequency)  (wavelength) • But the speed of light is a constant, c. • For light: f  = c • For light, the higher f is, the smaller  is, and vice versa. • Our eyes recognize f (or ) as color!

6. Light as a Particle • Light can also be treated as photons – packets of energy. • The energy carried by each photon depends on its frequency (color) • E = hf [“h” is called Planck’s Constant] • Bluer light carries more energy per photon.

7. Electromagnetic Radiation • Energy constantly emitted from every surface • Can be in many different forms, e.g. light or heat

8. Electromagnetic Spectrum

9. Temperature & Radiation • 1. Hotter objects radiate more energy than cooler • Radiation intensity = constant X T4 (Stefan-Boltzmann equation) • Double the temp of object, radiation intensity rises 16X • 2. Hotter objects radiate shorter wavelengths • Wein’s Law : wavelength = constant / temperature • Sun – surface temp 6000°C – emits shortwave radiation • Most Earth objects are cooler, emit longwave radiation

10. Solar Radiation • Shortwave Radiation from Sun (dark purple) • Absorption of UV by O3 • Absorption by CO2 and water vapor (H2O↑) shown as valleys • Longwave Radiation from Earth (dark red) • Much absorbed by CO2 & H2O↑

11. What causes the atmosphere to be opaque?

12. Scattering • Solar radiation can be scattered by atmosphere • Deflected off a molecule, cloud droplet, or particle • May go up toward space, or down toward Earth • Scattering most prevalent in blue wavelengths • Thus, clear, blue skies • Some solar radiation goes directly to surface • Called transmission • Solar radiation arrives as 0.3μm to 3μm wavelengths • This is shortwave radiation

13. Longwave Radiation • Energy emitted from Earth’s surface • 3-30 μm wavelength • Strong absorption by CO2 and H2O↑ • This absorption part of greenhouse effect • Longwave radiation is emitted by Earth back to space. • Thus, Earth’s temp remains fairly constant.

14. Remember you live on a rotating sphere

15. Geographic Variation in Solar Energy • Insolation – Incoming solar radiation • More intense where sun angle is highest • Less intense with lower sun angle • Same energy spread over a larger area

16. Insolation • Daily insolation – avg radiation total in 24 hours • Depends on : • Sun angle – higher sun angle → greater insolation • Length of day – higher latitudes get long summer days • Annual insolation – avg radiation total for year • Also depends on sun angle and length of day • Both of these determined by latitude • So, latitude determines annual insolation

17. Net Radiation • Energy not usually balanced at any location • Net Radiation - Difference between incoming and outgoing radiation • Between 40°N and 40°S, incoming > outgoing • Creates energy surplus • Poleward of 40°N & S, outgoing > incoming • Creates energy deficit • Deficit = Surplus, so net radiation for Earth = 0

18. Poleward Heat Transport • Surplus energy moves toward poles (deficit regions) • Carried by: • Warm, moist air • Warm sea water • Tropical cyclones • Poleward heat transport is driving force behind: • Global atmospheric circulation • Weather phenomena • Ocean currents

19. Why are there seasons? • The Earth is tilted 23.5° from it orbital plane • Combine tilt with orbit • Northern hemisphere gets more direct Sun part of year - summer • Southern hemisphere gets more direct Sun part of year – winter • Tilt & orbit create seasons, not distance to Sun

20. Northern Summer

21. Northern Winter

22. Solstices & Equinoxes

23. Path of the Sun in the Sky 40° North • June solstice: • Sun rises north of east & sets north of west • Peaks at 73.5° above horizon at noon • 15 hours of daylight • Highest daily insolation of year

24. Path of the Sun in the Sky (40° North)

25. Path of the Sun in the Sky (Equator)

26. Path of the Sun in the Sky (North Pole)

27. Daily Insolation through the Year • Yearly change in insolation greatest toward poles • In Arctic & Antarctic Circles, Sun is below horizon part of year • At Equator, 2 maxs & 2 mins for daily insolation • At equinoxes & solstices • Between tropics, also 2 maxs & 2 mins per year • Yearly insolation change important to climate

28. Annual Insolation by Latitude • Tilted Earth shown as red line • Equator greatest annual insolation • Considerable insolation at highest latitudes • Untilted Earth (blue line) • Equator greatest annual insolation • Highest latitudes little insolation • Big changes in climate • Very cold pole • Massive poleward heat transport

29. Sensible & Latent Heat Transfer Conduction • Surplus energy is transported in two forms: • Sensible Heat – can be felt & measured • Transferred by conduction (touching surface) • Transferred by convection (carried by rising air) • Latent Heat – cannot be felt or measured • Stored as molecular motion when water changes phase • Absorbed in evaporation, melting, and sublimation • Released in condensation, freezing, and deposition • Very important form of heat transfer over long distances Convection Latent heat absorbed in evaporation

30. Global Energy System • Solar energy losses in the atmosphere • Scattering due to: • Gas molecules • Dust or other particles • O2, O3, & H2O↑ most important absorbers of insolation • Global avg – 49% of insolation makes it to surface

31. Albedo • Proportion of shortwave radiation reflected • Shown as a proportion (0-1) • Examples: • Snowfield 0.45-0.85 • Black pavement 0.03 • Clouds 0.30-0.60 • Water (calm, high angle 0.02), (low angle 0.80) • Avg for Earth and atmosphere 0.29-0.34

32. Counterradiation • Counterradiation – heat absorbed by atmosphere reflected down to surface A – energy radiated to space from surface B – energy from surface absorbed by atmosphere C – energy radiated to space from atmosphere D – Counterradiation

33. Greenhouse Effect • Longwave radiation absorbed & re-radiated to surface by atmosphere • Lower atmosphere acts like blanket

34. Global Energy Budget Energy balanced for each level: surface, atmosphere, & space

35. Global Radiation Balance Shortwave radiation absorbed by Earth’s atmosphere & surface… Roughly equals…

36. Climate & Global Change • Quantifying human impacts on climate difficult • Climate and society have complex relationship • e.g., Industrial processes • add CO2 to atmosphere (warming) • add aerosols to atmosphere (cooling)