What is the Greenhouse Effect?

1. What is the Greenhouse Effect?

2. Review of last lecture • The two basic motions of the Earth • What causes the four seasons: the Earth’s tilt and the 3 ways it affects the solar insolation (change of length of the day, beam spreading, beam depletion) • Change of the Earth’s orbit at longer time scales (Milankovitch cycles): eccentricity, axial tilt, and precession

3. Satellite Measurements of the Earth’s Radiation Budget NASA’s Earth Radiation Budget Satellite (ERBS) 1985-1989

4. Earth’s energy budget (averaged over the whole globe and over a long time) Yellow: shortwave Red: longwave • At the top of the atmosphere: Incoming shortwave = Reflected Shortwave + Emitted longwave • At the surface: Incoming shortwave = Reflected shortwave + Net emitted longwave (emitted - incoming) + Latent heat flux + sensible heat flux Sensible heat 7% Net Longwave 21% Latent heat 23%

5. Atmospheric influences on radiation Absorption (absorber warms) Reflection Scattering

6. Reflection • reflection – redirection of energy w/o absorption • all objects reflect visible light  effectiveness varies • albedo – % of visible light reflected • There are two types of reflection (solid surface): • Specular: light is reflected with equal intensity (e.g. mirror) • Diffuse reflection OR scattering: light is reflected in multiple directions, weakly (e.g. snow)

8. Atmospheric Scattering 3 Types of Scattering: • Raleigh • Mie • Non-Selective A discussion of each type follows…

9. Rayleigh Scattering • involves gases smaller than insolation wavelength • scatters light in all directions • most effective at short wavelengths (violet, blue)… hence, blue sky The Earth has an atmosphere. So it has Rayleigh scattering and its sky appears blue The Moon has no atmosphere. So it has no Rayleigh scattering and its sky appears dark

10. Rayleigh scattering also explains reddish-orange sunsets when light travels through thick slice of atmosphere

11. Monet: Impressions, Sunrise

12. Grainstacks at different time of the day

13. 2) Mie scattering • involves aerosols (e.g. dust, smoke) larger than gas molecules • forward scatter • equally effective across visible spectrum • explains hazy, gray days

14. 3) Non-selective scattering • Happens when atmospheric particles are much larger than the wavelength of incoming radiation (e.g. water droplets in clouds) • Act like lenses; scatter all wavelengths equally to create a white appearance • That’s why clouds appear white

15. Video: The greenhouse effect • http://www.youtube.com/watch?v=ZzCA60WnoMk

16. Atmospheric Absorption - The Greenhouse Effect Transparent to solar (shortwave) radiation Opaque to earth’s (longwave) radiation Major GH gases: CO2, H20(v), CH4

17. The importance of methane (CH4) • 23 times more powerful as a greenhouse gas than CO2 • The livestock sector is a major player, which accounts for 35-40% global anthropogenic emissions of methane (their burps!) • The livestock sector is responsible for 18% of total greenhouse gas emissions • Therefore, consuming less meat can help reducing global warming than not driving cars.

18. Seasonal variation of surface radiation

19. Surface “Sensible” and “Latent” heat transfers First, recall 2 other methods of energy transfer in addition to radiation: • Conduction • This is how excess heat in ground is transferred to the atmosphere via an extremely thin layer of air in contact with the surface • Convection • Once the heat is transferred from the surface to the air via conduction, convection takes over from here via “sensible” and “latent” heat transfers

20. Sensible Heat • Heat energy which is readily detected • Magnitude is related to an object’s specific heat • The amount of energy needed to change the temperature of an object a particular amount in J/kg/K • Related to mass • Higher mass requires more energy for heating • Sensible heat transfer occurs from warmer to cooler areas (i.e., from ground upward)

21. Latent Heat • Energy required to induce changes of state in a substance • In atmospheric processes, invariably involves water • When water is present, latent heat of evaporation redirects some energy which would be used for sensible heat • Wet environments are cooler relative to their insolation amounts • Latent heat of evaporation is stored in water vapor • Released as latent heat of condensation when that change of state is induced • Latent heat transfer occurs from regions of wetter-to-drier

22. Seasonal variation of surface energy budget Storage change = net radiation - latent heat flux - sensible heat flux

23. Summary • Earth’s energy balance at the top of the atmosphere and at the surface. What percentage of solar energy is absorbed by the surface? • Atmospheric influences on radiation (3 ways) • The three types of atmospheric scattering. What causes the blue sky? Why causes the reddish-orange sunsets? • What cause the greenhouse effect? What are the major greenhouse gases? Why is methane important? • Sensible heat flux (dry flux from warm to cold regions) and latent heat flux (wet flux from wet to dry regions)