Planetary Surface Temperature

1. Planetary Surface Temperature What determines the mean surface temperature of a planet?

2. Planetary Surface Temperature • Source of energy: the sun

3. Sun radiates energy; Energy diminishes with distance Sun (Math: E ~ 1/r2)

4. Albedo The fraction of light reflected by an object. Increasing albedo

5. Albedo Earth Venus Albedo = 70% Albedo = 30%

6. Earth Nitrogen (N2) Oxygen (O2) Argon (Ar) Water Vapor (H2O) Carbon Dioxide (CO2) Venus Carbon Dioxide Nitrogen Atmospheres of Earth and Venus (Gases)

7. Earth Venus The Numbers GAS MASS ~500, 000(96%) CO2 N2 ~20, 000 (4%) TOTAL 520, 000 Compare! Unit: 1018 g

8. Main Conclusion Venus’s atmosphere contains an enormous amount of CO2 CO2 in Venus’ atmosphere Entire mass of Earth’s atmosphere

9. Comparison of Greenhouse Effects • Venus’ atmosphere contains much more greenhouse gas than Earth’s! • Thus, Venus has a much stronger greenhouse effect than Earth. • Explains why Venus is so much hotter than Earth.

10. Hypothesis • Surface temperature determined by • Distance to sun • Albedo • Amount of greenhouse gas in atmosphere

11. Electromagnetic Radiation • All objects emit radiation. • Warmer objects emit more radiation than cold objects. • Warmer objects emit at shorter wavelengths than cold objects.

12. Solar Radiation (Sunlight) • Solar radiation is mostly visible (VIS) and IR (infrared) (around 90%) • Less than 10% is ultraviolet (UV) • Order of wavelengths: UV < VIS < IR

13. Solar Radiation Unit: 1 m = 0.000001 m

14. Planetary Radiation • Planets also emit radiation

15. Solar vs. Planetary Radiation • The sun is much hotter than the planets • Therefore…

16. Absorption of Radiation by Atmospheric Gases • Little aborption of shortwave radiation occurs • Longwave radiation is strongly absorbed by greenhouse gases • Other gases do not absorb longwave radiation

17. Absorption of Radiation in Earth’s Atmosphere Fig. 2.9 From Essentials of Meteorology, by C. Donald Ahrens

18. Kirchhoff’s Law • A good absorber is a good emitter • Thus, greenhouse gases also strongly emitlongwave radiation

19. This radiation escapes to space Some radiation escapes to space Greenhouse gasesemit longwave upward and downward Absorption in atmosphere Longwave radiation is emitted from surface. Surface absorbs downward longwave

20. Other Methods of Vertical Heat Transport • Energy is also exchanged between the surface and the atmosphere by • Conduction • Convection

21. Earth’s Energy Balance – don’t memorize!

22. Energy Fluxes and Temperature • Look at surface energy fluxes • If fluxes equal, no change in temperature Earth’s Surface

23. Effect of Clouds • Clouds absorb and emit longwave • Removing clouds would have a cooling effect • Clouds reflect shortwave • Removing clouds would have a warming effect • Net effect of removing clouds: slight warming • Conclusion: Clouds have a net cooling effect

24. Age of the Earth • Accepted age of Earth: 4.5 billion years • Method: Radiometric Dating

25. The Human Presence on Earth • Earliest evidence for human beings: ~ 100, 000 years ago (Very late Cenozoic Era) Age of Earth Human presence

26. Earth’s Temperature over Geologic Time • Mostly warmer than today • Periods of extreme warmth • Periods of extreme cold

27. Time Scales -- 1 • Time scale: The time required for a significant change to occur

28. Climate and Life -- 1 • Slow processes (e.g., millions of years) can affect evolution of life • Example • 10 million years ago, Africa covered by forests • 5 million years ago, climate drier • Forests replaced by grassland • New species appeared, including distant ancestors of human beings

29. 2 • Faster climate changes can affect human and animal populations • Example • North Africa was wet 9, 000 years ago • People grew crops, fished from lakes • Hippos, giraffes, etc. lived • Climate became drier over next 6, 000 years, turned into desert (Sahara) • Farming became concentrated along Nile; many people became nomadic • Animals migrated southward

30. Determination of Past Global Temperatures • For last ~150 years: • Meteorological records, i.e., thermometer readings • Before that, need “proxy” climate data (“proxy” is Latin for “substitute”)

31. Ice as a Proxy • Basic idea: More global ice  colder Earth

32. Glaciers • Large masses of continental ice • Formed by long-term accumulation of snow • Requires: annual snowfall > annual snow melt • Area must be cold and get lots of snow • Very large glaciers called ice sheets • Antarctica and Greenland

33. Sea Ice • Frozen sea water • Not made from snow

34. Ice Today • Most ice is on Antarctica (90%) and Greenland (10%) (high latitudes) • In lower latitudes, glaciers exist mainly at high elevation

35. Glacial Evidence • Glaciers leave evidence behind • Examples: • moraines • erratic boulders • glacial striations (scratched rocks) • dropstones

36. Preservation • Glacial evidence can be preserved for a very long time • Gives us a “glacial history” going back hundreds of millions of years

37. No ice Lots of ice No ice

38. Early Archaean • No evidence for ice • Interpretation: Earth was fairly warm

39. The Faint-Sun Paradox • Geologic evidence  Earth was warm in early Archaean • Astrophysics theory  the early Archaean sun was much weaker than today’s sun

40. Most Likely Resolution of the Paradox • There was much more greenhouse gas in the early atmosphere than in today’s. • (Originally proposed by Carl Sagan.)

41. Early CO2 • Early atmosphere probably contained much moreCO2than today’s atmosphere • The stronger greenhouse effect compensated for the weaker sun

42. Question • Why does CO2 concentration change? • Answer depends on time scale • We will first look at very long time scales, i.e., millions of years

43. Where is the Carbon? • Carbonates: Compounds containing carbon and oxygen • Prime example: Calcium Carbonate (CaCO3)

44. Putting CO2 into atmosphere Atmospheric CO2 Carbonates

45. Formation of Calcium Carbonate • Carbonates are formed by Silicate-to-Carbonate Conversion

46. Silicates • Compounds containing silicon and oxygen • Example: calcium silicate, CaSiO3 • (Si = silicon) CaSiO3 + CO2 + H2O CaCO3 + SiO2 + H2O

47. Long-Term Carbon Cycle • Silicate-to-carbonate conversion removes CO2 from atmosphere • Volcanoes put CO2 into atmosphere

48. Geologic Control of Atmospheric CO2 Concentration

49. CO2 flux imbalances Atmosphere (CO2) Volcanic emissions Silicate-to-carbonate conversion Carbonates What would happen to atmospheric CO2? Increase.

50. CO2 flux imbalances Atmosphere (CO2) Volcanic emissions Silicate-to-carbonate conversion Carbonates What would happen to atmospheric CO2? Decrease.