MET 12 Global Climate Change – Lecture 3

1. MET 12 Global Climate Change – Lecture 3 “Greenhouse Effect: Earth and Venus” Shaun Tanner San Jose State University • By the end of this chapter you should: • Understand the differences in the atmospheres of Venus and Earth • Know how a changing albedo can change climate • Understand the role of greenhouses gases and the runaway greenhouse effect

2. The Solar System • “The nine planets”

3. The Solar System “The nine planets” 3

4. Average Surface Temperature of Earth and Venus: Factors to consider • Which planet is closer to the Sun? • Which planet is more reflective? • Which planet has a larger ability to absorb and retain solar energy?

5. Source of Energy: The Sun Sun radiates energy; Energy “radiates out” with distance

6. Earth and Venus Based on solely the position of Venus and the Earth, which planet would be warmer? Venus Sunlight striking Venus is 93% stronger than sunlight striking Earth, thus we would expect Venus to be warmer Earth

7. Temperature Based Only on Distance to the Sun • Based only on distance to the sun, • Venus would be somewhat warmer than Earth • Average surface temperatures would be as follows if the planets absorbed all of the incoming sunlight: • Earth: 5°C • Venus: 55°C • However …

8. Venus and Earth do not absorb all of the incoming sunlight • Part of the incoming sunlight is reflected back into space • Earth reflects 30% • Venus reflects 70% • This leads to an important concept

9. Definition • Albedo: The fraction of light reflected by an object or a surface • White objects have high albedos • Black objects have low albedos Increasing albedo

10. Comparison of Earth and Venus Earth Venus Venus is completely covered by highly reflective clouds; Earth is partially covered by clouds. Which planet has a higher albedo? _______________ Venus

11. Comparison of Earth and Venus • Earth’s albedo is 30% • Venus’ albedo is 70% Earth absorbs 70 % of the sunlight that strikes it. Venus absorbs 30 % of the sunlight that strikes it. Let’s re-examine the surface temperatures with this information.

12. Absorption of Solar Radiation Taking into Account Distance to Sun and Albedo • Let intensity of sunlight striking Earth be denoted by 100 • In these units, intensity of sunlight striking Venus is 193 Earth absorbs 100 x 0.70 = 70 units Venus absorbs 193 x 0.30 =58 units Thus, Earth absorbs sunlight at a greater rate than Venus does!

13. Temperature Taking Into Account Distance to Sun and Albedo • If a planet’s mean surface temperature were determined solely by the rate of absorption of solar radiation, • Venus would be colderthan Earth.

14. Actual Surface Temperatures • Based on average measurements combining surface air temperature and sea surface temperatures: • Earth’s mean surface temperature is about 15°C • Venus’ mean surface temperature is 480°C! • So… • Venus absorbs less sunlight than Earth, yet • Venus’ surface is much hotter than Earth’s • Why?

15. Recap • Rate of absorption of sunlight can be calculated from outer space; distance to sun and albedo are all that are required • Surface temperature is not correctly predicted from rate of absorption of sunlight • What could cause such a difference? • The respective atmospheres of the Earth and Venus

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

17. Earth Venus Principal Atmospheric Gases Compare the amount of CO2 Unit: 1018 g

18. Comparison of Atmospheres Conclusions: • Venus has a much more massive atmosphere than Earth. • Venus’ atmosphere is mostly CO2. • The mass of CO2 in Venus’ atmosphere is almost 100 times greater than the total mass of Earth’s atmosphere.

19. Why is CO2 So Important? • Carbon Dioxide is a greenhouse gas. • Greenhouse gases are those gases that cause the greenhouse effect. • The greenhouse effect makes a planet’s surface temperature warmer than it would otherwise be. • The stronger the greenhouse effect, the warmer the surface (other factors being equal). • Consider the blanket analogy

20. Greenhouse Gases • Water Vapor • Carbon Dioxide • Methane (CH4) • Nitrous Oxide (N2O) • Ozone (O3) • CFC’s (chlorofluorocarbons)

21. Comparison of Greenhouse Effects • Venus’ atmosphere contains ~ 25, 000 times as much greenhouse gas as Earth’s! • So, Venus has a much stronger greenhouse effect than Earth. • Thus, Venus has a much hotter surface than Earth, despite the fact that Venus absorbs less sunlight than Earth.

22. What does this have to do with climate? • Greenhouse gases like CO2 play an important role in determining surface temperatures. • Changes in greenhouse gases, thus can potentially increase or decrease surface temperatures. • Therefore, the temperature of Earth has more to do with the Earth’s atmosphere than how close the Earth is to the sun.

23. Activity 1b Answer each question and turn in at the beginning of the next class • If the Earth’s albedo was to increase due to more clouds, how would the Earth’s temperature change? • Today, greenhouse gases like CO2 are increasing in the atmosphere. How would that would change the Earth’s temperature? • If increases in greenhouse gases act to produce more clouds, how would that change the Earth’s temperature (consider your answers to 1 and 2).