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
Welcome to Lecture 2. Sorry to miss the first class, but I expect you’ll understand that I’m attending a conference on climate change and hope to learning something interesting that I can share with our class.
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”
Ok, we are going to begin our understanding of our planet, by comparing some basic processes of Earth with another planet, Venus.

3. The Solar System “The nine planets”
Ok, we are going to begin our understanding of our planet, by comparing some basic processes of Earth with another planet, Venus. 3 3

4. Average Surface Temperature of Earth and Venus: Factors to consider
What if we wanted to determine which planet was warmer, Earth or Venus. What factors would we consider?

5. 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?
Right!

6. Source of Energy: The Sun
Sun radiates energy; Energy “radiates out” with distance
Let’s think about how the sun heats both the sun and earth…

7. Earth and Venus
Based on solely the position of Venus and the Earth, which planet would be warmer?
Venus
Earth
Imagine that the temperature of Venus/Earth would only determined by the distance to the earth. If each arrow represents the intensity of sunlight, and since Venus is closer to the sun than the earth…

8. 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
Ok…

9. Temperature 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 …
So, this should make sense. Since Venus is closer to the earth, it should be warmer than the Earth.
But, as you will see in this class (and in life in general), things are not always as simple as they first seem…

10. Albedo Venus and Earth do not absorb all of the incoming sunlight
Part of the incoming sunlight is reflected back into space
Earth reflects ______
Venus reflects ______
Which brings up a definition …

11. 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

12. Albedo: The fraction of light reflected by an object or a surface
White objects have ___________
Black objects have ___________
This definition is very important and straightforward. The lighter in color an object, the more energy (light) the object reflects. This is why dark objects absorb more energy (a black coat) compared to white colored objects (white coat).

13. Albedo: The fraction of light reflected by an object or a surface
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

14. Comparison of Earth and Venus
Ok, test yourself with this question?
Venus
Venus is completely covered by highly reflective clouds; Earth is partially covered by clouds. Which planet has a higher albedo? _______________

15. Comparison of Earth and Venus
You got it!
Oh, in a couple of weeks, we’ll be using the clickers for questions like this, so I can tell if we all really did get it right.
Venus
Venus is completely covered by highly reflective clouds; Earth is partially covered by clouds. Which planet has a higher albedo? _______________
Venus

16. Comparison of Earth and Venus
Earth’s albedo is ____
Venus’ albedo is ____
Earth absorbs __
% of the sunlight that strikes it.
Venus absorbs __
% of the sunlight that strikes it.
Let’s re-examine the surface temperatures with this information.

17. 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.
You don’t need to memorize numbers like this, but rather just have a general idea that the albedo of Venus is much higher than the albedo of the Earth.

18. 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
units
Venus absorbs
units
Ok, here is some fun math problem…
If the earth receives 100 units of energy, and Venus receives 193 units (because its closer to the sun), than how much energy does the Earth and Venus absorb at the surface. Hint. We need to use the albedo to figure our how much energy is absorbed.
Thus, Earth absorbs sunlight at a greater rate than Venus does. Amazing, but true!

19. 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
So, now let’s think about the answer a second. The earth now absorbs more energy from the sun than Venus.
What do you think about this?
Thus, Earth absorbs sunlight at a greater rate than Venus does!

20. 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 ______ than Earth.
Warmer, colder or the same?

21. 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 colder than Earth.
Now, if you are following along, this might really make sense to you. At least many of us often think that Venus is a hot place, but not according to this analysis…

22. Actual Surface Temperatures
Based on average measurements combining surface air temperature and sea surface temperatures:
Earth’s mean surface temperature is about ______
Venus’ mean surface temperature is ______
So…
Venus absorbs less sunlight than Earth, yet
Venus’ surface is much hotter than Earth’s
Why?
Let’s see what the real temperatures are.

23. 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?
Wow, you were right. Venus really is very hot!

24. 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?

25. The respective atmospheres of the Earth and Venus
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
So, the reason our analysis didn’t work out was because we didn’t consider the affect of the atmosphere. Now, we must think about how the atmosphere affects the temperature of our planet.

26. Atmospheres of Earth and Venus (Gases)
Ok, let’s look at each atmosphere and what comprises each atmosphere. Essentially, what makes up our air?

27. Atmospheres of Earth and Venus (Gases)
Nitrogen (N2)
Oxygen (O2)
Argon (Ar)
Water Vapor (H2O)
Carbon Dioxide (CO2)
Venus
Carbon Dioxide
Nitrogen
Let’s stop for a second, because this is a point that can be difficult for some of us.
Think about this. When you take a breath of air, what are you breathing in? The basic answer, gases. And what is a gas? Well, gases are molecules that you can’t see, but are there. Some examples, oxygen (our favorite gas), or say helium (used for party balloons). Recall that water for example, can be a liquid (water), solid (ice) or gas (water vapor). So, in the air you are breathing in right now, you are inhaling mostly nitrogen and oxygen, with some water vapor, carbon dioxide and some other treats.

28. Principal Atmospheric Gases
Earth
Venus
GAS
MASS
Nitrogen
3870 (78%)
Oxygen
1185 (21%)
Argon
(<1%)
Water Vapor
(0.3%)
CO2
(0.06%)
TOTAL
5140
GAS
MASS
CO2
~500, 000 (96%) N2
~20, (4%)
TOTAL
520, 000
Here are the numbers for both Earth and Venus
Compare the amount of CO2
Unit: 1018 g

29. 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.

30. 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
So, we’ve introduced an obviously important term. The greenhouse effect.
If you’ve ever seen a greenhouse, it’s a glass house used to grow plants usually in the winter when it’s very cold. Inside a greenhouse, the sun’s energy can come through the glass, but the energy trying to leave the greenhouse cannot escape. While this is the basic idea, we’ll see that it’s actually a bit more complicated than this, but for now this is ok.
So, there are certain type of gases that we call greenhouse gases. These greenhouse gases are special because sunlight passes through them, but they trap heat trying to leave the planet, similar to a greenhouse.

31. Here is a cartoon of how the greenhouse effect works
Here is a cartoon of how the greenhouse effect works. See if you can follow this…

32. Greenhouse Gases Water Vapor Carbon Dioxide Methane (CH4)
Nitrous Oxide (N2O)
Ozone (O3)
CFC’s (chlorofluorocarbons)
Here are the major greenhouse gases that exist in our atmosphere?

33. 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.

34. 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.
Ok, so the main point. Although the distance between the Earth/Venus and the sun is important to the temperature, the composition of the atmosphere is ultimately more important to determining the temperature of the planet.
Please see the reading for Week 2 on the website, and complete Activity 1 (next slide).

35. 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).
Ok, that’s it! I hope this makes sense, and I look forward to meeting you next week!