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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 Understand the differences in the atmospheres of Venus and Earthand Earth

Know how a changing albedo can change climateKnow how a changing albedo can change climate

Understand the role of greenhouses gases and the Understand the role of greenhouses gases and the runaway greenhouse effectrunaway greenhouse effect

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The Solar System

“The nine planets”

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The Solar System

“The nine planets”

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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?

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Source of Energy: The Sun

Sun radiates energy; Energy “radiates out” with distance

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Earth and Venus

Venus

Earth

Sunlight striking Venus is 93% stronger than sunlight striking Earth, thus we would expect Venus to be warmer

Based on solely the position of Venus and the Earth, which planet would be warmer?

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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 …

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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

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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

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Comparison of Earth and Venus

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

Venus Earth

Venus

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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.

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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!

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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.

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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?

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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

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Atmospheres of Earth and Venus (Gases)

Earth1. Nitrogen (N2)

2. Oxygen (O2)

3. Argon (Ar)

4. Water Vapor (H2O)

5. Carbon Dioxide (CO2)

Venus1. Carbon Dioxide

2. Nitrogen

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Principal Atmospheric Gases

Earth Venus

GAS MASS

Nitrogen 3870 (78%)

Oxygen 1185 (21%)

Argon 67 (<1%)

Water Vapor 17 (0.3%)

CO2 3 (0.06%)

TOTAL 5140

GAS MASS

CO2 ~500, 000 (96%)

N2 ~20, 000 (4%)

TOTAL 520, 000

Unit: 1018 g

Compare the amount of CO2

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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.

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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

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Greenhouse Gases

Water Vapor

Carbon Dioxide

Methane (CH4)

Nitrous Oxide (N2O)

Ozone (O3)

CFC’s (chlorofluorocarbons)

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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.

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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.

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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).