Greenhouse Effect Text:

On a sunny winter day, you park your car in the sun, lock it, and go cruise the local shopping mall for a couple of hours. When you come back later and climb into your car, what do you notice? Is the interior of the car hot or cold? How does the temperature inside of your car compare to the temperature outside?

If you have ever been inside of a greenhouse, especially when the sun is out, what have you observed? It is definitely warm, and usually the air is moist. A greenhouse is commonly warmer than the outside air because a greenhouse has the ability to trap heat inside of it. This phenomenon is called the greenhouse effect.

This same phenomenon occurs on the Earth. What causes this? Well, let’s take a look at how a greenhouse works to give you some ideas.

When sunlight shines through the glass roof of a greenhouse, it heats the benches, plants, etc. inside. The warmed interior of the greenhouse will radiate infrared radiation back into the interior of the greenhouse, but this radiation cannot escape. Look at the diagram: Can you see how the short-wavelength light can enter a greenhouse, but the longer-wavelength infrared radiation cannot escape? This trapped heat energy is what causes the temperature inside of the greenhouse to climb until the glass heats up, getting warm enough to radiate heat away from the greenhouse as fast as sunlight enters.

This model can be similarly applied to the Earth. Here are some guided questions to help you figure it all out:

Many scientists and governments express concern over HOW the composition of the atmosphere will affect global temperature and weather patterns. For example, if the average temperature increases by 10 F globally, how will that affect agriculture and the food supply? What about changes in weather patterns? Too much water = flooding, and too little water = drought. Do humans contribute to this problem? Sure! Any increase in greenhouse gases will increase the effectiveness of the atmosphere at trapping heat, and industrial nations that burn fossil fuels (coal, oil, and gas) emit carbon dioxide into the atmosphere in HUGE quantities every year. If the temperature on Earth continues to rise, is there a possibility that we could be making Earth too hot to like on? Is there any way we could "cool off" the Earth, or make the increase in temperature more gradual and slow?

Perhaps if we look at how the Earth’s atmosphere compares with the atmospheres of Venus and Mars, you will come up with some ideas about how atmosphere and temperature can be brought into better control. Let’s check it out.

This same phenomenon occurs on the Earth. What causes this? Well, let’s take a look at how a greenhouse works to give you some ideas.

DAY FOUR: Composition of the Atmosphere/Greenhouse Effect

Atmosphere is a pretty important thing to have if you plan on living, breathing, and not being pelted by meteors. What determines whether or not a planet will have an atmosphere?

The atmosphere around a planet is determined by many factors, most of them occur early in a planet’s life while the it is forming. You will learn more about this process in the "protoplanetary disc" activities. Let’s look at the differences between the planets closest in size and distance from the Sun as our Earth.

You have already gathered data on the atmospheres of Venus, Earth, and Mars. Now you’ll need to analyze that data to see what patterns you can find regarding the atmosphere. The first thing we need to explore would be the atmospheric pressure.

Venus: 92 bars

Earth: 1014 millibars

Earth’s Moon: 3 x 10 ^-15 bar (or 0.000000000003 millibars)

Mars: 6.9 mb to 9 mb (Viking 1 Lander site)

These pressures are at the surface of each planet.

Which planet has the greatest pressure? VENUS.

Remember that atmospheric pressure is a measure of the weight of the air above. If Venus has the greatest atmospheric pressure, then is must have the (thickest, thinnest) atmosphere. (THICKEST)

Which of planets (not including the moon) has the lowest atmospheric pressure? (MARS)

Remember that atmospheric pressure is a measure of the weight of the air above. If Mars has the least atmospheric pressure, then is must have the (thickest, thinnest) atmosphere. (THINNEST)

Earth’s atmosphere is still pretty thin. The majority of the Earth’s atmosphere is within 20 km of the Earth’s surface, and it is equivalent to a coating of aluminum foil on a basketball. Considering life would not exist without an atmosphere, that’s a pretty thin margin to live in!

OK, so Venus has a thick atmosphere, Earth a thin atmosphere, and Mars a really thin atmosphere. Do you think this has any relationship to the temperatures seen on the planets? Sure it does!

OK, first a basic thought. The closer a planet is to the sun, the __________ solar energy it would receive.

1. More. (Right on!)
2. Less. (Nope – it’s like being close to a fire – the closer you are, the more heat you feel.)

What is the average distance away from the sun for each of the planets?

Venus: 0.7233 AU

Earth: 1.0 AU (everything is in reference to the Earth)

Mars: 1.5327 AU

So you would expect that Venus would be hotter, Earth a little cooler, and Mars even cooler than Earth. Does your data show that? YES! But, it’s not that simple (sorry).

Part of how hot or cold a planet is depends upon what the atmosphere is made of and how much heat (solar energy) it can hold onto.

What components of an atmosphere do you need to look at to determine how hot or cold it will be?

1. Carbon dioxide (CO₂)
2. Presence or absence of ozone.
3. Albedo (cloud cover)

Let’s look at carbon dioxide first. Carbon dioxide is important to an atmosphere because it can trap heat in a process called the greenhouse effect. When sunlight shines through the roof of a greenhouse, it heats the plants and equipment inside. The contents of the greenhouse will radiate heat, but this cannot escape. Short-wavelength light (sunlight) can enter the greenhouse, but longer wavelength infrared (heat) cannot escape the greenhouse. Heat is trapped until the temperature of the glass gets hot enough to radiate heat away as fast as sunlight enters.

How does this apply to an atmosphere? Let’s look at the Earth’s atmosphere first to explain this. In the Earth’s atmosphere, the carbon dioxide lets sunlight through and the Earth’s surface heats up. However, when the Earth heats up and radiates heat (infrared wavelengths) away, the heat cannot escape. Carbon dioxide is a good thing, because it helps to keep the planet warm.

OK, so carbon dioxide is a greenhouse gas that traps heat. What are the concentrations of carbon dioxide gas for the three planets?

VENUS: 96.5% Carbon Dioxide (CO₂)

EARTH: 350 ppm Carbon Dioxide (CO₂) (less than 1%)

MARS: 95.32% Carbon Dioxide (CO₂)

If carbon dioxide were the only factor influencing heat, you would expect that:

1. Venus would be hotter than Earth.
2. Mars would be hotter than Earth.
3. Earth would be hotter than Mars.
4. A and B. (CORRECT)
5. None of these.

Is this the case? No! Mars is measurably cooler than Earth’s average surface temperature. Does it have something to do with distance? Sure. Mars would receive less solar radiation being further away from the Sun. But that’s not the only thing you need to consider.

You may be wondering WHY Earth has so little carbon dioxide in the atmosphere when Venus and Mars have so much. Well, it’s like this: Outgassing from a forming planet such as Earth would release mostly carbon dioxide and water vapor. Carbon dioxide is very soluble in water, and the earth’s surface temperature is just right for it to be covered with water. Much of the carbon dioxide released in outgassing dissolves in oceans and combines with minerals in seawater to form deposits of silicon dioxide, limestone, etc. So the carbon dioxide has been removed from Earth’s atmosphere and buried in the Earth’s crust (rocks).

Let’s look at ozone next and see what’s up with that. Ozone is an unstable molecule in some atmospheres. It is made of three oxygen atoms (O₃). What are the benefits of ozone in an atmosphere? The ozone layer absorbs UV (ultraviolet) light and prevents them from reaching the ground. Ozone allows the energy (heat) from UV light to be trapped in the atmosphere, increasing the heat somewhat (but not nearly as much as carbon dioxide).

Which of the three planets contain ozone, and if a planet contains ozone, in what amounts does ozone exist?

VENUS: No ozone. (There’s not enough time to die of radiation poisoning here – the temperature and pressure would kill you relatively quickly.)

EARTH: Ozone layer approximately 25 km above the Earth’s surface (stratosphere).

MARS: No ozone. (Don’t try getting a tan here – without ozone a few minutes in the sun would expose you to so much radiation you’d be dead.)

Ozone does help trap heat, but that is not the primary reason Earth’s inhabitants are glad ozone exists on planet Earth. The main benefit to living organisms it that ozone blocks harmful UV light that damages DNA and causes skin cancers, etc.

The human race is changing the Earth’s atmosphere in two dramatic ways: the greenhouse effect appears to be increasing, and the ozone layer is depleting. The greenhouse effect is produced by increasing levels of carbon dioxide in the atmosphere, trapping more heat and making the Earth warmer. Even a small rise in the average temperature of the Earth could cause HUGE changes in agriculture, affecting the world’s food supply, melting polar ice caps, raising sea level (and causing much flooding), etc.

Remember that carbon dioxide has been buried in the Earth’s crust in the form of oil, coal, and natural gas deposits. As humans burn these fuels to power the world’s activities, carbon dioxide gas is released into the atmosphere. This wouldn’t be a problem if the Earth were covered with tons of forests – plants remove carbon dioxide from the atmosphere, rip off a carbon, and release what’s left (oxygen) into the atmosphere. However, the Earth’s forested areas are diminishing.

Carbon dioxide is a problem, but so is the depletion of the ozone layer. Ozone is an unstable molecule in some atmospheres. It is made of three oxygen atoms (O₃). Ozone is produced naturally in the Earth’s atmosphere, and it absorbs UV light. Pollution with chlorofluorocarbons (CFCs) has destroyed much of the ozone layer, and thus there has been an increase in the UV radiation at the Earth’s surface.

Let’s look at albedo – the fraction of light striking a planet that is reflected back into space by the planet. Another way of saying albedo is cloud tops. Clouds help reflect light back into space. The albedo of a perfect reflector – meaning that ALL light is reflected back into space – is 1. What is the albedo for each of the planets?

VENUS: Average albedo is 0.76 (76% of all incoming light is reflected back into space.)

EARTH: Average albedo is 0.39 (39% of all incoming light is reflected back into space.)

MARS: Average albedo is 0.16 (16% of all incoming light is reflected back into space.)

The climate (temperature and amount of precipitation) is SUPER-SENSITIVE to many different factors. One of these factors is CLOUDS (albedo). The formation of clouds depends largely on the presence of water vapor in the upper atmosphere, the temperature of the upper atmosphere, and the patterns of circulation in the atmosphere. Small changes in cloud cover will affect a planet’s albedo, and thus its climate.

Which of the three planets has the greatest cloud cover (highest albedo)? VENUS.

Which of the three planets has the lowest cloud cover (lowest albedo)? MARS

What is the approximate water content in the atmosphere of these three planets?

VENUS: 20 ppm Water (H₂O) (very little)

EARTH: 0.04 – 4% (weight by volume)

MARS: 210 ppm Water (H₂O) (very little, but more than Venus)

Why do you suppose that the water content in the atmosphere of Venus is so low, and yet there is a high albedo (cloud cover)? HINT: It has to do with the temperature of Venus.

(Student fill in the blank).

When looking at temperature ranges for Venus, Earth, and Mars, some scientists have explained the phenomenon as the "Goldilocks Effect." Write a brief paragraph explaining the analogy of how the Goldilocks Effect applies to the temperatures of these three planets.

Goldilocks was journeying through the woods. She became hungry and tired. She saw a cottage, entered in, and looked around. She decided to try the porridge on the table. The first was too hot, the second too cold, and the third just right. Then she decided to take a nap. She wanted to rest, so she tired out each of the beds in the cottage. The first one was too hard, the second too soft, and the third just right. The bears came home, frightened Goldilocks, and she ran away.

Summary:

Climate – weather over long periods of time – depends upon the average temperature of a planet. Earth’s temperature stays relatively constant (when compared to other planets) because of the greenhouse effect. This is good – the Earth continues to re-radiate light even during the night-time hours when one side of the Earth is not receiving sunlight. (Look at Mars for temperature variations. Do you think that Mars re-radiates a lot of energy? Do you think Mars’ atmosphere can "trap" it? Look at the temperature variations from night to day? Is the difference large or small (relatively)?)

The greenhouse effect is of concern because it may be creating problems that are long-term and hard to repair. The buildup of greenhouse gases will make the atmosphere better at "trapping" terrestrial radiation, leading to global warming and possible climate change. If the worst predictions come true, the resulting problems could be ours to solve:

*Melting polar ice caps

*Rising sea levels

*Uninhabitable coastal areas (where half of the worlds population now lives – see photo from NASA).

*Weather patterns will change (wild, unpredictable storms)

*Agricultural region changes (some fertile areas become desserts, while some barren areas could become fertile.)

Some scientists are not convinced that global warming is actually occurring, or that greenhouse gases cause it. For example, oceans absorb a large amount of carbon dioxide gas (it dissolves ito the water easily, which is why carbonated beverages like Pepsi and Coke are cheap to make). Higher temperatures will cause more water to evaporate into clouds, which "shade" the Earth from sunlight. This could help cool the Earth (in theory).

Much of the information scientists have about the greenhouse effect and global warming come from computer models that predict climate change. Estimations may be off – it’s kind of tough to predict what will happen in the atmosphere, since the atmosphere is a pretty large area to cover.

Some questions for further thought: (Students fill in the blanks).

1. Sometimes a waste product or pollutant can be recycled or used another way. Can you think of some other uses for greenhouse gases?
2. How can scientists be sure that human activity is really contributing to global warming? If scientists can determine FOR SURE that humans are contributing to global warming, what do you think should be done about it?

TEACHER PAGE:

Lab Activity that goes along with Greenhouse Effect/Global Warming:

Background Information:

Carbon dioxide can be found in other locations, not just as a greenhouse gas in the atmosphere. It is part of the global "carbon cycle." Carbon travels through the living and non-living aspects of the environment. Humans are even carbon-based life forms (Ace Ventura, and in this case he is correct, even if he is somewhat on the edge.) Carbon can be found trapped in rocks or minerals, and it is most often combined with oxygen and a metal in a compound called carbonate. You can identify a carbonate-containing material because carbonate reacts with acid to yield carbon dioxide gas (which you can see as bubbles).

The chemical reaction that describes this process is:

Na₂CO₃ + 2HC₂H₃O₂ > CO₂ + H₂O + 2NaC₂H₃O₂

Materials:

Vinegar (acetic acid)

Eyedropper

Paper towels

Shallow container (glass or ceramic dish works best) for testing

Balance (to weight material before and after)

Materials to test (suggested list- make sure a mixture of living/non-living items are represented):

Eggshells, chalk, rocks, sand, bones, baking soda, salt, sugar, flour, metals, etc.

CAUTION: Do not test precious objects (like your Mom’s favorite pearl necklace or a cool-looking seashell you cannot replace or fix). Acid can permanently damage them.

Procedure:

1. Weigh each material sample to be tested on a balance. Record the weight of each sample in grams to the nearest tenth. (one decimal place, like 10.2 g).

2. Put each sample material in the dish and place several drops of vinegar on it.

3. Observe whether or not bubbles form. If bubbles do form, keep adding several drops of vinegar at a time until the material stops producing bubbles.

4. When the bubbles stop forming, dry off the material with a paper towel (if there is any material left.)

5. Weigh each sample material on the balance again to determine what changes, if any occurred. Record the weight of each sample the second time.

6. Determine the difference in weight for samples that contained carbonate. Do this by subtracting the second weight from the first weight.

7. After recording and analyzing your results, answer the questions in the results and conclusions section of this lab report.

Results and Conclusions:

1. Which materials tested positive for carbonate (they bubbled)?
2. How can you be sure that any bubbles you say were carbon dioxide and not some other gas? (Think of other ways to test a gas for carbon.)
3. Of the materials that contained carbonate, what was the percentage of the material that was carbonate? (Find this by taking the weight change and dividing it by the initial weight for each sample.)
4. How else could you measure the amount of gas given off? (Give at least two ideas.)
5. Do you think the some of the Earth’s atmospheric carbon comes from the chemical weathering of rocks? How would acid rain and other pollution contribute to this problem?