Climate Change: Winners and Losers
Lesson 1: The Greenhouse Effect
Page 1 Page 2 Page 3 Page 4: The Greenhouse Effect ReviewThe Greenhouse Effect
Imagine pouring hot coffee or tea into a cup. The cup heats up because of the hot liquid. Now imagine placing your hand very close to the cup, so close that you can feel some of the heat that the cup is giving off without actually touching it. The cup is absorbing energy from the liquid and then sending some of it into the small space between the cup and your hand as Infra-Red (IR) radiation.
Something similar happens when Earth receives energy from the Sun. EM radiation from the Sun arrives at Earth. Most of this energy is in the form of visible light waves. Some of it gets reflected away, but most of it hits Earth's surface and is absorbed. This heats our planet and makes Earth hotter than the space outside of our atmopshere. Since the planet is hotter than the space that surounds it, Earth radiates energy back out into space as IR waves. (IR waves are what makes your hands warm if you hold them near a hot radiator on a cold day. That's why they're called "radiators"--they radiate heat energy as IR waves.) So, we have visible light energy coming from the Sun, passing through the gases in the atmosphere, arriving at Earth, and then going back into space as IR wave energy.
But something really interesting happens when IR radiation goes into our atmosphere. It turns out that IR radiation is just the right type of energy to get certain molecules in our atmosphere moving faster. And what happens when molecules move faster? Whatever those molecules are part of becomes warmer--just like the water in the glasses from Page 2.
Result? These molecules absorb IR radiation and heat up, trapping this heat in our atmosphere. This is what causes the greenhouse effect.
As this drawing shows, some IR radiation (called "heat" in the drawing) escapes into space, and some of it sticks around to warm the atmosphere. For humans, this is a good thing. (You'll see why in a moment.)
Our atmosphere contains many different gases. It is mostly (close to 99%) nitrogen and oxygen, but it also contains traces of other gases like argon, water vapor, carbon dioxide (CO2), and methane. There is actually very little CO2, and even less methane, in our atmosphere. But CO2 and methane molecules are affected much more by the IR radiation than nitrogen or oxygen molecules.
So it's a very small fraction of the molecules in our atmosphere that are most responsible for greenhouse warming. The more CO2 and methane that we put into the atmosphere, the more our atmosphere responds to IR radiation, and the more it heats up. That's why these gases are called "greenhouse" gases. There are other greenhouse gases, including nitrous oxide, ozone and CFCs. Water vapor can also act to trap heat in the atmosphere, but its role in global warming is much more complicated and beyond the scope of this lesson.
As I said, the greenhouse effect is a good thing. Without the greenhouse warming of our atmosphere, our planet would be too cold for us to live on comfortably. In fact, it's very likely that humans would never have evolved on Earth if it didn't have a greenhouse effect.
But too much greenhouse effect can make our planet inhabitable.
You can see the impact of the greenhouse effect on Earth's average surface temperature by using the Planet Temperature Calculator. This calculator was developed to predict average surface temperatures on other planets, but you can use it to see what Earth would be like under different amounts of greenhouse warming. To use the calculator, you select values for the mass (size) of a planet's nearest star, the distance of that star from the planet, the planet's bond albedo--which is the amount of energy from the star that is reflected away from the planet--and the strength of the planet's greenhouse effect.
When you go to the Planet Temperature Calculator, click on the "Build" button at the top right of your screen. You can then set values for Mass, Distance, Bond Albedo and Greenhouse Effect. Start by using Earth values for Mass (1), Distance (1), Bond Albedo (29) and Greenhouse Effect (1). Next go to the Summary page by clicking the "Summary" button on the top right of your screen, check to make sure you have entered the correct values, and then click on the "Results" button on the top right. You will see that right now Earth's average surface temperature--summer temperatures, winter temperatures, temperatures at the Equator, in the Arctic, and all other temperatures in between averaged together--is about 59 degrees Fahrenheit (about 15 degrees Celsius).
Now comes the fun part. Go back to the Summary page ("Summary" button on the top left). Keep the same values for Mass, Distance, and Bond Albedo, but change Greenhouse Effect to 0--no greenhouse warming. Click the "Results" button to see how cold our planet would be without a greenhouse effect.
Congratulations. You have just ended life as we know it on Earth.
Feeling particularly destructive? How hot would Earth be if we had a greenhouse effect as strong as Venus'? Go back to the Summary and enter 200 for the greenhouse effect. Ouch!
Experiment with different amounts of greenhouse warming. Double it. Triple it. You can enter decimal amounts, too, like 1.3 or 48.6. About how much greenhouse warming can we take before we all fry?
One Important Point: Doubling the amount of CO2 in the atmosphere is not the same thing as doubling the greenhouse effect. CO2 makes up only a tiny fraction of the gases in our atmosphere, and there are other gases that contribute to greenhouse warming. But increasing the amount of CO2 does increase the strength of the greenhouse effect.
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