Shaun Tanner has been a meteorologist at Weather Underground since 2004.
By: Shaun Tanner , 4:56 AM GMT on October 11, 2011
The topic of climate change is loaded with charged words that provoke emotions on both sides of the "argument" (I say "argument" because, in fact, there is no argument at all. 97% of climate scientist acknowledge that anthropogenic climate change is real and current). "Fossil fuels", "natural", and "green energy" are just a few of these words or terms. There is one term, however, that gets caught up in the argument but really should not be.
The term "greenhouse effect" gets batted around like a tennis ball in the media. As far as the media is concerned, it is a dirty word. Greenhouse effect = death of the planet. This is wrong, for the most part. As you will learn by the end of this part of the lesson, the greenhouse effect is essential for life on Earth.
Our dear Mother Earth strives for balance. When she gets thrown a curve ball in terms of climate (a volcano erupts or a meteoroid runs into the surface), Earth takes the consequences of that curve ball and strives for a resulting balance. Thus, we have the idea of radiative equilibrium. Think of equilibrium as balance. Ying = Yang, In = Out, Luke Skywalker = Darth Vader, you get the idea. One aspect the Earth strives for balance in is temperature. To obtain radiative equilibrium, the amount of sunlight the Earth is absorbing has to equal the amount of radiation the Earth is emitting. If the Earth emits more radiation than it was absorbing, then it will cool down. If it absorbs more than it was emitting, it would heat up. The radiative equilibrium temperature of Earth if 0°F. Cold! Now, here comes the curve ball. If you were to take the temperature of every thermometer on the planet for numerous years and average all of these values, you would get 59°F. So, I am telling you that based on the radiation we receive from the Sun, the temperature should be 0°F, yet the ACTUAL average surface temperature is 59°F. What gives?
The answer is blackbodies and selective absorbers.
A blackbody is an object that is a perfect emitter and a perfect absorber. This is a fancy way of saying that all of the radiation that hits a blackbody is absorbed. There are several important blackbodies in the Earth system. The surface, is probably the most important of them all. How do we know that the surface is a blackbody? Well, when it is daylight on one side of the planet, it is night on the other. That is to say, the radiation from the Sun does not pass all the way through the planet. Instead, it is absorbed by the surface and prevented from passing any further. You are also a blackbody, which is why you cast a shadow.
The real reason for the difference between radiative equilibrium (0°F) and actual observed surface temperature (59°F) is due to something called selective absorbers.
A selective absorbers is something that only absorbs certain wavelengths and usually emits at similar wavelengths. For instance, an object may absorb only infrared radiation and will emit only infrared radiation. Since the radiation from the Sun passes right through the atmosphere on its way to being absorbed by the surface, the ATMOSPHERE must be a selective absorber since it does not absorb sunlight very well. The "shortwave radiation" from the Sun passes right through the atmosphere. So what does the atmosphere absorb? It does a very good job of absorbing "longwave radiation" from the surface.
The best way to think of this is a blanket. When you go to bed at night, you pull a blanket over yourself. Why? Well, the natural answer is, "to keep warm." How does the blanket keep you warm? Your body is an engine that generates a lot of heat. Without the blanket covering you, the heat that your body generates would be emitted out into the room and be lost. You would get cold. If you throw a blanket over you, your body heat is emitted and then absorbed by the blanket almost immediately. This will warm the blanket a little and keep you significantly warmer. Thus, you are warmer with that blanket than without it.
Now, let's translate this to the atmosphere. The blanket is the atmosphere and your skin is the surface of the Earth. Sunlight passes through the atmosphere and warms the surface. In turn, the surface emits radiation (like your skin) which is trapped by the atmosphere (the blanket). So, without the atmosphere, the surface of the Earth will be a lot colder (0°F) than it currently is (59°F). Get it? This, ladies and gentlemen, is the greenhouse effect. Without the greenhouse effect, the surface of the planet would be 0°F and too cold for life to have evolved how we know it today. Because of the greenhouse effect, the surface temperature is 59°F, which is pleasant enough for life to have evolved.
For those of you more inclined to learn via a picture, behold Figure 1.
Figure 1. The greenhouse effect as we know it.
Now, here is the problem. As we learned in the very first lesson, the atmosphere is made up of numerous different gases (nitrogen, oxygen, carbon dioxide, water vapor, etc.). Some of these gases are much better at absorbing the "longwave radiation" coming from the surface of the Earth. The more radiation these gases absorb, the warmer the atmosphere and surface. Back to our blanket analogy, it would be the equivalent of throwing another blanket on your bed. The extra blanket would act to trap more of your body heat, thus warming you more. At some point, it would become too hot for you to be comfortable.
The gases responsible for absorbing "longwave radiation" from the surface are known as greenhouse gases. To name a few greenhouse gases:
Water vapor is perhaps the most important greenhouse gas. The more water vapor in the atmosphere, the more radiation it traps and the warmer the surface. I guarantee you have direct experience with this. Think about a morning when the sky is covered in clouds. It is completely overcast. Now, think of a morning when the sky is completely clear. Which morning is warmer?
Of course, the answer is the overcast morning. This is because water vapor is a greenhouse gas. Thus, the more of it that is in the atmosphere, the more infrared radiation from the surface it will absorb and the warmer the surface will be. Think of the extra blanket on the bed, again.
Carbon dioxide, of course, gets all the headlines as being the most diabolical greenhouse gas. This is because humans are directly involved in pumping this gas into the atmosphere. The more carbon dioxide, the thicker the blanket on the bed. Problem is, we cannot just roll out of this bed to get away from the heat.
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