Weather school: Why does it get warm?

A greenhouse is an inexact metaphor for how Earth absorbs and emits energy

Last time in weather school we looked at how heat moves around by examining conduction, convection, advection and latent heat transfer. This week in our weather lesson we’ll start to explore the Earth’s energy balance, or what’s often referred to as our energy budget.

Before we can dive into that topic, however, we must first take a look at the greenhouse effect. So far, we’ve learned the Earth absorbs shortwave energy coming from the sun. This energy is turned into heat and is moved around by the processes we examined last class. If we constantly absorbed shortwave radiation from the sun and did not give off any radiation of our own, we would continually heat up — getting hotter and hotter each day. Since this isn’t the case, the Earth must be releasing this heat energy. It does this by emitting long-wave radiation.

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So, the Earth absorbs shortwave radiation, heats up, then cools off by emitting long-wave radiation back into space — so where does the greenhouse effect come into play?

As it turns out, our atmosphere is fairly transparent to shortwave radiation coming in from the sun, much like the glass on a greenhouse. Most of the sun’s energy makes it through the atmosphere to reach the Earth’s surface, where it is converted to heat, then radiated back toward space as long-wave radiation. In a greenhouse, the glass does not allow the long-wave radiation to pass through it; instead, it traps the heat energy inside the greenhouse. The same thing happens in your car when it is sitting in direct sunshine.

Our atmosphere acts kind of like the glass in a greenhouse or car, although it is not exactly the same. Gases such as carbon dioxide, water vapour, methane, nitrous oxide and others absorb some of the outgoing long-wave radiation and re-emit them back toward the Earth’s surface. So, the heat is not trapped like it is in a greenhouse, but rather it’s delayed in its passage to space, and this delay is what makes the Earth a habitable place for life. Without this greenhouse effect, the average temperature on Earth would be a rather chilly -18 C, a far cry from the balmy +16 C it is now!

Now that we know the greenhouse effect is a good thing, and that without it we would not be able to survive, why is there all this negative talk about global warming and the greenhouse effect in the news out there? The reason for this is that, like so many other things in this world, the wrong information or term gets used, and all of a sudden it becomes entrenched in the way things are reported. Just like the term “normal” temperature for a day — there is no such thing as a “normal temperature.” This term was created using a math term that is used to describe the average temperature… but I digress. When scientists or media talk about global warming and the greenhouse effect, what they should really be saying is the “enhanced” greenhouse effect.

Enhanced effect

The idea behind the enhanced greenhouse effect is that if we are adding more gases to the atmosphere than what would normally be there, and these gases absorb long-wave radiation and re-emit them back to Earth, then logically we should see an increase in the overall temperature of the Earth. I don’t think anyone out there, whether they are for or against global warming, disagrees on this fact — that increasing the amount of greenhouse gases will allow for more long-wave radiation to be absorbed. Then why would there be people who do not think that the Earth is undergoing warming due to this enhanced greenhouse effect? The answer is that it’s not as simple as just absorbing more of the long-wave radiation and then re-emitting it back to Earth.

Simply absorbing more long-wave radiation doesn’t necessarily mean the Earth will warm up. This is due to all the different feedback mechanisms that come into play. For example, as more long-wave energy is absorbed and then re-emitted, there is a good chance that more water will evaporate into the atmosphere, increasing the amount of water vapour. More water vapour means more long-wave radiation will be absorbed and re-emitted — so we should get even warmer — but more water vapour can also mean more clouds. More clouds result in more of the shortwave energy from the sun being reflected back to space before it even has a chance of becoming long-wave radiation; more clouds mean colder temperatures. Now, what if the increase in clouds occurs more at night when the sun is not shining? Then the clouds don’t block incoming shortwave radiation, but they do help to absorb outgoing long-wave radiation — so now that means warmer temperatures!

As you can see, it is not a simple matter, and this is only one part to the whole global warming/enhanced greenhouse effect puzzle, which we will continue to look at as we explore the Earth’s energy budget.

About the author

Co-operator contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.

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