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06.08.08
So I plan to talk about this picture for a bit. It shows energy arriving at the earth from the sun, bouncing around a bit, and making its way back into space. The first thing you will notice that is different is that is is uglier than the usual one (it was made by scientists not by graphics professionals). The second is that one reason for the ugliness is that it is cluttered up by numbers. Understanding climate begins by understanding what these numbers mean.
The numbers are in watts, the same sort of watts that you measure household appliances with. The larger the number of watts, the larger the associated flow of energy. Now, one common to keep the numbers manageable is, rather than adding all the wattage from the sun, to indicate the wattage per unit area. In this case the unit area is a square meter.
Astronomers will immediately notice that the solar constant as used by climatologists is exactly a quarter of the solar constant as used by astronomers. That is because the solar constant as used by astronomers is per square meter of a plane facing the sun. By averaging over a sphere, some of which is in the dark, it turns out that the number is divided by exactly four, the ratio of the surface area of a sphere to that of its bisecting plane.
Sometimes when astronomers advocate the sun as a potential source of recent climate change they forget to divide by four. This is usually an indicator that they haven't worked through this diagram and aren;t familiar with the sorts of numbers to expect in the climate system.
Now, it's necessary to admit that these numbers are not known exactly. Errors of a few watts per square meter in most of the flows are possible.Nevertheless you will see a perfect balance at each layer. For instance, at the top of the atmosphere, 342 watts come in and 237 + 105 = 342 watts go out. Is this a realistic constraint?
Yes. Yes it is. Even though we don't know the 237 and the 105 exactly, we know they add up to exactly 342.
Next time we'll discuss why, and some of the implications.
Energy in the Climate System
by You've probably seen some version of this cartoon in discussions of global climate change. For instance, something similar is seen in An Inconvenient Truth. Looking at it as a diagram of climate change misses the point. It is actually a diagram of the climate system as a whole. Understanding this picture is the first step toward understanding the climate.
So I plan to talk about this picture for a bit. It shows energy arriving at the earth from the sun, bouncing around a bit, and making its way back into space. The first thing you will notice that is different is that is is uglier than the usual one (it was made by scientists not by graphics professionals). The second is that one reason for the ugliness is that it is cluttered up by numbers. Understanding climate begins by understanding what these numbers mean.
The numbers are in watts, the same sort of watts that you measure household appliances with. The larger the number of watts, the larger the associated flow of energy. Now, one common to keep the numbers manageable is, rather than adding all the wattage from the sun, to indicate the wattage per unit area. In this case the unit area is a square meter.
Astronomers will immediately notice that the solar constant as used by climatologists is exactly a quarter of the solar constant as used by astronomers. That is because the solar constant as used by astronomers is per square meter of a plane facing the sun. By averaging over a sphere, some of which is in the dark, it turns out that the number is divided by exactly four, the ratio of the surface area of a sphere to that of its bisecting plane.
Sometimes when astronomers advocate the sun as a potential source of recent climate change they forget to divide by four. This is usually an indicator that they haven't worked through this diagram and aren;t familiar with the sorts of numbers to expect in the climate system.
Now, it's necessary to admit that these numbers are not known exactly. Errors of a few watts per square meter in most of the flows are possible.Nevertheless you will see a perfect balance at each layer. For instance, at the top of the atmosphere, 342 watts come in and 237 + 105 = 342 watts go out. Is this a realistic constraint?
Yes. Yes it is. Even though we don't know the 237 and the 105 exactly, we know they add up to exactly 342.
Next time we'll discuss why, and some of the implications.
4 Comments
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June 9, 2008 2:49 AM
Julius de Hond
But what is then the problem with global warming? I mean if a certain amount of energy enters the atmosphere but also leaves it? The problem with warming is that more energy than usual is kept inside, right?
June 9, 2008 5:54 AM
MIchael Tobis
Julius, that is roughly correct. Note that while related, temperature is not the same thing as energy, though.
Your confusion is not unusual. It may be a result of the fear of any numbers other than dollar amounts and sports scores in the press.
I will explain in more detail in a subsequent article, in the way that I think the person on the street ought to be able to think about the problem.
June 11, 2008 1:33 PM
Chad
"The numbers are in watts, the same sort of watts that you measure household appliances with."
Aren't the numbers in watts/square meter?
June 11, 2008 2:46 PM
Michael Tobis
Yes watts per square meter. That's hairsplitting at the intended level of accessibility, though. I specified "per square meter" in the text.
There's always a tradeoff between accessibility and precision.
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