The greenhouse effect and the 2nd law of thermodynamics
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The 2nd law of thermodynamics is consistent with the greenhouse effect which is directly observed. |
Climate Myth...
2nd law of thermodynamics contradicts greenhouse theory
"The atmospheric greenhouse effect, an idea that many authors trace back to the traditional works of Fourier 1824, Tyndall 1861, and Arrhenius 1896, and which is still supported in global climatology, essentially describes a fictitious mechanism, in which a planetary atmosphere acts as a heat pump driven by an environment that is radiatively interacting with but radiatively equilibrated to the atmospheric system. According to the second law of thermodynamics such a planetary machine can never exist." (Gerhard Gerlich)
Skeptics sometimes claim that the explanation for global warming contradicts the second law of thermodynamics. But does it? To answer that, first, we need to know how global warming works. Then, we need to know what the second law of thermodynamics is, and how it applies to global warming. Global warming, in a nutshell, works like this:
The sun warms the Earth. The Earth and its atmosphere radiate heat away into space. They radiate most of the heat that is received from the sun, so the average temperature of the Earth stays more or less constant. Greenhouse gases trap some of the escaping heat closer to the Earth's surface, making it harder for it to shed that heat, so the Earth warms up in order to radiate the heat more effectively. So the greenhouse gases make the Earth warmer - like a blanket conserving body heat - and voila, you have global warming. See What is Global Warming and the Greenhouse Effect for a more detailed explanation.
The second law of thermodynamics has been stated in many ways. For us, Rudolf Clausius said it best:
"Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature."
So if you put something hot next to something cold, the hot thing won't get hotter, and the cold thing won't get colder. That's so obvious that it hardly needs a scientist to say it, we know this from our daily lives. If you put an ice-cube into your drink, the drink doesn't boil!
The skeptic tells us that, because the air, including the greenhouse gasses, is cooler than the surface of the Earth, it cannot warm the Earth. If it did, they say, that means heat would have to flow from cold to hot, in apparent violation of the second law of thermodynamics.
So have climate scientists made an elementary mistake? Of course not! The skeptic is ignoring the fact that the Earth is being warmed by the sun, which makes all the difference.
To see why, consider that blanket that keeps you warm. If your skin feels cold, wrapping yourself in a blanket can make you warmer. Why? Because your body is generating heat, and that heat is escaping from your body into the environment. When you wrap yourself in a blanket, the loss of heat is reduced, some is retained at the surface of your body, and you warm up. You get warmer because the heat that your body is generating cannot escape as fast as before.
If you put the blanket on a tailors dummy, which does not generate heat, it will have no effect. The dummy will not spontaneously get warmer. That's obvious too!
Is using a blanket an accurate model for global warming by greenhouse gases? Certainly there are differences in how the heat is created and lost, and our body can produce varying amounts of heat, unlike the near-constant heat we receive from the sun. But as far as the second law of thermodynamics goes, where we are only talking about the flow of heat, the comparison is good. The second law says nothing about how the heat is produced, only about how it flows between things.
To summarise: Heat from the sun warms the Earth, as heat from your body keeps you warm. The Earth loses heat to space, and your body loses heat to the environment. Greenhouse gases slow down the rate of heat-loss from the surface of the Earth, like a blanket that slows down the rate at which your body loses heat. The result is the same in both cases, the surface of the Earth, or of your body, gets warmer.
So global warming does not violate the second law of thermodynamics. And if someone tells you otherwise, just remember that you're a warm human being, and certainly nobody's dummy.
Basic rebuttal written by Tony Wildish
Update July 2015:
Here is the relevant lecture-video from Denial101x - Making Sense of Climate Science Denial
Update October 2017:
Here is a walk-through explanation of the Greenhouse Effect for bunnies, by none other than Eli, over at Rabbit Run.
Last updated on 7 October 2017 by skeptickev. View Archives
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[DB] Please note that the NOTES section described by muoncounter is not viewable to the lay reader, so the relevant section is reproduced below:
Good response by John Farley: Cockburn is impressed by a scientific argument, claiming that the greenhouse effect violates the Second Law of Thermodynamics. He relies on a publication by Gerlich and Tscheuschner (GT), "Falsification of the Atmospheric CO2 Greenhouse Effects within the Frame of Physics."5 However, the greenhouse effect can be easily demonstrated in the laboratory. The BBC broadcast a tabletop demonstration of the greenhouse effect, which can be found at the BBC website (at <news.bbc.co.uk/2/hi/science/nature/8394168.stm>). The video, a little over 2 minutes long, is well worth watching. Physics is an experimental science, and if theory disagrees with experiment, the theory must be flawed.
But beyond noting that the GT theory is refuted by experiment, it is worthwhile examining where GT went wrong. They claim that greenhouse gases in cold upper atmosphere cannot possibly transfer heat to the warmer earth, without violating the Second Law of Thermodynamics.
Let's be clear about what Second Law of Thermodynamics does and does not say. Suppose that you have two objects at two different temperatures, and suppose that light (visible or infrared) from either object can reach the other object. There will be a flow of heat from the hot object to the cold object and a smaller flow of heat from the cold object to the hot object. There are thus heat flows in both directions: from hot to cold and from cold to hot.
The Second Law says that the flow of heat from hot to cold is greater than the flow of heat from cold to hot. Hence the net flow of heat is from the hot object to the cold object. Note that the existence of a smaller flow of heat from the cold object to the hot object does not refute the Second Law.
At this point, we return to Cockburn's argument (from GT). Heat flows from the warm earth to the cold atmosphere and also from the cold atmosphere to the warm earth. (Heat also flows from the cold atmosphere to outer space, which is even colder.) The flow of heat from the earth to the atmosphere is greater than the flow of heat from the atmosphere to the earth, so the net flow of heat is from the earth to the atmosphere.
But there is also a (smaller) flow from the atmosphere to the earth. This smaller flow keeps the earth warmer than it would be if there were no greenhouse gases in the atmosphere. This is what the greenhouse effect is all about. On this point, Cockburn has been misled by GT, who have advanced degrees in physics but have made a serious mistake in thermodynamics.
Readers with a background in physics and calculus can read a comprehensive refutation of the GT paper by Arthur P. Smith, "Proof of the Atmospheric Greenhouse Effect." Smith's article begins: "The results presented here are not new." Indeed, they are over a century old and found in standard textbooks. Smith has presented the subject in great detail in order to answer objections raised by GT to the treatment found in standard textbooks.
The greenhouse effect has been known for over a century. The greenhouse effect is quite a big effect: the Earth's surface is about 59 F warmer than it would be in the absence of greenhouse gases in the atmosphere. The greenhouse effect was entirely natural until the industrial revolution. In the last two centuries, the burning of fossil fuels has added a manmade contribution to the greenhouse effect. It is surprising that the GT paper survived peer review, which is a quality-control policy that makes it harder to publish erroneous papers. Harder, but evidently not impossible.
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