Apr 03 2007
Greenhouse vs. Global Warming
In The Pig Teaches, Dr. Rabbett sounds like he wants to give an A+ for this work: Cap Answers it All. While it is explained simply, I would add a few minor comments.
First, explain what a greenhouse is and how it works (by suppressing convection). And how can one pass up an opportunity to link to Bad Greenhouse, part of Bad Meteorology? And I think most of the post is a little too complicated, and not detailed in certain portions.
Greenhouse gases, such as carbon dioxide, are radiatively inactive in the visible wavelength - most of the radiation from the sun is in the visible portion of the spectrum. However, they are radiatively active in the infrared portion of the spectrum. The reasons which will be glossed over here, just as at CAP. The earth just so happens to emit radiation at the infrared wavelengths. The visible radiation from the sun reaches the earth’s surface where it is mostly absorbed. Because the earth has a temperature above absolute zero, it radiates according to Planck’s law. Remember those greenhouse gases that were active at infrared wavelengths? They absorbed the outgoing radiation emited by the earth. Then, because they too have a temperature above absolute zero, they emit radiation as well. The trick is that they (as well as the earth and sun and you and I and everything else) radiate isotropically, which basically means that half radiates to space and have radiates back toward the earth. (It’s actually slightly more complicated than this obviously.)
In short, “The surface of the Earth is warmer than it would be in the absence of an atmosphere because it receives energy from two sources: the Sun and the atmosphere.” (Bad Meteorology)
But don’t confuse the greenhouse effect with global warming. An increase in greenhouse gases will necessarily increase the ill-termed greenhouse effect, which will cause the temperature at the surface of the earth to rise. It will also cause the temperature in the upper atmosphere to cool. Which is fairly unrelated to global warming from a human perspective, unless we can find a way to repeal the law of gravitation so everyone can float! But, an increase in carbon dioxide won’t cause a huge increase in the global temperatures. Neither will methane. Or lots of other greenhouse gases. The most important greenhouse gas is water vapor. And it’s usually included in climate models in “feedbacks”, which are just was in which we think the water will react to increased surface temperature. Water feedbacks are not as well known as how the temperature will react to increased CO2.
Perhaps the most unknown feedbacks are related to aerosols. As far as I can recall, the indirect effects of aerosols on cloud albedo represented the largest source of uncertainty in the IPCC2001 and the 2007 SPM (the last one is one my desk somewhere, hidden under several large piles of papers).
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In short, “The surface of the Earth is warmer than it would be in the absence of an atmosphere because it receives energy from two sources: the Sun and the atmosphere.” (Bad Meteorology)
I notice elsewhere in the blog that the thermodynamic argument is being taken seriously. The Earth will receive background radiation from space. It will not receive heat from space.
A Greenhouse explanation is needed which does not require a heat sink, the troposphere, to warm its source, the earth. Otherwise, as Professors Gerlich and Tscheuschner point out, we have a source of unlimited perpetual motion.
Professor Thieme suggest the following: use the radiation balance equations to calculate the temperature at the tropopause - about 255 degrees K. Then calculate an adiabatic temerature increase to the surface using the pressure increase due to gravity and the gas laws.
The result is what we experience.