May 14 2007
Understanding the Greenhouse Effect
Yesterday Fermi Paradox had a post about Understanding the Greenhouse Effect. I think s/he is making it a lot harder and longer than it needs to be (and longer than the attention span of those that don’t already understand it).
I’ll try to explain the greenhouse effect in a short, simple way in this post.
I began by searching Google for “Understanding the Greenhouse Effect” (no quotes), the first result I get is from UCAR. I was surprised to see what they said.
Instead, parts of our atmosphere act as an insulating blanket of just the right thickness, trapping sufficient solar energy to keep the global average temperature in a pleasant range. The Martian blanket is too thin, and the Venusian blanket is way too thick! The ‘blanket’ here is a collection of atmospheric gases called ‘greenhouse gases’ based on the idea that the gases also ‘trap’ heat like the glass walls of a greenhouse do.
- Blankets act to supress convection, as do greenhouses.
- Greenhouse gases don’t trap anything, nor do they supress convection.
- The thickness of Martian and Venusian atmospheres have little to do with their temperature. What’s important is their optical thicknesses.
The rest of the page is better. Anyway, here’s my go at the greenhouse effect.
Greenhouse gases are radiatively inactive in the visible spectrum, and radiatively active in the infrared. The majority of incoming solar radiation is in the visible spectrum. Most of this shortwave radiation passes through the atmosphere and is absorbed by the surface. The earth, like every other object with a temperature greater than absolute zero, radiates. The amount and at what frequency is determined by its temperature. The higher the temperature, the more an object radiates, and at higher frequencies. The earth is a lot cooler than the sun, thus the earth emits its radiation at a lower frequency - in the infrared.
As I said before, greenhouse gases are active in the infrared, which means that the radiation that the earth emits will be absorbed by these gases. Then, because the gases have a temperature above absolute zero, they emit radiation. But, this emited radation is radiated isotropically - meaning in all directions. Thus, for each photon that is absorbed by a greenhouse gas, half of the energy will be sent back towards the surface. So the surface of the earth is heated by the sun and the atmosphere and its temperature is greater than if it was heated by the sun alone.
Wikipedia offers a more detailed explaination.
(1 votes, average: 1 out of 5)
Loading ...Related Posts:
7 Responses to “Understanding the Greenhouse Effect”
To reduce spam, comments are automatically closed 30 days after the last comment. If you would like to comment on any closed thread, please use the contact form at the top of this page.
Hi Atmoz,
that post is just a start. The explanation you give is okay, but it quickly gets hairy the more you go into the details. This post starts simple, but the next in the series will be more complex.
One question that bugged me was that the absorption by both CO2 and H20 are already quite high, so what difference does a little more make? H20 absorbs much more, so why is the CO2 still important? There is an answer, but it wasn’t that easy to find.
Btw, I am male
Stay tuned…
Just to clarify, I thought your post was great. But when talking to someone who doesn’t understand complex scientific topics, I think it’s best to start simple. Then when they understand that, add more details.
One obvious hole in my explaination is the amount of temperature increase due only to anthropogenic CO2 sources. It’s quite small, and it is the water vapor feedback that causes most of the temperature increase - see Real Climate: Water vapour: feedback or forcing?.
I look forward to your future articles.
[...] Understanding the Greenhouse Effect [...]
“So the surface of the earth is heated by the sun and the atmosphere and its temperature is greater than if it was heated by the sun alone”
But the atmosphere is colder than the surface, no energy is being created by external work, so, sadly, this explanation contradicts the second law of thermodynamics.
If the down radiation cancelled part of the earth’s radiation, the earth could gain heat from the sun, and warm until the original balance was re-established.
In a glass greenhouse this effect must exist, but it is not detectable. Since most of teh earth’s surface is water, and the heat transfer to the atmosphere is latent heat, is there any data to suggest that AGW really exists?
[Response: This is clearly wrong, and any second-year undergraduate in physics would be able to tell you why. I've already debunked it above, but clearly you weren't an English major either.]
“If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations — then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.”
– Sir Arthur Eddington
Another retired physiscist.
The cooler atmosphere cannot heat the warmer earth. It can, and does, slow down the cooling of the earth, but whether or not radaiative absorption adds much to this effect is my query.
Hi Amoz, the greenhouse effect as defined violates the second law of thermodynamics. By applying this law such a planetary machine can never exist even though it is taken for granted in widespread secondary literature. Rather than making charges and counter charges all parties need to get back to applying the scientific method. To do otherwise or claim “consenses” we are headed down the slippery slope of witch hunts. To put it another way why take drastic action which will do great harm to the ecconomic system of the US when most of the rest of the world will not make the same sacrifice?
[Reply: Please show quantitatively that the greenhouse effect violates the second law of thermodynamics.]
“qauntitatively” There are many different heat transfer phenomena, radiative transfer, heat transfer, mass transfer, energy transfer, etc AND many types of interfaces, static and moving, between solids, fluids, gases, plasmas etc, for which one cannot even write down boundry conditions. Therefore the “calulation”of the IPCC report to show the simple greenhouse effect is no calculation and does not work, as it is illustrated in a one dimentional framework, and makes the model they use fiction and not science in any sense. If anyone can take the above interfaces and produce calculations to back them up using the scientific method they will overturn the laws of physics. Until this is done it is fiction for political policy makers th claim that they can compute the influence of carbon dioxide on climate.