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Today there is a lot of discussion about the role of CO2 in contributing to Anthropogenic Global Warming (AGW). However, the question is, what is the foundational physics for this contention? Most of the sites and papers that discuss this question harken back to Svante Arrhenius and a paper that he wrote in 1900 regarding CO2 buildup due to the burning of fossil fuels. However, Arrhenius knew very little about the actual mechanism of how CO2 operates in absorbing radiation. This is because quantum mechanics had not been invented yet.
Over the years there has been much discussion about absorption of infrared radiation and there are much better expositions about it than mine such as this link.
However, what needs to be understood is that on realclimate and other sites it is said that CO2 is not at extinction, that is CO2 has not absorbed all of the energy from the sun and from the heat radiated from the Earth at the wavelengths that CO2 typically absorbs. This argument is linked here.
Here is the relevant excerpt from the realclimate website:
In any event, modern measurements show that there is not nearly enough CO2 in the atmosphere to block most of the infrared radiation in the bands of the spectrum where the gas absorbs. That's even the case for water vapor in places where the air is very dry. (When night falls in a desert, the temperature can quickly drop from warm to freezing. Radiation from the surface escapes directly into space unless there are clouds to block it.)
This is manifestly incorrect. In the picture that accompanies this article is a set of measurements taken in the late 1940's by both Russian and American scientists studying the upper atmosphere (to figure out how to build heat seeking missiles) that shows the amout of absorption of CO2, H20, N2O, CH4, and other molecules from the ultraviolet band on the left to the mid infrared on the right.
The top part of the chart shows the relative contribution in terms of temperature of the sun (left most hump), and the heat of the earth (right most hump). Even though both are shown as the same height, the sun is at a temperature far higher than the earth and its blackbody curve (the hump) is shown on the left side because it is at far higher energies than the infrared heat radiated from the Earth's surface.
The middle graph shows the absorption percentage at ground level with zero being none and 100% being all radiation absorbed. As can be clearly seen, both water vapor and CO2 are 100% absorbed. If you look toward the left side you will see several absorption features of water that are in energy bands of much higher energy value than the infrared absorption of CO2. CO2 has no absorption features in the visible light energy bands, which are orders of magnitude more energetic than infrared.
Now the lower graph shows the absorption of energy by water vapor and CO2 and Ozone at 11 kilometers, or 6.6 miles or 35,000 feet, which is about as high as 1940's era aircraft could reach. If you look closely the primary CO2 absorption feature at 15 microns (toward the right of the graph) is almost fully absorbed. Ozone at ~10 microns is about 60% absorbed. Now as realclimate accurately states, water vapor is not fully at extinction (absorbed) at that altitude. This means nothing as it is at the ground where all of the temperature rise is!. Now what is important to note is that CO2 is fully absorbed at ~15 microns and about 30% absorbed at the next major line at about ~4.7 microns.
What does this mean to climate science? The graph shown here was taken in the 1940's, well before the growth in CO2 as recorded at Mauna Loa and the subject of so much discussion over climate change. If CO2 at ground level was fully absorbed, then it does not matter how much more (up to a point of several percent of the total atmosphere which nobody claims) CO2 is injected into the atmosphere, there will be no further heating. Also, even at high altitudes, CO2 at ~15 microns is almost completely absorbed.
Now a technical person who is well versed in spectroscopy will say that the graphs are not representative as they do not show the narrowband features and they would be right. However, there is high resolution data available as well that shows the same thing that will be the subject of a follow on article.
