Global Warming Greenhouse Gas Science

The purpose of this page is to provide a simple explanation of why some gases are Global Warming (GW) gases and others are not. Quite a few people have wondered why carbon dioxide (CO2) and methane (CH4) are called greenhouse gases when they are in very low concentration in the atmosphere while nitrogen (N2) and oxygen (O2) which make up 99%+ of the atmospheric gases are not. To understand why this is so you need to understand the mechanism of the greenhouse effect.

It is well known in physics that there are three mechanisms of heat transfer from one body to another. They are:

Because the Earth is surround by an almost perfect vacuum we can ignore conduction and convection as they require either a physical contact between the two bodies (conduction) or the movement of a heat containing fluid (convection).

That leaves radiation as the only mechanism by which heat can enter and leave the Earth. The Earth heats up due to radiation from the Sun and the Earth cools by radiating heat out into space. Because of the properties of the gases that make up the atmosphere the light that comes from the Sun, which is mostly in what we call visible wavelengths, is not absorbed by the atmosphere. However the Earth radiates most of its heat away in what we call the infra-red wavelengths.

The wavelengths that a hot body radiates at is determined by its temperature and can be described by the Planck Radiation Formula which is way too complex to discuss here but if you are interested there is a nice explanation at:

Wikipedia: Planck's law or
HyperPhysics (GSU): Blackbody Radiation

In a nutshell the cooler a body is the lower the wavelength of the radiation it will emit. As the Earth has a fairly cool absolute temperature of around 300K it radiates most of its energy in the infra-red wavelength. The greenhouse effect is a result of the Earth radiating in the infra-red region. To understand why you need to know a bit about how gases absorb and emit radiation.

The reason that CO2, CH4 etc. are greenhouse gases and the main atmospheric gases (O2,N2) are not is because they absorb radiation at the wavelengths that the Earth emits at. That greenhouse gases absorb in the infrared region is because infrared absorption is due to intramolecular motion (vibration, rotation, and bending). The diatomic gases (N2,O2) are to 'stiff' to flex easily this way. The triatomic CO2 and penta-atomic CH4 are much more flexible and therefore they do absorb in the infra-red.

An infrared spectrophotometer (a very common laboratory instrument) can be used to measure and quantify the IR absorption. You can see the relevant spectra on the web at:
Iowa State
or
brneurosci.org

To summarize:

All these are facts that can be easily demonstrated by experiment using equipment available in most high school physics labs.

The only non-obvious facts are that CO2, CH4 etc. are greenhouse gases and the main atmospheric gases (O2,N2) are not and this is due to their chemical structures as explained above.

The idea that CO2 in the air is an important determinant of global temperature is actually quite old, it was first theorized by Svante Arrhenius (1859-1927) in a paper published in Philosophical Magazine 41, 237-276 (1896)[1].

He is one of the most respected of scientists and in fact the symbol 'A' is used for the activation energy of a chemical reaction in his honor. He even related the concentration of CO2 to the past geological variation in the Earths temperature.

"One may now ask, How much must the carbonic acid vary according to our figures, in order that the temperature should attain the same values as in the Tertiary and Ice ages respectively? A simple calculation shows that the temperature in the arctic regions would rise about 8 deg. to 9 deg. C., if the carbonic acid increased to 2.5 or 3 times its present value. In order to get the temperature of the ice age between the 40th and 50th parallels, the carbonic acid in the air should sink to 0.62--0.55 of its present value (lowering of temperature 4-5 deg. C.)."

Ref: Le Moyne College

Not bad work, comparing it to the current supercomputer models, considering it was published in 1896.

His work was never considered to be very controversial, the controversy has only come about because humans are causing a rise CO2 now. Even though the majority of annual emissions do come from natural sources, these natural sources have long been in equilibrium with natural sinks of CO2, the manmade emissions have upset that balance. The annual accumulation is easy to measure and it is due to humans: Graph courtesy of NOAA, Mauna Loa Observatory Atmospheric Carbon Dioxide - Mauna Loa