This section deals with two of the lesser important greenhouse gases, methane and nitrous oxide, and others which are even less important; the chlorofluorhydrocarbons

The graphs show how the greenhouse gases, methane, nitrous oxide, and two of the chlorofluorohydrocarbons have become more importent in recent times.

The rise of CO2 is also shown. The most important gas, water vapour, is missing but has been dealt with in previous pages. There is considerable discussion happening about the roles of methane and nitrous oxide in global warming. Their concentrations are much lower than that of CO2 and although the concentration of nitrous oxide is rising, that of methane seems to be levelling off.

The methane 'scare' arises from the observations that the gas is produced by both ends of cattle as they digest their food and from landfill sites where our rejected food decomposes. Methane is regarded by the IPCC as being 15-20 times more effective as a greenhouse gas than is CO2. This exaggeration contains an error only understood by chemists. The effectiveness of a greenhouse gas is regarded as the effect produced by a mass of the gas compared to the same mass of CO2. Since the relative molar masses of CH4 and CO2 are 16 and 44 respectively the method of comparing masses gives methane an advantage of a factor of 44/16 = 2.75 advantage over CO2. The other unfairness about the comparison is that methane is on the part of its absorption curve that is rising considerably with increasing concentration, CO2 is on a much flatter curve.

A global distribution graph of methane concentration over time is shown below.

 

It's clear that the production of methane is mainly in the northern hemisphere where most agriculture occurs. The seasonal changes are also obvious.

Nitrous oxide is derived from nitrogenous fertilizers and is on the increase. Here the mass error does not arise since the relative molar mass of N2O is 44. It is on the rapidly rising part of its absorption curve and should contribute more to any extra global warming that might be observed.

The chlorofluorohydrocarbons are present in extremely small quantities and can be discounted in the global warming stakes. They do, however, present a possible threat to the ozone in the stratosphere if they can get that high.