Here is a summary of calculations that provide some indication of the relative importance of the greenhouse gases in warming the atmosphere. Additionally there is a decription of the forcing that increases in carbon dioxide exerts on the temperature of the system


This is taken from Appendix 4 of Aspects of the Greenhouse Effect by Jack Barrett available as an ebook from Amazon (£1.97).

The latest paper [J. Geophys. Res., 115, 20106, (2010)] concerning the attribution of greenhouse gas contributions to the greenhouse effect gives results shown in the Table.

Table Major contributions to the greenhouse effect

Absorber

Clear Sky

All Sky

H2O vapour

67%

50%

CO2

24%

19%

Clouds

25%

All others

9%

7%

The authors [Schmidt, Ruedy, Miller & Lacis] comment that 'since the attribution of CO2 is closer to 20% than 2%, it might make more intuitive sense that changes in CO2 could be important for climate change'. This is a reference to Richard Lindzen's unsubstantiated comment in a review of the 1991 IPCC publication, Climate Change: The IPCC Scientific Assessment in which he claimed that '98% of the natural greenhouse effect' is due to water vapour and stratiform clouds, and 'less than 2%' for CO2. This comment has been proliferated by extreme sceptics of climate change projections even though there seems to be no justification for it except from Lindzen's statement.

 


There are those who doubt that such small concentrations of greenhouse gases can have such large effects, but they should think about the addition of a small amount of a dye to a bucketful of water. They would not like 380 ppm of arsenic in their tea either! [We are indebted to Craig Bohren for the last remark.] The following shot shows the effect of 380 ppm of potassium permanganate on the visible appearance of water! The normal pressure units for gas concentrations are ppmv [parts per million by volume] and are equivalent to mole fractions. These are ratios of the number of moles of a substance in a mixture to the total number of moles of all substances in the mixture. The solution of potassium permanganate has a mole fraction of the substance of 380 ppm. The green part of visible light is absorbed resulting in the perceived purple [red + blue] colour.

 

 

 

                                                            

This graph shows the vertical distribution of temperature in the first 100 km of the atmosphere. The troposphere [0-15 km] has a negative temperature gradient as may be experienced when ascending in an airplane. Essentially this is because the troposphere is largely warmed from the Earth's surface with some warming from the direct absorption of some solar radiation. The tropopause separates the troposphere from the next layer; the stratosphere. The latter has a positive temperature gradient because it is warmed by absorption of solar radiation which tails off as the altitude decreases. At around 50 km altitude there is another U-turn in temperature at what is known as the stratopause and is the division between the stratosphere and the next higher layer which is the mesosphere. That is also warmed from above by the absorption of solar radiation. At around 90 km altitude there is another U-turn in temperature at the mesopause and the beginning of the thermosphere.

            Radiation emitted by the troposphere originates at fairly high altitudes, with that from the strongly absorbing bands coming from the highest points. In the case of radiation emitted by CO2, the radiation originates towards the top of the troposphere. The injection of more CO2 causes the radiation emitted upwards from the gas to come from slightly higher altitudes. Because of the negative temperature gradient this means that the intensity of the radiation will be lower, in spite of the greater CO2 concentration. Thus, the extra absorption leads to a positive forcing of the troposphere and the temperature of that layer would be expected to rise to restore radiation balance across the tropopause.

            Because of the U-turn at the tropopause the argument is just the reverse for radiation emission in the stratosphere. The radiation from the stratosphere includes some which is transmitted by the stratosphere and some which originates in the stratosphere at around 35 km altitude. Now, because more CO2 would cause this emission level to move to higher altitude the emission intensity will increase at the slightly higher temperatures. This has a negative forcing effect which encourages the stratosphere to cool to restore radiation balance between it and space.

            That such changes are occurring may be seen from the satellite-derived data shown in the diagrams below. The lower stratosphere has a cooling trend, although there has been little change since 1992 after the Mount Pinatubo eruption. The other regions of the atmosphere are showing slight warming trends, but again it must be stated that not very much change has occurred since the 1998 El Niño event.