The MODTRAN programme is described on a previous page. Here are some calculations which put the suggested effect of doubling the atmospheric carbon dioxide concentration into perspective

This is a plot of some MODTRAN results for the temperature of the atmosphere in which the CO2 concentration varies from zero to 1000 ppmv. The intention is to show the logarithmic nature of the relationship between CO2 and surface temperature, i.e., the temperature rises non-linearly with every successive addition of CO2 causing smaller effects.

One method of estimating the contri­bution made by the presence of CO2 to the total 34·5°C of global warming is by the use of the MODTRAN programme and database, which contains all the spectral information about greenhouse gases and allows the calculation of fluxes at any alti­tude, looking downwards to the surface or upwards towards space.

Keeping everything constant except for the CO2 concentration and consid­ering the transfer of energy across the troposphere at an altitude of 15 km, the results of Modtran calculations are shown in the graph. For each point the temperature was reduced until radiative balance was re-established.

   The widely proph­esied doubling in CO2 concentration from the pre-industrial value of 285 ppmv to 570 ppmv would be associated with an increase of just 1·5°C.

 

   As ever, these figures are to be considered with caution since they represent the instantaneous effects of CO2 changes. They do not include the ameliorating effects of clouds, nor do they include the eventual global consequences of the instantaneous changes.