The graph below shows the relationship between downwelling longwave radiation in the tropics (i.e. the greenhouse effect) and atmospheric CO2. I used RRTM_LW as the radiative transfer model, which is the model used by NCAR.
TROPICAL ATMOSPHERE ADJUSTED TO RIDGEWAY’S GLA MODEL
As you can see, CO2 is a very minor component of the greenhouse effect in the tropics, and almost all of its effect is in the first 20 PPM CO2. Once you get above 100 PPM CO2, adding more CO2 has almost no effect. Water vapor completely dominates, and other gases are almost irrelevant. An increase from 50 PPM to 1,000 PPM CO2 only increases downwelling longwave radiation by 1%.
The same model is used by the following organizations, and can be downloaded from this link : RRTM/RRTMG
RRTMG Global and Regional Model Applications
- ECMWF (European Centre for Medium-Range Weather Forecasts), Reading, UK
- Integrated Forecast System (IFS), ERA40…
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