Continuing with runs of CRM, I recently set up a comparison of water vapor. In this comparison are runs of increased water vapor for the entire troposphere and then for just the lowest two levels. For these runs, dewpoint depression is chosen to keep the total precipitable water similar between the two distributions.
Here is the run of various humidities spread through out the troposphere as constant dewpoint depressions:
For these profiles, the net radiance difference at the tropopause ( that is, the radiative forcing ) is relatively large and increases with increasing humidity.
Here is the run of various humidities restricted to just the lowest two levels:
The effect of increasing humidity, as per constant total precipitable water, is much smaller when only the low levels are humidified.
This result might be expected. Water vapor can obscure infrared effectively such that the temperature of the ‘top-of-the-water-vapor’ determines the energy emitted to space.
Paltridge et. al. 2009 found evidence that lower level humidities have increased in the radiosonde record, but that middle and upper humidities had not increased significantly. If the profile of water vapor feedback to global warming is such that only the low levels of the atmosphere experience an increase, the extent of the radiative forcing feedback may be reduced as in this example.