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This is what I got PDF
Thanks for this comment paper muf. Now I see how the IPCC sees 'water vapour (WV) feedback' as the mediator for this "hot-spot". This has always befuddled me, as I can only see WV's actions as a 'negative forcing'!
The Radiative Case For WV:
WV is the most abundant poly-atomic molecule (more than two atoms) in Earth's atmosphere and at an average approximate percentage of 1% of the total atmospheric gas mixture pretty well saturates the many radiative 'windows' of the spectra that it is associated with. Thus, a doubling of WV to 2% of the total atmospheric gas mix would have 'negligible radiative effect' to the already saturated "radiative 'windows' of the spectra that it is associated with". There can not be any 'discernible' "positive radiative feedback" caused by a doubling of WV. However, any 'large reduction' of WV in the current average atmospheric gas mix may well lead to a 'strong negative radiative feedback'!
The Latency Case For WV:
WV is a lighter than air gas with a large latent heat of phase change between a liquid (water) and gas (WV) and the compound is found at most of Earth's atmospheric temperatures, as it fairly easily "sublimates" from its solid phase (ice). Though in the upper stratosphere, and above, becomes the product of its component atoms (hydrogen (possibly in monatomic form) and monatomic-ozone, with compound variations between the component atoms and other inclusions). However, the "lighter than air" property of WV ensures its short life span of about 9 days in the troposphere, when its exclusion is expedited by precipitation to the surface. Thus, no lasting effect is forced upon climate, but precipitated WV is continually replaced by surface evapotranspiration ensuring an unending supply of WV to the atmosphere relative to the surface insolation of the time! Thus, a 'running WV continuum' is established for atmospheric WV content! Now, WV has a high value of latent heat of phase change. Thus, precipitation levels indicate the energy released to a region of the troposphere that is open to radiate to space, but more to this, the diurnal effect of WV both absorbs energy when H2O presents in liquid phase at sunrise, and emits from WV phase after about 4 PM local, thus, is a buffer to temperature change in Earth's atmosphere. H2O is absolutely a negative forcing against temperature change!
Did you want the H2O case, or was it CO2? Did I mention that CO2 is "trapped" in the troposphere by H2O?
PS. If this doesn't make sense I'll blame Mum! She's had a bad evening.

Best regards, suricat.