Key finding:
"The finding of an observationally measured increase in downwelling radiation of 3.3W/m2 since 1988, in addition to the increase in downwelling radiation over the period as calculated by the IPCC, with little corresponding change in temperature, calls into question the applicability of the concept of “climate sensitivity”.
From the paper:
"That is to say, we get a bit over two watts per square meter of increased absorption for every additional kilogram of atmospheric water per square meter.That is an interesting finding which we can combine with the following look at the change in global average total precipitable water since 1988: There is a clear trend in the TPW data. The total change over the period is ~ 1.5 kg/m2, centered around the long-term mean of 28.7 kg/m2. Utilizing the relationship between water content and atmospheric absorption derived above, this indicates an increase in downwelling radiation of 3.3 W/m2 over the period. Note that this 3.3 W/m2 increased forcing from the long-term increase in water vapor since 1988 is in addition to the IPCC-claimed 2.3W/m2 increase since 1750 in all other forcings (see Figure SPM-5, IPCC AR5 SPM 5). The IPCC counts as forcings the long-term changes in the following: CO2, CH4, Halocarbons, N2O, CO, NMVOC, NOx, mineral dust, SO2, NH3, organic carbon, black carbon, land use, and changes in solar irradiance, but not the long-term changes in water vapor."
"This leads us to a curious position where we have had a larger change in forcing from water vapor since 1988 than from all the other IPCC-listed forcings since 1750."
In other words, All other forgings have acted at less than 20% of the IPCC lowest C-scenario assessment. They also document energy escape showing that water is most definitely a negative forcing at combating heat build up on earth.
