A compelling argument for the positive longwave response is a leading
alternate to Lindzen's IRIS although it receives less attention, and is known as the FAT
hypothesis (
from Dennis Hartmann) and arises
from the fundamental physics of convection only heating the atmosphere where radiative cooling is efficient, and thus the temperature at the top of convective cloudiness should be near constant as it becomes warmer.
My understanding is that the notion of null versus
alternate hypotheses came
from the statistical testing field where in general a null
hypothesis is an assertion that some phenomenon will be constrained to a subset, often a subspace, of the total universe of possible observations.