It is just a first +
second order approximation of the trending.
Thus in
the second order approximation, the Earth's global temperature depends linearly on both the mean radiative forcing and the standard deviation of radiative forcing.
Well one way to see how reasonable the first order approximation is to perform
the second order approximation, which is
Not exact matches
One is using
approximations, and usually these are first (the mean) or at most
second order term
approximations from a putative expansion in the variables into a series that hopefully converges.