X-axis: surface temperature,
Y-axis temperature offset for 3.7 W/m2
Following the discussion with Lord Mockton on Wattupwiththat. I calculated the Stefan-Boltzmann temperature needed to adjust for a forcing of 3.7 W/m2.
It turns out that the hotter the surface, the less temperature increase is needed to offset the forcing.
Stefan-Boltzmann law: I = σT4
What the graph is saying is that a constant 3.7 W/m2 is inverse proportional to the first derivative of the Stefan-Boltzmann law
dI/dT = 4 σT3
Which gives dT = dI/(4 σT3),
or in approximation
ΔT = 3.7/(4 σT3)