The confusion is yours. Radiative forcing is measured in terms of the

change in radiative forcing since 1750 purely because tis is the most

useful measure for understanding the effect of CO2 concentration changed

by human processes related to industrialisation. The model is used to

estimate the degree of radiative forcing in subsequent years relative to

1750. The model simply measures radiative forcing. It measure

something that exists. If it is zero at year one that is the same as

taking a reference year as the base of measurement For example a

temperature of 0 degrees Fahrenheit or Celsius does not indicate that

heat in the atmosphere does not exist. Zero degrees Kelvin represents

the absence of heat energy, but that does not mean that heat energy

does not exist, just that it is not present at that measured

temperature. You are thus mixing up a measurement scale with a physical

state. It is quite possible that if CO2 concentration is reduced below

that of 1750 that the model would produce a negative measurement. Just

as a really cold day can produce a negative temperature compared with

the defined 0 degrees F or C.

Radiative forcing, like many other phenomena, cannot be measured

directly because there is no suitable instrument. It requires

mathematical transformation to filter out other influences to produce

the measurement number. The problem is not that it does not exist.

Your argument that the measurement of radiative forcing does not provide

evidence of a greenhouse effect is what is a real stretch. It has

been shown that higher atmospheric concentrations of CO2 result in

increased heat energy being trapped inside the atmospheric envelope.

Radiative forcing measures merely show how this varies with CO2

concentration. The model that is used for historical measurement can be

used to project future estimates of radiative forcing given differing

concentrations of atmospheric CO2.