Why does the Threshold Detector control output have an error associated with it?

The control output is also the product of an atmospheric model.

In detail

The control output from the Threshold Detector should not be confused with the observed data. Although the Weather Generator (the product of which is being analysed by the Threshold Detector) is trained using 1961–1995 observed data, the control climate is actually a modelled climate constrained by this observed data. Hence, the control runs need to be run multiple times, as does the future climate. This means that when the Threshold Detector analyses the output there is an error associated with both the control and future climates.