Editing Statistical Downscaling (section)

=== Delta Change and Linear Scaling ===
The simplest approach is the '''delta-change method''' (or perturbation method). A baseline historical climate record (e.g., from a local weather station) is adjusted by adding the projected mean change (delta) calculated by a GCM between a future simulation and the model's historical baseline. 
* '''For temperature:''' Future Temperature = Observed Temperature + (GCM Future Temperature - GCM Historical Temperature)
* '''For precipitation:''' Future Precipitation = Observed Precipitation * (GCM Future Precipitation / GCM Historical Precipitation)
While intuitive and preserving the observed historical variance, the delta method assumes that local weather variability, extreme event frequencies, and spatial correlation patterns will remain strictly identical to the historical baseline.