What is the Delta downscaling method?
In short: The Delta method, also called the change-factor method, transfers the GCM-projected change from a historical period to a future period onto an observed baseline. Temperature change is commonly applied as an additive difference, while precipitation change is commonly applied as a multiplicative ratio. It is transparent and robust for mean changes but does not fully correct every aspect of a distribution.
Basic calculation
For temperature, a modeled future-minus-historical change is added to the observed baseline. For precipitation, a modeled future-to-historical ratio is commonly multiplied by the observed baseline, with care required where historical precipitation is zero or very small.
Strengths and limitations
Delta downscaling is easy to interpret and preserves the modeled average change signal. A simple implementation may preserve much of the observed variability rather than representing projected changes in variance, frequency, or distribution tails.
Run the Delta change-factor method for CMIP5 and CMIP6 projections with SD-GCM.