These differences or ratios are then applied to the observed baseline climate whether mean values, monthly or a daily time - series. (ipcc.ch)
Further uncertainties are therefore introduced into a climate scenario because observed data sets seldom capture the full range of natural decadal - scale climate variability, because of errors in gridded regional or global baseline climate data sets, and because different methods are used to combine model and observed climate data. (ipcc.ch)
Sounds like lots of baseline climate change data to me (sarcasm!). (wattsupwiththat.com)