 Climate models are used for many types of purpose; just a few will be mentioned here (see also Petersen 2012: Ch.5). One important use is in characterizing features of the climate system that are difficult to learn about via available observations. For example, the internal variability of the climate system is often estimated from the variability seen in long GCM/ESM simulations in which external conditions are held constant at pre-industrial levels (Bindoff et al. 2013). Internal variability is difficult to estimate from the instrumental record, because the record is relatively short and reflects the influence not just of natural internal processes but also of changing external conditions, such as rising greenhouse gas concentrations. Estimates of internal variability in turn play an important role in studies that seek to detect climate change in observations (see  Section 5.1).