Extrapolation means making a guess or prediction about something based on existing information and patterns. It's like figuring out what might happen in the future by using what we already know. Full definition
We hypothesized differential distributions of the various attachment styles as a function of age and ethnicity, on the basis of extrapolation from attachment theory's formulations concerning (a) the functions of attachment, (b) stages of human development, and (c) the effect of environmental perturbations on family structures and attachment bonds. (academic.oup.com)
Therefore we get polar areas that are covered by extrapolation by GISS and not covered at all by HadCRUT. (clearclimatecode.org)
The drop is mostly due to lower statistical and mixed statistical / heuristic contributions (Figure 2), because these methods are generally at least partially based on extrapolation from current / previous conditions whereas modeling contributions are generally not. (neven1.typepad.com)