text: The most important measure of model skill is of course its ability to predict previously unmeasured (or unnoticed) phenomena or connections in ways that are more accurate than some simpler heuristic. Many examples exist, from straightforward predictions (ahead of time) of the likely impact of the Pinatubo eruption (Hansen et al 1992), the skillful projection of the last three decades of warming (Hansen et al 1988; Hargreaves 2010) and correctly predicting the resolution of disagreements between different sources of observation data e.g., between ocean and land temperature reconstructions in the last glacial period (Rind and Peteet 1985), or the satellite and surface temperature records in the 1990s (Mears et al 2003; Thorne et al 2011). Against this must be balanced predictions that did not match subsequent observationsfor instance the underestimate of the rate of Arctic sea ice loss in CMIP3 (Stroeve et al 2007).