2015 was the warmest year in the instrumental record (dating back to the mid/late 19th century) in all the major surface temperature datasets including NASA’s GISTEMP:
However, 2015 still falls below the CMIP5 climate model mean value (left panel below). The difference between observations and the mean value from climate models is often used as an estimate of the ‘unforced’ or ‘internal’ variability in global temperature (right panel blow). It is apparent from this estimate that there was an unforced cooling event from ~1998 to ~2013. Thus the 2015 record temperature does not ‘erase’ the hiatus – it is totally legitimate to study why observations diverged from the model mean over this time period.
Because of the on-going El Nino event, 2016 will likely be even warmer than 2015 and thus 2016 may be above the climate model mean value for the 1st time since 1998. It will be very interesting to see what happens in 2017 and 2018. When neutral or La-Nina conditions return, will observations keep up with the steep rate of warming predicted by climate models?