It would be useful to see what ERA-Interim does get for the global average surface absolute T for your 1989-99 base period. The work that estimated the 14 number (it was 14.02 for the globe) made lots of assumptions over the sea ice areas and the Antarctic. I always thought this was accurate to about +/- 0.5 deg C. It was impossible to explain this accuracy of the absolute vs the much smaller error bars on the anomalies to any journalists.
Scientists Locate The Missing Heat
14 hours ago