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.
A conversation with Nima Arkani-Hamed
55 minutes ago