Q.*Why are some current station records different from what was shown before 2012?
A.*UK media reports in January 2015 erroneously claimed that differences between the raw GHCN v2 station data (archived here) and the current final GISTEMP adjusted data were due to unjustified positive adjustments made in the GISTEMP analysis. Rather, these differences are dominated by the inclusion of appropriate homogeneity corrections for non-climatic discontinuities made in GHCN v3.2 which span a range of negative and positive values depending on the regional analysis. The impact of all the adjustments can be substantial for some stations and regions, but is small in the global means. These changes occurred in 2011 and 2012 and were documented at that time.
To recap, from 2001 to 2011, GISS based its analysis on NOAA/NCDC’s temperature collection GHCN v2, the unadjusted version. That collection contained for many locations several records, and GISS used an automatic procedure to combine them into a single record, provided the various pieces had a big enough overlap to estimate the respective offsets; non-overlapping pieces were combined if it did not create discontinuities. In cases of a documented station move, the appropriate offset was applied. No attempt was made to automatically detect and correct inhomogeneities, assuming that because of their random nature they would have little effect on the global mean.
After October 2011, NCDC (now NCEI) added no more data to GHCN v2, so GISS used the replacement GHCN v3.1 as the base data. One of its differences from GHCN v2 is that multiple records are replaced by a single record, obtained by using for each month the report from the highest ranked source without applying any offsets when switching from one source to another. The resulting discontinuities are handled by NCEI when creating the adjusted version. Since the multiple records used by the GISS procedure no longer were available, GISS switched to using the adjusted instead of the unadjusted version of GHCN v3.1.
So the differences between the station records of the old v2 and the current site have mainly two causes:
a. the different combination procedures used by GISS and NCEI for stations with multiple records,
b. homogenization applied by NCEI to any station (single or multiple records), whereas none was applied by GISS.
As part of the v2->v3.1 transition in December 2011, GISS showed its impact*on this page. Though substantial for some stations and small regions, it barely changed any global mean trends. However, a few months later, NCEI applied some fixes and refinements to the adjustment scheme, replacing GHCN v3.1 by GHCN v3.2; that change did increase the global trend as documented and*explained here. Independent replication of the current GISS results is, e.g., provided by the Berkeley Earth dataset, created without the use of NCEI's adjustments. A list of all changes to the GISS analysis and their impacts is presented in the*History Section.
