Guest post by John Goetz
The GISStemp Step 1 code combines “scribal records” (multiple temperature records collected at presumably the same station) into a single, continuous record. There are multiple detailed posts on Climate Audit (including this one) that describe the Step 1 process, known affectionately as The Bias Method.
On the surface seems like a reasonable concept, and in reading HL87 the description of the algorithm makes complete sense. In simple terms, HL87 says that:
- The longest available record is compared with the next longest record, and the period of overlap between the two records is identified.
- The average temperature during the period of overlap is calculated for each station.
- The difference between the average temperature for the longer station and shorter station is calculated, and that difference (a bias) is added to all temperatures of the shorter station to bias it – bringing it in line with the longer station.
- The two records can now be combined as one, and the process repeats for additional records.
In looking at numerous stations with multiple records, more often than not the temperatures during the period of overlap are identical, so one would expect the bias to be zero. However, we often see a slight bias existing in the GISS results for such stations, and over the course of combining multiple records, that bias can be several tenths of a degree.
This was one of Steve McIntyre’s many puzzles, and we eventually figured out why we were getting bias when two records with identical overlap periods were combined: GISStemp estimates the averages during the overlap period. Read the rest of this entry »
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