Guest Post by Willis Eschenbach
Anthony has covered the National Geographic Northern Hemisphere temperature graph here. This is the graph under discussion.
Figure 1. Graph from November 1976 National Geographic article
Then I graphed it against the GISS Northern Hemisphere data. Here is that graph:
Figure 2. (Upper) Data from November 1976 National Geographic article, and GISS temperature data. Both datasets are for the Northern Hemisphere. (Lower) Difference between the two datasets (right scale).
A few notes, in no particular order.
1. Most of the ~ 0.2°C difference between the NatGeo and Giss data in the recent record is likely from the 1941 sea surface temperature (SST) adjustment. See here, here, and here for discussion of this adjustment. There is an abrupt jump 1940-1941.
2. For the middle part of the record, they track each other pretty closely.
3. There is another adjustment, again of ~ 0.2°C ,for unknown reasons, in the period from the start of the record to 1906. Again, there is an abrupt drop 1906-1907.
4. The existence of these two adjustments is shown by a discontinuity analysis (described here on page 2845 paragraph 2). This is the result of that analysis:
So we have two major adjustments of ~ 0.2°C, for one of which we likely know the justification, and for the other, I have no idea.
Go figure …