I’ve tried to re-plotted the current Hansen data – right frame – in the same format as Hansen et al 1981 Figure 3. This is not as easy as it seems because Hansen et al 1981 does not include information on centering; no digital version seems to be available; and the modern zonal versions commence in 1900 rather than 1880.
I’ve updated the graphic to also illustrate an interesting 1998 version of Hansen’s data which has proved helpful in trying to figure things out. The left panel below is a plot of the 1998 version (black) centered on 1901-1970. After thinking about this for a while, it became clear that the reference period had to cover most of the century or else the position of the NH series in Hansen et al 1981 could not be replicated. I think that the black version of the NH series is pretty well matched to Hansen et al 1998. Notice that Hansen et al 1981 picked 1880 as a starting point and that this neatly truncates from view a temperature decline from 1866 to 1880 (which was in the original data version). Just a coincidence, I’m sure.
This shows that there doesn’t seem to have been much adjusting of the north (N of 24N) series up to 1998, but it has been adjusted down between 1998 and 2007. However, there do seem to have been adjustments to the other two series between 1981 and 1998, as well as between 1998 and 2007. One of the very important reporting changes between 1998 and 2007 is the truncation of the period 1880-1900. Notice the particular impact on the tropics, where 1998 results show late 19th century values similar to closing 1980 values, resulting in no net change between 1880 and 1980 in the tropics. What do you suppose the reason was for eliminating the reporting of zonal measurements for 1880-1900 in the most recent results?
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