If anything is imperfect it is the temperature series and the tree ring reconstructions of temperature. In the chart above I have the Taymyr Peninsular tree ring series that I got from the NOAA data base provided by Jacoby et. al, the Central England Temperature series provided by UK MET and the GISS global average temperature anomaly series provided by NASA GISS. There is also an average series which is the average of the Taymyr and CET for the period 1600 to 1970 where both series had data. The mean for each series (horizontal lines) were lined up by shifting the Taymyr and Giss series up to the mean of the CET based on the full length of each series. The GISS series has a much greater upward slope in this chart.
In this chart the start date is 1800 which is when I suspect agricultural expansion may have played some part in the warming in the northern hemisphere. In this case, GISS and the average are very similar in slope. The taymyr slope is the greatest which I suspect is due to better growing conditions in part due to agricultural expansion in Europe. Note that the means of the series are different because of the changed periods.
This chart has the 1880 start time of all series. Note that the means are even more different because of the reduced period.
This chart has the 1880 start with the means adjusted for the period. The GISS mean had to be shift upward by about 0.4 degrees to adjust the means and the Taymyr required downward adjustment of about 0.1 degrees.
While this proves absolutely nothing, it does indicate to me that there should be some uncertainty in what "average" is for the global and northern hemisphere temperatures.
Update: The following is full series length chart but with the 1880 to end of series mean used to align the series.
Note: chart was updated due to error.
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