The Cowtan & Way version of Hadcrut4 uses a kriging technique to extrapolate values into those parts of the world where there are no direct measurements. Originally I had assumed that this fit was guided by the use of UAH satellite tropospheric temperature data and therefore more valid than that say used by GISS. However this is not really true as kriging has been used on all Hadcrut4 gridded data back to 1850, and the difference between the Hybrid (satellite) and kriging only results since 1979 is minimal. The net effect of this is to airbrush away regional variability. This can be seen by comparing the meridional temperature profiles of the original Hadcrut4.5 data with those of Cowtan & Way.
All profiles have been nicely smoothed out with warming concentrated at the North Pole and the Antarctic. The Antarctic profiles are all sorted so that the warmest also agree with those at the North Pole. What a tidy picture – but is it true or just a by-product of the fit?
Here are the annual averaged anomalies calculated using the above profiles.
The boost in warming from 2000 onwards is because the high latitude temperature profiles have been untangled by the algorithm. Kriging forces a smoothed ‘spline type’ dependence with latitude all the way to 90N.