A strange thing has been happening to the historic temperature dataset.  NOAA/NCDC have been frequently revising the temperatures, and not just for recent data, but for temperatures going all the way back to 1880.  The folks at C3Headlines have been keeping track of temperature data sets for several years.  They noticed that NOAA has been revising the dataset almost monthly, and noted that there were six revisions in December and three so far in January.  The revisions have a particular pattern.  Temperatures from dates before 1940 are being gradually made cooler, while temperatures since 1951 are being made warmer.  The effect is to make recent warming seem more alarming.  Other agencies that keep temperature data sets rarely make revisions.  See the story from C3Headlines here.