We know a lot of the history of how Hansen and USHCN corrupted the US temperature record in the year 2000, but much less is understood how he wiped out the northern hemisphere warmth from 1900 to 1940. The graph below shows National Academy of Sciences northern hemisphere temperatures (black) published in 1975, overlaid with the current GISS northern hemisphere plot (red). Hansen generally wiped 0-2-0.3 degrees off all pre-1940 temperatures.
Does anyone have any information how and when this was done?
Prior to both cheats, the northern hemisphere graph (black below) looked very similar to the US temperature graph (red below).
The cheat occurred in two stages. First Hansen corrupted the northern hemisphere data to make it appear different from the US data. Then he used that difference as an excuse to corrupt the US data.