Within the last few days, NCDC released a revised global temperature data set stretching all the way back to 1880. The chart below shows the monthly changes NCDC made to temperature anomalies based on the previous data set, which was the global temperature anomaly record through October 2010. (click on images to enlarge)
Amazingly, they found mucho, newly discovered degrees of global warming during the late 19th century. (How did they miss all this "warming" during their last fabrication revision of global temperatures way back in 2009?)
What the chart above also clearly indicates are major changes (cooling adjustments) to the global temperature record starting around 1938. The cooling adjustments continue up through 1965. After 1965, the majority of the adjustments made to the old 2009 temperature record are of "global warming" nature.
The chart below shows the cumulative adjustments for the two periods: 1938-1965 and 1966-2010. Based on this simple analysis, it is clear why NOAA/NCDC continues to fabricate revise global temperature records - they need to "cool" down the 1930 and 1940's and "heat" up modern temperatures to lend credence to the very wobbly AGW hypothesis.