One clue we can see is that NASA has been reworking recent temperatures upwards and older temperatures downwards - which creates a greater slope and the appearance of warming. Canadian statistician Steve McIntyre has been tracking the changes closely on his Climate Audit site, and reports that NASA is Rewriting History, Time and Time Again. The recent changes can be seen by comparing the NASA 1999 and 2007 US temperature graphs. Below is the 1999 version, and below that is the reworked 2007 version.

NASA's original data: 1999

NASA's reworked data: 2007

In order to visualize the changes, I overlaid the 2007 version on top of the 1999 version, above, and a clear pattern emerged. The pre-1970 temperatures have been nearly uniformly adjusted downwards (red below green) - and the post 1970 temperatures have been adjusted upwards (red above green.) Some of the yearly temperatures have been adjusted by as much as 0.5 degrees. That is a huge total change for a country the size of the US with thousands of separate temperature records.

How could it be determined that so many thermometers were wrong by an average of 0.5 degrees in one particular year several decades ago, and an accurate retrofit be made? Why is the adjustment 0.5 degrees one year, and 0.1 degrees the next?

Describing this more succinctly, the 2007 version of the data appears to have been sheared vertically across 1970 to create the appearance of a warming trend. We can approximate shear by applying a small rotation, so I tried "un-rotating" the 2007 graph clockwise around 1970 until I got a reasonably good visual fit at six degrees.  Read rest...

Average user rating