It seems like a simple question: What is the temperature. Well, we know now that surface temperature measurement is really hard, since its hard to get good geographic coverage when oceans cover 3/4 of the world and biases are a huge problem when most of the measurement points we had in the year 1900 have been engulfed by cities and their urban heat islands.
But John Goetz brings us a new answer to the question, what is the temperature? Answer: Whatever the GISS wants it to be, and they seem to change their minds a lot. He only has the last 2-1/2 years of GISS data but finds an astounding amount of variation in the data over these couple of years. Excerpt:
On average 20% of the historical record was modified 16 times in the last 2 1/2 years. The largest single jump was 0.27 C. This occurred between the Oct 13, 2006 and Jan 15, 2007 records when Aug 2006 changed from an anomoly of +0.43C to +0.70C, a change of nearly 68%.
I was surprised at how much of the pre-Y2K temperature record changed! My personal favorite change was between the August 16, 2007 file and the March 29, 2008 file. Suddenly, in the later file, the J-D annual temperature for 1880 could now be calculated. In all previous versions the temperature could not be determined.