In February I wrote a post asking How much Estimation is too much Estimation? I pointed out that a large number of station records contained estimates for the annual average. Furthermore, the number of stations used to calculate the annual average had been dropping precipitously for the past 20 years. One was left to wonder just how accurate the reported global average really was and how meaningful rankings of the warmest years had become.

One question that popped into my mind back then was whether or not - with all of the estimation going on - the historical record was static. One could reasonably expect that the record is static. After all, once an estimate for a given year is calculated there is no reason to change it, correct? That would be true if your estimate did not rely on new data added to the record, in particular temperatures collected at a future date. But in the case of GISStemp, this is exactly what is done. Last September I noted that an estimate of a seasonal or quarterly temperature when one month is missing from the record depends heavily on averages for all three months in that quarter.

On March 29 I downloaded the GLB.Ts.txt file from GISS and compared it to a copy I had from late August 2007. I was surprised to find several hundred differences in monthly temperature. Intrigued, I decided to take a trip back in time via the “Way Back Machine”. Here I found 32 versions of GLB.Ts.txt going back to September 24, 2005.

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%. Wow.

The next question I had was “how often are the months within specific years modified?” As can be seen in the next chart, a surprising number of the earliest monthly averages are modified time and again.

See larger graph here

It is at this point I would like to ask, does anyone have a copy of the GISS monthly and annual temperatures - the equivalent to GLB.Ts.txt - from a date earlier than Sep. 24, 2005? In the meantime, will the real historical record please stand up?

Read more here.