Excerpt from The Inconvenient Skeptic by John Kehr
The longer I am involved in the global warming debate the more frustrated I am getting with the CRU temperature data. This is the one of the most commonly cited sources of global temperature data, but the numbers just don’t stay put. Each and every month the past monthly temperatures are revised. Since I enter the data into a spreadsheet each month I am constantly seeing the shift in the data. If it was the third significant digit it wouldn’t bother me (very much), but it is much more than that.
For example, I have two very different values for January of 2010 since September 2010. Here are the values for January based on the date I gathered it.
Sep 10th, 2010: January 2010 anomaly was 0.707 °C
Jan 30th, 2011: January 2010 anomaly is now 0.675 °C
That is a 5% shift in the value for last January that has taken place in the past 4 months. All of the initial months of the year show a fairly significant shift in temperature.
Read the entire post here
Some of this may be related to late reporting of GHCN stations, a problem we’ve also seen with GISS. Both GISS and CRU use GHCN station data which are received via CLIMAT reports. Some countries report better than others, some update quickly, some take weeks or months to report in to NOAA/NCDC who manage GHCN.
The data trickle-in problem can have effects on determining the global temperature and making pronouncements about it. What might be a record month at the time of the announcement may not be a few months later when all of the surface data is in. It might be valuable to go back and look at such claims later to see how much monthly temperature values quoted in news reports of the past have shifted in the present.
More on CLIMAT reports here in the WMO technical manual.
UPDATE: For more on the continually shifting data issue, see this WUWT post from John Goetz on what he sees in the GISS surface temperature record: