UPDATE 23/3/12: I have discovered that 110 stations have changed numbering between CRUTEM3 and CRUTEM4 ! Therefore the map below has been revised to show just the new and discarded stations excluding those renumbered. The list of renumbered stations can be found here.
The new data from the Hadley Centre and UEA CRU (HadCrut4) has been released. The temperature anomalies calculated just from the station data is called CRUTEM4. I have downloaded all the new station data and compared the results with the previous iteration CRUTEM3. What has changed?
The most obvious difference is that many new stations have been added , while many others have been dropped. There are now 5549 stations in the set compared to 5097 in CRUTEM3. 738 628 new stations have been added while 286176 stations have been discarded. We can see exactly which stations have been added and lost in the map below. The red points are the new stations added in CRUTEM4, while the blue points are those discarded from CRUTEM3. Drag a rectangle in the map to zoom in. Click on a station to view data.
Clearly the new points are all in the northern Arctic regions especially over Russia. Some of the dropped stations have few updates since 2000, but whether that means they no longer exist is unclear. Many stations have been dropped from Northern America.
What is the effect of this on temperature anomalies ? I calculated the anomalies for both data sets using the Hadley provided scripts and compare both results below.
There is no real statistical difference beween the two results. Small differences that are apparent are concentrated at end of the 19th century (slightly lower) and more contentiously (slightly higher) during the last 10 years. HADCRUT3 has shown no warming since 1998 and this has become something of an embarrassment to the AGW narative. A zoom in to the last 10 years identifies the subtle new change.
CRUTEM4 now places 2010 a tiny bit warmer than 1998, but there is no statistical significance to this despite the propaganda value. Note also that this result covers just the land station data. HADCRUT4 also includes sea surface temperature data form the Hadley Centre which have also been updated. This may then explain other differences as discussed for example here. For details of how the sea surface data has been updated – see the Met Office Web site.