NCDC Cooling The Past Again
By Paul Homewood
Climate At A Glance
Yesterday, WUWT picked up the story of how the NCDC have been cooling past temperatures in the US, thereby creating a warming trend. Essentially the state temperatures now shown by NCDC are lower than the originally recorded ones, which are still available online.
The explanation given is that “they changed from the ‘Traditional Climate Division Data Set’ (TCDD) to a new ‘Gridded Divisional Dataset’ (GrDD) that takes into account inconsistencies in the TCDD. “. (In simple terms, the old system simply averaged together all stations in a state, ignoring any potential weighting issues, where there was, for instance, a preponderance of stations in one particular area, or in warmer cities. The new system allows for the full area of each state to be adequately represented).
This sounds fine in theory, but how does it all work out in practice?
I have taken a detailed look at November temperatures in Alabama, comparing 1934 with 2011. Why Alabama – it was the first one on NCDC’s list! Why 1934? It was 1934 that Anthony’s article homed in on. Why November? November 2011 is the latest available monthly report available on NCDC’s website.
As illustrated on the above graph, according to NCDC, November temperatures in 1934 were 57.0F, and compare with 55.5F in 2011. (The full data is available here.) However the mean temperature originally published by the US Dept of Agriculture in 1934 is shown as 57.6F, as in Figure 1.
Furthermore the latest state report, before any adjustments, issued by NCDC for Nov 2011 indicate a statewide average of 55.3F. (This figure is arrived according to the “old method” and is available here.) In other words, NCDC show Nov 1934 as being 1.5F warmer than 2011, but the state reports show 1934 as being 2.3F warmer, a discrepancy of 0.8F. But are the 1934 and 2011 reports directly comparable?
In 1934, as Figure 1 shows, the statewide temperature was worked out by averaging 40 stations together. The current method, however, takes 132 stations, spread across 8 divisions. Each division is then individually averaged, and the average of each of these 8 divisions can then be amalgamated to arrive at the state figure. The divisions are each deemed to be climatically significant, rather than purely political boundaries. This, of course, should go a long way to addressing the problem of weighting.
So how do 2011 temperatures on the latest list of stations compare with their 1934 ones? These are listed in Figure 2 and sub totalled by division.
|ALL DIVISIONS AVERAGE||57.4||55.1||2.3|
Bearing in mind, the NCDC figures show a difference of 1.5F, it is immediately apparent that in every division, the difference is much greater. Over the state, there appears to be a discrepancy of 0.8F. Remember, we are not concerned with absolute temperature, but relative changes, so the argument about weighting should not really factor. A number of points seem to raise themselves:-
- Out of the 132 stations now currently monitored, only 19 have records back to 1934. Equally, most of the stations used in the original 1934 report no longer exist. This must surely make any comparison with earlier decades highly problematic, and must involve considerable “guesswork”.
- Out of the 132 stations, in Nov 2011, 50 have no data at all. (This applies when more than 10 daily figures are missing, and can be seen here).
- In Division 5, Piedmont Plateau, there are no stations at all that report back to 1934, which again must call into question how accurate NCDC’s temperature comparisons can be.
- The overall average of 2.3F calculated from the above sample, is exactly same as the figure arrived at by comparing the 1934 and 2011 state reports, i.e
55.3F v 57.6F57.6F v 55.3F. This indicates that the 1934 methodology was probably pretty sound.
It is abundantly clear that, in Alabama at least, the extra warming trend shown by NCDC cannot be explained by the change to a Gridded Divisional Dataset. Which, of course, brings us back to “Adjustments”.
The individual station temperatures, above, it should be remembered, are all “raw”. Rather than adding a cooling adjustment to allow for UHI, the NCDC mincing machine adds 0.8F of warming.