The contrarian tactic is to find some station that has been changed and beat the drum about rewriting history, or some such. It is usually one where the trend has changed from negative to positive. Since adjustment does change values, this can easily happen. I made a Google Maps gadget here which lets you see how the various GHCN gadgets are affected, and posted histograms here. This blog started its life following a classic 2009 WUWT sally here, based on Darwin. That was probably the most publicised case.
There have been others, and their names are bandied around in skeptic circles as if they were Agincourt and Bannockburn. Jennifer Marohasy has for some reason an irrepressible bee in her bonnet about Rutherglen, and I think we'll be hearing more of it soon. I have a post on that in the pipeline. One possible response is to analyse individual cases to show why the adjustments happened. An early case was David Wratt, of NIWA on Wellington, showing that the key adjustment happened with a move with a big altitude shift. I tried here to clear up Amberley. It's a frustrating task, because there is no acknowledgement - they just go on to something else. And sometimes there is no clear outcome, as with Rutherglen. Reykjavik, often cited, does seem to be a case where the algorithm mis-identified a genuine change.
The search for metadata reasons is against the spirit of homogenisation as applied. The idea of the pairwise algorithm (PHA) used by NOAA is that it should be independent of metadata and rely solely on numerical analysis. There are good reasons for this. Metedata means human intervention, with possible bias. It also inhibits reproducibility. Homogenisation is needed because of the possibility that the inhomogeneities may have a bias. Global averaging is very good at suppressing noise(see here and here), but vulnerable to bias. So identifying and removing possibly biased events is good. It comes with errors, which contribute noise. This is a good trade-off. It may also create a different bias, but because PHA is automatic, it can be tested for that on synthetic data.
So, with that preliminary, we come to Cape Town. There have been rumblings about this from Philip Lloyd at WUWT, most recently here. Sou dealt with it here, and Tamino touched on it here, and an earlier occurrence here. It turns out that it can be completely resolved with metadata, as I explain at WUWT here. It's quite interesting, and I have found out more, which I'll describe below the jump.
Cape Town (CT) has a long record, from about 1857. The GHCN datasheet for the station is here. I showed the graphs from it:
You can see in the top graph a sudden descent in about 1960. GHCN said this was spurious, and made the adjustment seen in the bottom graph. This had the effect of turning a neutral trend into an uptrend, which makes it red meat for WUWT. So how did this happen?
An important clue is that the new Cape Town airport was completed in 1960, and that is the current location of the station. But of course, it wasn't in 1857. In fact, the observations were taken for nearly a century from the then Royal Observatory, founded in 1820. This is notable enough that an inner suburb is named after it. But it seems that in Jan 1961, it trekked out to the new site:
How do we know, exactly? Well, the site at the observatory did not cease reporting. The record continues as another GHCN site, CAPE TOWN-SAAO:CSIR. The important clues here are:
- SAAO stands for South African Astronomical Observatory (its current name). CSIR is the SA govt research outfit (similar to CSIRO here), which would have been the operator. Berkeley Earth records SAAO:CSIR as a former name of Cape Town station, and says there was a move in 1861.
- The GHCN number of Cape Town is 14168816000 and of SAAO:CSIR is 14168816002. The last three digits were used in V2 of GHCN to mark duplicates - ie different records from what it regarded as the same place.
So I looked up the GHCN records for both locations. The record for SAAO starts in 1960, and runs parallel to Cape Town for a year, and diverges substantially in Jan 1961 - a divergence that is then maintained. Here is a table:
It looks as if the two sets of instruments were on the same site in 1960, and then separated, with Cape Town at the new airport being substantially cooler. Here now is a plot of CT unadjusted (pink), CT adjusted, and CT Observatory (since 1960):
What was objected to was the fact that unadjusted CT seemed to have no trend, while the adjusted had an uptrend. But if you consider the composite of pink to 1960 and then blue, that is the actual unadjusted record of the Observatory site. And the adjusted curve looks very similar to that record. There is further adjustment around 1890, which I haven't looked into; the shift in 1960 was the main issue.
Does it matter that they are separated by about 1.2°C? No, because both places (Obs and airport) are equally representative of the region.
Why didn't GHCN do what I did - make a full record of the Obs site, and regard the airport as a new site? Presumably the SA Met sent the composite Obs/airport record as the official CT, probably believing that the airport would be better supported in future. It seems this worked out, as the Obs record trails off in 2002.