In my discussion of Peruvian stations, I noted several examples of negative urban adjustments. A couple of readers inquired as to whether there were any examples of the opposite effect. In fact, Hansen et al 1999, the primary reference for the present-day GISS adjustment methodology provides using two such examples (the only ones discussed): Phoenix and Tokyo. Here is their Figure 3 showing the adjustment process for both sites. Hansen provided the following discussion of these sites:
The measured and adjusted temperature records for Tokyo and Phoenix are shown in Figure 3. These are among the most extreme examples of urban warming but they illustrate a human influence that can be expected to exist to some degree in all population centers. Tokyo warmed relative to its rural neighbors in both the first and second halves of the century. The true nonclimatic warming in Tokyo may be even somewhat larger that suggested in Figure 3 because some “urban” effect is known to occur even in small towns and rural locations [Mitchell 1953; Landsburg 1981]. The urban effect in Phoenix occurs mainly in the second-half of the century. The urban-adjusted Phoenix record shows little temperature change.
Hansen et al 1999 Figure 3. (a, b) Measured time series of temperature for Tokyo, Japan, and for Phoenix, Arizona; (c, d) adjustments required for linear trends of measured temperatures to match rural neighbors for the periods before and after 1950; and (e, f) adjusted (homogenized) temperatures.
As an exercise, I updated Hansen et al 1999 Figure 3 using current GISS values for Phoenix and Tokyo, shown in the figure below, drawn after the style of the earlier figure. The Phoenix adjustment has been reduced slightly in the earlier portion of the series by about 0.3 deg C, resulting in a slightly increased adjusted Phoenix trend relative to 1999. My guess is that the increased Phoenix adjustment results from changes in the USHCN adjustments between 1999 and 2001. These changes ended up increasing the trend in the USHCN network – and presumably the trend in Code 1 stations used as a comparandum for Phoenix was also increased.
There has been a slightly larger reduction in the Tokyo adjustment – the current adjustment at 1905 is about 1.2 deg C, while the 1905 adjustment in the 1999 version looks to be about 1.7 deg C – for an reduction in UHI adjustment of about 0.5 deg C between 1999 and 2007. In addition, the adjusted series has been shortened from an 1885 start to a start around 1905. In both cities, the adjusted trend is higher in 2007 than it was in 1999.
I’m not suggesting any malfeasance – I presume that both these effects occur from the operation of the GISS algorithm; but exactly why the algorithm has produced such changes seems to be a relevant question and one that is not discussed anywhere by GISS.
I also checked the rankings of Tokyo and Phoenix among all UHI adjustments – ranking by the largest total adjustment for each station. In the present network, Phoenix ranks 3rd overall and, is, as Hansen says, an “extreme” case. Tokyo is in the top 80 or so, but is not in the top 10.
In my earlier post on Peruvian stations, I identified some high negative UHI adjustments and have been criticized for selecting extreme cases to discuss. I disagree with this comment from the point of view of data analysis, something that I pride myself on: analysis of extreme cases helps groundtruth any algorithm to see whether it makes sense. It’s something that I routinely do this on many data sets. In this case, Hansen had already done analysis of extreme positive cases and was able to develop a highly plausible interpretation of the results.
Interpreting the negative urban adjustments is more challenging – I’ll return to this tomorrow.