In last week’s post on the Horrors of Homogenization I presented examples of the large distortions caused by the adjustments applied by NOAA/NCDC to individual raw surface air temperature records. In this follow-up post I analyze the equally large distortions that similar adjustments applied by GISS introduce at the hemispheric and global scale. The analysis is performed by comparing the adjusted GISS series with unadjusted series I constructed from scratch some years ago using the same set of raw records as GISS and procedures which I describe briefly below.
Record selection: Over a period of months I went through thousands of GISTEMP raw records one by one, selecting those which I could verify by comparison with adjacent records and throwing out those that didn’t fit. At the end of this process I had selected 800 raw records, about 500 in the Northern Hemisphere and 300 in the Southern Hemisphere.
Projection distance: I found that there was no one-size fits-all answer, so I segregated the records into areas in which temperature trends were similar but different to the trends in adjoining areas, ultimately blocking out 64 areas ranging in size from a few hundred thousand to several million square miles.
Averaging: I then averaged the records in each area and area-weighted the averages to construct global and hemispheric temperature time series.
Finally I compared my global series with the published GISS “meteorological station data only” global surface air temperature series, which was constructed from homogeneity-adjusted versions of the same set of GISTEMP raw records that I had used. I don’t have the original GISS series to hand so in Figure 1 I compare it with the current GISS global series, which is not significantly different (the data are expressed as anomalies relative to 1890-1910 means so that the series start off around zero. My series on this and other Figures are labeled “RA”):
Figure 1: GISS vs RA surface air temperature series, Global
The peaks and troughs match up well but GISS shows about 0.3C more overall warming, which as illustrated by the GISS-minus-RA difference plot is added quite regularly. Since both GISS and I used the same raw data set we can reasonably assume that this added warming was a product of the GISS homogeneity adjustments.
The question now becomes, is the added warming real or was it manufactured by the GISS homogenization algorithm? Comparing the hemispheric series gives the answer.
Figure 2 compares my Northern Hemisphere series with the GISS Northern Hemisphere series. The two are very similar, showing a close peak-to-trough match and the same amount of overall warming. (The match is in fact close enough to allow my results to be considered an independent verification of GISS’s.) Clearly the GISS homogenization process has not added any significant amount of warming in the Northern Hemisphere:
Figure 2: GISS vs RA surface air temperature series, North Hemisphere
Which of course means that it must have added it in the Southern Hemisphere. And indeed it has. About six-tenths of a degree C since 1900:
Figure 3: GISS vs RA surface air temperature series, South Hemisphere
Is there any chance this added warming is real? None. Figure 4, which plots 1970-2000 warming calculated from raw GHCN v2 surface air temperature records at 495 stations against latitude, confirms a large warming differential between the hemispheres. I spent a considerable amount of time looking into this differential and confirmed that there is no way the Southern Hemisphere raw records can legitimately be adjusted to the point where they show as much warming as the Northern Hemisphere records:
Figure 4: 1970-2000 warming from 495 raw surface air temperature records plotted against latitude
Yet this is what the GISS adjustments have done. The GISS Southern Hemisphere series shows the same amount of warming since 1890 as the GISS Northern Hemisphere series (Figure 5). The trend lines in fact have almost identical gradients. If this is a coincidence it’s a very remarkable one:
Figure 5: GISS North and South Hemisphere surface air temperature time series
Not even climate models can match GISS’s Southern Hemisphere series. I downloaded the multi-model surface air temperature (tas) means from the CMIP5 suite of models the IPCC used in the AR5 from KNMI Climate Explorer and plotted them against the GISS and RA series. There’s a good match with both in the Northern Hemisphere:
Figure 6: GISS (red) & RA (blue) surface air temperature series vs. CMIP5 climate model means (black), North Hemisphere
But the climate models fall far short of matching the warming shown by the GISS series in the Southern Hemisphere. If anything they are a closer match to mine:
Figure 7: GISS (red) & RA (blue) surface air temperature series vs. CMIP5 climate model means (black), South Hemisphere
How did GISS achieve this result? As Euan pointed out to me in correspondence: “it should be obvious that the S hemisphere has been homogenised to match a nearby set of stations – the N hemisphere.” And if one starts in an area of the N. Hemisphere that shows significant warming, homogenizes the surrounding raw records with it, and then moves progressively south homogenizing raw records with the already-homogenized records to the north it’s possible to see how something like this could happen. Confirming that it did happen, however, would take a lot of work.