Urban corrections in Gistemp (and GHCN-M), “then and now”

Further content coming soon. This placeholder post published now to allow a link to the coming post to be included in an online comment.

Hansen et al. 1999 (Hansen, J., R. Ruedy, J. Glascoe, and M. Sato, 1999: GISS analysis of surface temperature change. J. Geophys. Res., 104, 30997-31022, doi:10.1029/1999JD900835) illustrates the Gistemp urban adjustment with two examples, Tokyo and Phoenix.



This post first of all updates these two examples. First consider recent data, Gistemp up to September 2015 (the “now”). In all plots below the values in the key show the temperature rise, in degrees C per decade for the corresponding data set. Data series based on GHCN-M v2 (the “then”) are shown as dotted lines:



Most of the adjustment comes from the adjusted GHCN-M data now used by Gistemp as input. Gistemp itself makes a smaller contribution to the overall adjustment.



In this case GHCN-M adjusted and Gistemp contribute roughly equally to the overall adjustment.

The overall adjustment for Tokyo, 16 years on, is somewhat less that Hansen et al. 1999 had shown, although they suggested that “The true nonclimatic warming in Tokyo may be even somewhat larger than suggested by Figure 3”, while that for Phoenix is almost the same 16 years on.

Now consider an older Gistemp data set, with data up to March 2009 (date chosen as the archived data is conveniently to hand on disk). In this case Gistemp was still using the unadjusted GHCN-M data as input.





The broken-line Gistemp adjustments are seen more clearly here, since the inclusion of the GHCN-M adjustments on the earlier plot tends to obscure the broken-line nature of that Gistemp adjustment.

Finally consider both dates plotted together:





We see

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2 Responses to Urban corrections in Gistemp (and GHCN-M), “then and now”

  1. I’m still struggling with these temperature adjustments and the various sources of data, but does Hansen explain why temperatures in say 1900 are adjusted by 2 deg K. Do they increase the maximum or reduce the minimum to get an increase? I assume that in 1900 they logged max and min to get an average, rather than having integrating thermocouples like now.

    I guess that I’ll need to look into all this GHCN and GISTEMP a bit more sometime.

    • Unfortunately, understanding does entail some work, and it really is best to read some of the relevant papers describing GHCN and Gistemp.

      Your example of changed hardware illustrated one reason why temperature records may need to be adjusted. Changed time of day for observation is another example, relocation of the station yet another, but unfortunately, however desirable this may obviously be in hindsight, there is often no record of when such changes occurred, and even less often overlapping records of “old” and “new” which would for example enable the change arising from a change of equipment or location to be evaluated. Hence the use of automated methods to try and detect and correct for such changes by comparison with nearby stations.

      Gistemp specifically attempts to correct for urban effects such as urban heat islands, where urbanization gives rise to an additional rise in temperature over that which would be observed in nearby rural areas. Hansen et al.in various papers do explain how they adjust. You will have to make your own mind up regarding whether these adjustments are reasonable.

      Adjustments are made on a station by station basis, and adjusted station records are combined into zonal, hemisphere and global means. There is not for example an adjustment of say 2 deg applied to all stations for 1900. (It is unlikely that you intended your question to be read as if there was such a single adjustment, but I want to keep this explanation both clear and as short as possible. They neither increase the maximum nor reduce the minimum when adjusting. Unfortunately there is a common misconception that the raw data is changed and the original data lost. This misconception often arises when people use data without checking exactly what the data shows. An example of this is when people use the GISS website to download what they believe to be old GHCN version 2 and current GHCN version 3 unadjusted data. What they download is old GHCN version 2 unadjusted data but current GHCN version 3 adjusted data. To download current GHCN version 3 unadjusted data you need to go to the NOAA FTP site. It has not been replaced by adjusted data, other than in the minds of those who do not take the trouble to check exactly what they are downloading.

      I hope this helps to get you started downloading the appropriate data.

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