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 (, , J. Glascoe, and , 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: