Why Do GHCN Adjust Temperatures?
By Paul Homewood
There has been much discussion recently about how GHCN have adjusted temperatures at many stations around the Arctic Circle. There have been certain misconceptions about why this should have happened, so I want to explain the logic behind what both GHCN and GISS do.
Let’s start with GHCN.
I have seen it claimed that GHCN adjustments are designed to “average out” regional temperatures – so, say, it is OK to alter Reykjavik’s temperature record if it shows a different trend to other stations in Iceland. I have also seen it claimed that the sole objective is to arrive at a “global average temperature”, and that, therefore, individual stations are irrelevant. Neither of these claims are true.
In their technical report GHCN make it absolutely clear why they perform “homogeneity adjustments”.
Surface weather stations are frequently subject to minor relocations throughout their history of operation. Observing stations may also undergo changes in instrumentation as measurement technology evolves. Furthermore, observing practices may vary through time, and the land use/land cover in the vicinity of an observing site can be altered by either natural or man-made causes. Any such modifications to the circumstances behind temperature measurements have the potential to alter a thermometer’s microclimate exposure characteristics or otherwise change the bias of measurements relative to those taken under previous circumstances. The manifestation of such changes is often an abrupt shift in the mean level of temperature readings that is unrelated to true climate variations and trends. Ultimately, these artifacts (also known as inhomogeneities) confound attempts to quantify climate variability and change because the magnitude of the artifact can be as large as or larger than the true background climate signal. The process of removing the impact of non-climatic changes in climate series is called homogenization, an essential but sometimes overlooked component of climate analysis.
To isolate these “abrupt shifts”, they use an algorithm. And it was changes to this algorithm in July 2011 by a Google Summer Student, Daniel Rothenberg, that suddenly produced this swathe of anomalous adjustments in Greenland, Iceland and Siberia. The Icelandic Met have confirmed that there have been no station moves or other non-climatic factors, which would have created the need for the adjustments in Iceland, and of course the algorithms in use previously in GHCN V2 and V3 did not spot anything unusual in the temperature data. So it appears that the new algorithm is not working correctly.
It is also perhaps worth pointing out that as the algorithm only looks for “abrupt shifts”, there is no UHI adjustment in GHCN, as the UHI effect would occur much too slowly.
Let’s move onto GISS then. GISS start with the GHCN adjusted data, but then carry out their own “homogeneity adjustment”. This is designed to take account of the UHI effect. Usually the older temperatures are increased at urban and peri-urban sites, so that at Reykjavik, for example, the 1940 temperature is adjusted upwards by 0.6C. The effect can be seen below.
Before UHI Adj After UHI Adj
Whether 0.6C is enough is another question!!