Anthony Watts of Watts Up with That contributed to Climate Change with a chapter “Creating a False Warming Signal in the US Temperature Record.”
Several groups around the world track the global temperature using various methods, all of which involve adjustments to the raw data. Some massaging can be justified but Watts notes that most tend to exaggerate the urban heat island effect rather than correcting for it.
If this were forensic science such polluted and corrupted data would be tossed out as being unsuitable for the purpose of making a legal decision. Yet entire economies and national policies are being modified, based on the trends seen in this data.
The author traces the long and winding path from the thermometers in the field through many stages of processing, filing, converting, adjusting, to the final product that is released to the public. The system started with human beings making daily max and min readings, writing the numbers on pieces of paper and sending by snail mail to Head Office. Over the decades the process evolved with computerization and thermometers which record the temperature continuously so readings can be captured at any interval.
There is a discussion of the vexed topic of homogenization of data which will also came up in some of the other chapters.
He provided some graphic examples of the way the data are misleading. The Chicago O’Hare International Airport became the busiest port in the world in 2014, a big change from the time it was the Orchard Place/Douglas Field. The initials ORD persist as the International Civil Aviation Organization identifier. It was surrounded by farmlands for miles until over the decades it morphed into suburban megacomplex of concrete and tarmac. The average annual temperature increased by 2.35C per century from 1961 to 2013 but the method of measurement did not change while the immediate surrounds of the weather station changed out of recognition.
The extent of the potential for bias in that situation is revealed by the data from the McCarran Airport at Las Vegas. Between 1940 and 2013 the average temperature increased substantially – indeed by 4 degrees F in the last four decades that was a period of very rapid population growth. But the average conceals a very different story that is told by the average for the daily minimum and the daily maximum over the same period. The minimum has gone up but the daily maximum is as flat as a tack (or as flat as a zig zag horizontal line can be). Indeed the record high temperature set in July 1942 has not been beaten. So it seems that the urban heat island effect at that location makes a big difference at the lower end of the scale during the night (and hence lifts the average for the day) but not at the upper end during the day. Presumably it is routinely so hot during the day that the heat island effect makes no difference.