Perhaps the key point discovered by Smith was that by 1990, NOAA had deleted from its datasets all but 1,500 of the 6,000 thermometers in service around the globe.Now, 75% represents quite a drop in sampling population, particularly considering that these stations provide the readings used to compile both the Global Historical Climatology Network (GHCN) and United States Historical Climatology Network (USHCN) datasets. These are the same datasets, incidentally, which serve as primary sources of temperature data not only for climate researchers and universities worldwide, but also for the many international agencies using the data to create analytical temperature anomaly maps and charts.Yet as disturbing as the number of dropped stations was, it is the nature of NOAA’s “selection bias” that Smith found infinitely more troubling.It seems that stations placed in historically cooler, rural areas of higher latitude and elevation were scrapped from the data series in favor of more urban locales at lower latitudes and elevations. Consequently, post-1990 readings have been biased to the warm side not only by selective geographic location, but also by the anthropogenic heating influence of a phenomenon known as the Urban Heat Island Effect (UHI).
...he was deadly serious after comparing current to previous versions of USHCN data and discovering that this “selection bias” creates a +0.6°C warming in U.S. temperature history.
Prior to publication, NOAA effects a number of “adjustments” to the cherry-picked stations’ data, supposedly to eliminate flagrant outliers, adjust for time of day heat variance, and “homogenize” stations with their neighbors in order to compensate for discontinuities. This last one, they state, is accomplished by essentially adjusting each to jive closely with the mean of its five closest “neighbors.” But given the plummeting number of stations, and the likely disregard for the latitude, elevation, or UHI of such neighbors, it’s no surprise that such “homogenizing” seems to always result in warmer readings.
WUWT’s editor, Anthony Watts, has calculated the overall U.S. homogeneity bias to be 0.5°F to the positive, which alone accounts for almost one half of the 1.2°F warming over the last century. Add Smith’s selection bias to the mix and poof – actual warming completely disappears!
The scientists at NASA’s GISS are widely considered to be the world’s leading researchers into atmospheric and climate changes. And their Surface Temperature (GISTemp) analysis system is undoubtedly the premiere source for global surface temperature anomaly reports.In creating its widely disseminated maps and charts, the program merges station readings collected from the Scientific Committee on Antarctic Research (SCAR) with GHCN and USHCN data from NOAA.It then puts the merged data through a few “adjustments” of its own.
Smith attributes up to 3°F (more in some places) of added “warming trend” between NOAA’s data adjustment and GIStemp processing.
That’s over twice last century’s reported warming.