In a study presented at the American Geophysical Union on December 17, 2015, Watts et al showed that the land-based meteorological sites used for measuring temperature in the continental United States (CONUS) yield more than 50% bias in the 30-year trend for temperature change. 
The study identified 410 “unperturbed” stations among the 1218 weather stations in the NOAA’s U.S. Historical Climatology Network using a metric for classification and assessment based on proximity to artificial heat sources and heat sinks which affect temperature measurement (Leroy 2010).
Unmodified data from the 410 unperturbed stations show a 30-year temperature trend from 1979 to 2008 of .204ºC while data from the excluded stations show a trend of .319ºC, more than 50% higher. NOAA published data for all 1218 stations, which is processed by numerous algorithms to “improve” the data, shows an even higher trend of .324ºC.
The authors conclude, “We believe the NOAA/NCDC homogenization adjustment causes well sited stations to be adjusted upwards to match the trends of poorly sited stations.”
For the complete study and press release, see the posting on WUWT.
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