Last year Anthony Watts said that it was a certainty that siting differences caused a warm bias:
“I can say with certainty that our findings show that there are
differences in siting that cause a difference in temperatures, not
only from a high and low type measurement but also from a trend
measurement and a trend calculation.”
“The early arguments against this project said that all of these
different biases are going to cancel themselves out and there would be
cool biases as well as warm biases, but we discovered that that wasn’t
the case. The vast majority of them are warm biases, and even such
things as people thinking a tree might in fact keep the temperature
cooler doesn’t really end up that way.”
Temperature trend estimates vary according to site classification, with poor siting leading to an overestimate of minimum temperature trends and an underestimate of maximum temperature trends, resulting in particular in a substantial difference in estimates of the diurnal temperature range trends. The opposite-signed differences of maximum and minimum temperature trends are similar in magnitude, so that the overall mean temperature trends are nearly identical across site classifications.
Mind you, if you read Pielke Sr’s spin, you might not notice:
Volunteer Study Finds Station Siting Problems Affect USA Multi-Decadal Surface Temperature Measurements
We found that the poor siting of a significant number of climate reference sites (USHCN) used by NOAA’s National Climatic Data Center (NCDC) to monitor surface air temperatures has led to inaccuracies and larger uncertainties in the analysis of multi-decadal surface temperature anomalies and trends than assumed by NCDC.
And on and on for over 600 words about alleged inaccuracies in poorly sited stations before grudgingly conceding
In the United States, where this study was conducted, the biases in maximum and minimum temperature trends are fortuitously of opposite sign, but about the same magnitude, so they cancel each other and the mean trends are not much different from siting class to siting class.
Hat tip: Steve.