https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2012JD018509
Homogenization of the monthly temperature data via NCDC's PHA removes the majority of this apparent urban bias, especially over the last 50–80 years. Moreover, results from the PHA using the full set of Coop station series as reference series and using only those series from stations currently classified as rural are broadly consistent, which provides strong evidence that the reduction of the urban warming signal by homogenization is a consequence of the real elimination of an urban warming bias present in the raw data rather than a consequence of simply forcing agreement between urban and rural station trends through a spreading of the urban signal to series from nearby stations.
