From the we told you so department and The Hockey Schtick: It is all about nighttime influence on minimum temperatures, mostly due to the heat sink effect of urbanization and nearby structures and paving.
New paper finds “surprisingly, there are many US weather stations that show cooling” over the past century
A paper published today in the Journal of Climate finds, contrary to popular belief, that US “monthly maximum temperatures are not often greatly changing — perhaps surprisingly, there are many stations that show some cooling [over the past century].
In contrast, the minimum temperatures show significant warming. Overall, the Southeastern United States shows the least warming (even some cooling), and the Western United States, Northern Midwest, and New England have experienced the most warming.”
In essence, this paper is saying the weather/climate has become less extreme, with little to no change in maximum temperatures “and even some cooling” of maximum temperatures in some stations, and warming of minimum temperatures. Thus the temperature range between minimum and maximum temperatures has decreased, a less extreme, more benign climate.
According to the paper, the warming in minimum temperatures is regional, with the SE US showing “the least warming (even some cooling),” suggesting that other processes such as ocean and atmospheric oscillations are responsible, rather than a uniform warming from AGW.
Note these results are after the huge up-justments made to the US temperature data and urban heat island [UHI] artificial warming, which could account for all or most of the warming of minimum temperatures.
From our 2012 draft paper: Acceptably placed thermometers away from common urban influences read much cooler nationwide:
A reanalysis of U.S. surface station temperatures has been performed using the recently WMO-approved Siting Classification System devised by METEO-France’s Michel Leroy. The new siting classification more accurately characterizes the quality of the location in terms of monitoring long-term spatially representative surface temperature trends. The new analysis demonstrates that reported 1979-2008 U.S. temperature trends are spuriously doubled, with 92% of that over-estimation resulting from erroneous NOAA adjustments of well-sited stations upward. The paper is the first to use the updated siting system which addresses USHCN siting issues and data adjustments.
The new improved assessment, for the years 1979 to 2008, yields a trend of +0.155C per decade from the high quality sites, a +0.248 C per decade trend for poorly sited locations, and a trend of +0.309 C per decade after NOAA adjusts the data. This issue of station siting quality is expected to be an issue with respect to the monitoring of land surface temperature throughout the Global Historical Climate Network and in the BEST network.