What was done
Based on temperature data obtained at what they call the "national reference and basic stations" at Beijing and Wuhan, China, plus similar data obtained at six rural stations near Beijing and four rural station near Wuhan, the authors calculated the rates of temperature rise over the periods 1961-2000 and 1981-2000, in an attempt to determine what portion of the observed warming at these two important stations is truly background warming and what is spurious urban-induced warming.

What was learned
The rate of increase in annual mean surface air temperature over the period 1961-2000 was 0.32°C/decade and 0.31°C/decade, respectively, for Beijing and Wuhan; but it was only 0.06°C/decade and 0.11°C/decade for the corresponding sets of rural stations that surround them. As a result, spurious urban warming was responsible for over 80% of the 1961-2000 temperature increase experienced at Beijing and a little more than 64% of the temperature increase experienced at Wuhan, while for the period 1981-2000, spurious urban warming accounted for 61% of the Beijing temperature increase and 40% of the Wuhan temperature increase. Furthermore, the researchers report that the Beijing and Wuhan stations are not located in the central parts of the cities, and that their findings are thus not representative of the cities' downtown areas, where urban heat island effects would be expected to be even greater.

What it means
Ren et al. note that the impact of urbanization on the surface air temperature trends of the two mega-city stations is much larger than what is reported for North China as a whole, as well as for Hubei Province. Consequently, they conclude "it is likely that a larger part of the surface air temperature increase in the country as obtained from .. national reference and basic stations has been caused by [an] enhanced urban heat island effect during the past decades." As a result, they say there is "a need for paying more attention to the selection of observational sites, and for further detecting and adjusting the urbanization-induced bias probably existing in surface air temperature records of city stations [our italics]," which would appear to us to be sound advice for all countries of the world. In fact, without continual checking for the type of spurious warming detected in Ren et al.'s study and appropriately adjusting for it when deemed necessary, public and scientific confidence in global temperature data sets cannot help but decline with each passing year.