Reference
Zhou, Y. and Ren, G. 2011. Change in extreme temperature event frequency over mainland China, 1961-2008. Climate Research 50: 125-139.
What was done
Working with the daily temperature records of 526 measurement stations included among the China Homogenized Historical Temperature Datasets compiled by the National Meteorological Information Center of the China Meteorological Administration, the authors evaluated trends in 15 different extreme temperature indices for the period 1961-2008; and based on the earlier findings of Zhou and Ren (2009) - which indicated that the contribution of urban warming to overall warming often exceeded 50% - they adjusted their results to account for the impact of each site's urban heat island effect.
What was learned
Zhou and Ren discovered that "urbanization intensified the downward trend in cold index series and the upward trend in warm indices related to minimum temperature." More specifically, they report that "the urbanization effect on the series of extreme temperature indices was statistically significant for the downward trends in frost days, daily temperature range, cool nights, and cool days," as well as for "the upward trends in summer days, tropical nights, daily maximum temperature, daily minimum temperature, and warm nights." And for these indices, they say that "the contributions of the urbanization effect to the overall trends ranged from 10 to 100%, with the largest contributions coming from tropical nights, daily temperature range, daily maximum temperature and daily minimum temperature," adding that "the decrease in daily temperature range at the national stations in North China was caused entirely by urbanization."
What it means
Speaking of urbanization or the urban heat island phenomenon, the two researchers concluded their paper by stating that "more attention needs to be given to the issue in future studies," which is something that IPCC contributors and reviewers need to look at much more closely in the future than they have in the past.
Reference
Zhou, Y.Q. and Ren, G.Y. 2009. The effect of urbanization on maximum and minimum temperatures and daily temperature range in North China. Plateau Meteorology 28: 1158-1166.