Assessment of urbanization effects in time series of surface air temperature over land

Records of hemispheric average temperatures from land regions for the past 100 years provide crucial input to the debate over global warming. Despite careful use of the basic station data in some of these compilations of hemispheric temperature, there have been suggestions that a proportion of the 0.5 °C warming seen on a century timescale may be related to urbanization influences - local warming caused by the effects of urban development. We examine here an extensive set of rural-station temperature data for three regions of the world: European parts of the Soviet Union, eastern Australia and eastern China. When combined with similar analyses for the contiguous United States the results are representative of 20% of the land area of the Northern Hemisphere and 10% of the Southern Hemisphere. The results show that the urbanization influence in two of the most widely used hemispheric data sets is, at most, an order of magnitude less than the warming seen on a century timescale