The effect of urban warming on the northern hemisphere temperature average

The significance of the urban warming effect on large-scale and hemispheric mean temperature series is assessed using estimates of the urbanization bias for stations in the United States produced by Karl et al. It is concluded that the Northern Hemisphere landmass average recently compiled by Jones et al. may contain a spurious warming trend which is, at the maximum, 0.1°C over the first eight decades of the twentieth century. This is considerably less than the long-term warming trend observed over the same period.