A data set derived from the United States Historical Climate Network has been compared to two global land-based temperature data sets that have been commonly cited in connection with the detection of the greenhouse effect and in other studies of climate change. Results indicate that in the United States the two global land-based temperature data sets have an urban bias between + 0.1°C and +0.4°C over the twentieth century (1901–84). This bias is as large or larger than the overall temperature trend in the United States during this time period, +0.16°C/84 yr. Temperature trends indicate an increasing temperature from the turn of the century to the 1930s but a decrease thereafter. By comparison, the global temperature trends during the same period are between +0.4°C/84 yr and +0.6°C/84 yr. At this time, we can only speculate on the magnitude of the urban bias in the global land-based data sets for other parts of the globe, but the magnitude of the bias in the United States compared to the overall temperature trend underscores the need for a thorough global study.

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