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Spurious Warming Signal Detected in USHCN Temperatures
Balling Jr., R.C. and Roy, S.S. 2004. A spatial entropy analysis of temperature trends in the United States. Geophysical Research Letters 31: 10./1029/2004GL019630.

The authors note that the United States Historical Climatology Network (USHCN) database has emerged as one of the most respected repositories of data that are used for regional-scale analyses of historical temperature trends, but that various adjustments that have been made "for station moves, instrument changes, urbanization, and/or time of observation biases" may have "created additional warming in the time series" and, therefore, that the validity of these adjustments is "a matter of considerable debate in the climate change community (e.g., Balling and Idso, 2002; Vose et al., 2003)."

What was done
To further explore this topic, the authors employed the concept of spatial entropy to estimate the degree of disorder in the pattern of temperature change across the 1221 USHCN stations over the period 1951-2000, examining the relationship between a station's entropy and the magnitude of the linear temperature change at that station.

What was learned
Balling and Roy found that "spatial entropy levels are significantly and positively related to the observed temperature trends," which suggests, in their words, that "stations most unlike their neighbors in terms of temperature change tend to have a higher temperature trend than their neighbors." In fact, they found that the warming trend of the highest of the seven spatial entropy classes they encountered was more than double that of the lowest spatial entropy class. Overall, they report a +0.06C temperature increase over the period of record for each unit increase in spatial entropy, where the average temperature increase for all stations over the period 1951-2000 registered +0.26C.

What it means
In the words of the authors, "these results suggest that the USHCN contains some questionable warming signals at some stations, despite the many attempts to quantitatively control for these contaminants." Hence, they conclude that "the adjusted records continue to contain any number of contaminants that increase the temperature trend at some stations."

Balling Jr., R.C. and Idso, C.D. 2002. Analysis of adjustments to the United States Historical Climatology Network (USHCN) temperature database. Geophysical Research Letters 29: 10.1029/2002GL014825.

Vose, R.S., Williams Jr., C.N., Peterson, T.C., Karl, T.R. and Easterling, D.R. 2003. An evaluation of the time of observation bias adjustment in the U.S. Historical Climatology Network. Geophysical Research Letters 30: 10.1029/2003GL018111.

Reviewed 26 May 2004