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A Tale of Two Temperature Trends
Reference
Knappenberger, P.C., Michaels, P.J. and Davis, R.E.  2001.  Nature of observed temperature changes across the United States during the 20th century.  Climate Research 17: 45-53.

What was done
Based on annual temperature data for the USA's 48 contiguous states, which were obtained from the U.S. Historical Climate Network database, the authors plotted the history of the country's mean annual temperature from 1910 to 1997.  This presentation revealed the existence of three distinct periods of climate change: a warming from 1910 to 1939, a cooling from 1940 to 1969, and another warming from 1970 to 1997.  For each of these periods, the authors eliminated all stations with more than 10% missing data, location moves of more than 0.1' of latitude or longitude, elevation changes of more than six meters, and time of observation changes of more than one hour, as well as all stations where liquid-in-glass thermometers in Cotton Region Shelters were replaced with newer thermistor-based temperature observing systems, which changeover was begun in the mid-1980s.

For each of the surviving stations of each of the three periods, all days of the year were ranked from coldest to warmest.  Linear regressions were then run on each station's maximum and minimum temperature for the 730 resulting time series.  The first two (maximum and minimum temperature) were for each year's coldest day, the second two were for each year's second coldest day, and so forth on up to each year's hottest day.  Finally, the resulting maximum temperature trends for all stations within each of seven roughly equal-area regions of the country were averaged, as were the resulting minimum temperature trends, after which national trends were calculated.

What was learned
The primary objective of this unique study was to compare the warmings of the first and third periods of the record, since the first period was more "natural," exhibiting a mean atmospheric CO2 concentration increase of about 0.32 ppm/year (an absolute increase of 9.3 ppm), while the last period was more anthropogenically-influenced, having a mean CO2 rate of rise of approximately 1.4 ppm/year (an absolute increase of 38.3 ppm), which is nearly 4.4 times greater (4.1 times on an absolute basis) than that of the first period.

In the case of the first or more natural period, the greatest rise in temperature occurred during the hottest days of the year.  By contrast, the warming of the last period - where the fingerprint of man would be expected to be more prominent, if it were detectable at all - occurred predominantly during the coolest days of the year, while days with the highest temperatures exhibited far less of an increase in temperature.  Hence, in the words of the authors, "the surface air temperature change that has occurred during the period of the greatest human influence on the climate is one in which increases of extremely low temperatures have dominated over those of high temperatures - a climate tending toward moderation rather than the extreme."

What it means
Political climatologists are always predicting greater extremes of weather as a consequence of rising atmospheric CO2 concentrations.  In analyzing the two periods of warming in this study, however, the period of time most likely to depict a CO2-induced increase in temperature extremes exhibited just the opposite behavior.  Hence, it would appear that this favorite claim of the climate alarmists is without basis in observational fact in the best temperature record currently available to scientists.