Russ Steele
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California is warming according to recent studies, but there seems to be some disagreement if the warming is because there is a stronger increase in the maximum daily temperature, or in the minimum daily temperature that is increasing. I have reported on a study, Recent California climate variability: spatial and temporal patterns in temperature trends by Steve LaDochy, Department of Geography & Urban Analysis, California State University, Los Angeles, in earlier posts here. From the LaDochy study Abstract:
We investigated air temperature patterns in California from 1950 to 2000. Statistical analysis were used to test the significance of temperature trends in California subregions in an attempt to clarify the spatial and temporal patterns of the occurrence and intensities of warming. Most regions showed a stronger increase in minimum temperatures than with mean and maximum temperatures.
And now this week I got a press release on yet another study of California global warming,
Human activities reshape California climate, This one led by Celine Bonfils, currently at the Lawrence Livermore National Laboratory.
On the shorter (50-year) timescale, results are sensitive to the choice of observational dataset. For both timescales, the most robust results are large positive trends in mean and maximum daily temperatures in late winter/early spring, as well as increases in minimum daily temperatures from January to September.
However, the Bonfils team did not find any increase is summer maximum temperatures.
We hypothesize that the lack of a detectable increase in summertime maximum temperature arises from a cooling associated with large-scale irrigation.
This seems to be inconsistent with the findings of a study lead by John Christy, Earth System Science Center, University of Alabama in Huntsville, when his team looked at temperatures in the California Central Valley.
Results show that twentieth-century Valley minimum temperatures are warming at a highly significant rate in all seasons, being greatest in summer and fall. A working hypothesis is that the relative positive trends in Valley . . . are related to the altered surface environment brought about by the growth of irrigated agriculture, essentially changing a high-albedo desert into a darker, moister, vegetated plain.
Now if I understand this correctly we have three studies that show warming and cooling at different times of the year, and one finds that irrigation contributed to California's Global Warming and the other one finds that irrigation contributed to the cooling, or in this case smaller increase in warming. Can We have it both ways? Can one study say the warming is due to rapid increase in minimum and another say it is due to strong increases in maximum temperatures. Which is right? Are any of them right? Both LaCochy and Bonfils used existing data sets from the Cooperative Network and the Historical Climatology Network. Networks that Anthony Watts and his team of volunteers have found significant station siting problems. Two separate analysis of the same network data concluded California is warming, but at different times of the year using different variables. On the other hand, Christy used the hand written station records for his analysis and validated the data from 1903 to 2003. More on problems with California Station data in future posts. I have e-mailed Celine Bonfils for more details on how they validated the station data reliability. LaDochy did not do any site data validation.
