Methodology and Results of Calculating Central California Surface Temperature Trends: Evidence of Human-Induced Climate Change?

A procedure is described to construct time series of regional surface temperatures and is then applied to interior central California stations to test the hypothesis that century-scale trend differences between irrigated and nonirrigated regions may be identified. The procedure requires documentation of every point in time at which a discontinuity in a station record may have occurred through (a) the examination of metadata forms (e.g., station moves) and (b) simple statistical tests. From this “homogeneous segments” of temperature records for each station are defined. Biases are determined for each segment relative to all others through a method employing mathematical graph theory. The debiased segments are then merged, forming a complete regional time series. Time series of daily maximum and minimum temperatures for stations in the irrigated San Joaquin Valley (Valley) and nearby nonirrigated Sierra Nevada (Sierra) were generated for 1910–2003. Results show that twentieth-centuryValleyminimum temperatures are warming at a highly significant rate in all seasons, being greatest in summer and fall (> +0.25°C decade−1). TheValleytrend of annual mean temperatures is +0.07° ± 0.07°C decade−1.Sierrasummer and fall minimum temperatures appear to be cooling, but at a less significant rate, while the trend of annual meanSierratemperatures is an unremarkable −0.02° ± 0.10°C decade−1. A working hypothesis is that the relative positive trends inValleyminusSierraminima (>0.4°C decade−1for summer and fall) 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.