Recent California climate variability: spatial and temporal patterns in temperature trends

With mounting evidence that global warming is taking place, the cause of this warming has come under vigorous scrutiny. Recent studies have lead to a debate over what contributes the most to regional temperature changes. We investigated air temperature patterns in California from 1950 to 2000. Statistical analyses 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. Areas of intensive urbanization showed the largest positive trends, while rural, non-agricultural regions showed the least warming. Strong correlations between temperatures and Pacific sea surface temperatures (SSTs) particularly Pacific Decadal Oscillation (PDO) values, also account for temperature variability throughout the state. The analysis of 331 state weather stations associated a number of factors with temperature trends, including urbanization, population, Pacific oceanic conditions and elevation. Using climatic division mean temperature trends, the state had an average warming of 0.99 degrees C (1.79 degrees F) over the 1950-2000 period, or 0.20 degrees C (0.36 degrees F) decade. Southern California had the highest rates of warming, while the NE Interior Basins division experienced cooling. Large urban sites showed rates over twice those for the state, for the mean maximum temperatures, and over 5 times the state's mean rate for the minimum temperatures. In comparison, irrigated cropland sites warmed about 0.13 degrees C decade(-1) annually, but near 0.40 degrees C for summer and fall minima. Offshore Pacific SSTs warmed 0.09 degrees C decade(-1) for the study period.