There is 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. This paper investigates air temperature patterns in California from 1950-2000. Statistical analyses can 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 show a stronger increase in minimum temperatures than with mean and maximum temperatures. Areas of intensive urbanization show the largest positive trends, while rural, non-agricultural regions show the least warming. Strong correlations between temperatures and Pacific sea surface temperatures, SSTs, particularly Pacific Decadal Oscillation (PDO) values, also contribute to temperature variability throughout the state. The analysis of 330 state weather stations attempts to associate a number of factors with temperature trends, including urbanization, population, landuse changes, Pacific oceanic conditions and elevation.