In the 1970s long-term increases in California (CA) 2-m air temperatures were quantified, while in the 1990s these changes were related to increased greenhouse gas concentrations. The CEC, DOE, NASA, and NSF have initiated recent climate modeling studies to understand regional climate impacts in CA from modeled global change simulations through application of statistical downscaling techniques on scales down to about 10 km horizontal resolution. These efforts are useful to estimate resulting impacts on water resources, health, and air quality.

The current observational study reports on analyses of 2 m land and sea-surface temperatures from about 300 sites throughout CA for the last 60 years. Results for all of CA showed daily min temperatures with a significant upward trend, but corresponding max values with a smaller upward trend. Sub-area analyses showed these same results at inland Central Valley (CV) sites, but while coastal-plain areas in the San Francisco Bay Area (SFBA) and South Coast Air Basin (SCAB) generally also showed increasing min values, they showed decreasing max temperatures.

The current observational analysis has thus shown that global warming at inland CV sites have increased summer daytime horizontal temperature gradients across the state, which has increased the strength and frequency of cool summer marine sea-breeze flows into heavily populated and highly polluted CA coastal plains. These results will thus provide increased understanding of past and present trends in summer time ozone levels, as well as of the global and mesoscale (e.g., marine, land use, and topographic) physical processes causing these trends.