SACRAMENTO — For most Californians, one effect of global warming will be the opposite of what they might expect: cooler summer days.

In the first localized study of temperature changes in California, a San Jose State meteorology professor has discovered that summer temperatures declined measurably from 1948 to 2005 in areas near the coast — specifically in the Los Angeles Basin and the San Francisco Bay Area, the state’s two largest population centers.

Robert Bornstein presented his study, which has been preliminarily accepted for publication in the Journal of Climate, at the annual California Climate Change Conference this week.

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Previous studies have shown average high temperatures statewide have gone up over the last several decades, but, Bornstein said, those studies used data points spread out over wide areas. A closer look, he said, reveals that inland temperatures have risen faster than the statewide average, and coastal temperatures have actually declined.

More sea breeze activity

The reason for the trend, he said, is that as inland areas have become hotter, the high pressure has had the effect of generating more sea breeze activity. An analysis of air pressure differentials between the coast and inland areas from 1979 to 2005, he said, confirms the pressure differential has increased.

• A d v e r t i s e m e n t

Areas that have registered the most significant declines are not those directly along the coast, where ocean breezes have historically moderated temperatures, but rather in areas several miles inland that were previously less affected by onshore winds.

“The maximum cooling is near the leading edge of penetration,” he said. “We don’t know whether the sea breezes are more frequent, colder, last longer or have deeper penetration. It could be any of those.”

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