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Mark Twain famously described his coldest winter as the summer he spent in San Francisco. One reason for the chill is the fog. It rolls in off the cold waters of the northeastern Pacific and regularly blankets northern California. It makes for pretty pictures and slow traffic. And it does something else that's largely unnoticed. It waters the big redwood trees along the coast.

Now, NPR's Christopher Joyce reports, scientists suspect the fog may be disappearing.

CHRISTOPHER JOYCE: Biologist James Johnstone grew up in Virginia, but when he moved to the University of California at Berkeley, he discovered what really weird weather can be.

Dr. JAMES JOHNSTONE (Biologist, University of California at Berkeley): It can be 55 degrees and humid along the coast. If you go up a mountain slope or go inland just say a few miles, you can go to 80, 90 degrees and 20 percent humidity.

JOYCE: This soggy to toasty divide scientists call it a gradient is part of what causes California's coastal fog. The other part is what's going on in the ocean.

Dr. JOHNSTONE: There's a systematic connection between fog and a variety of other ocean and atmosphere factors that seem to be ultimately driven by the position of the high pressure cell that resides off the coast of California in summer.

JOYCE: That atmospheric cell, along with cold ocean upwelling and the land gradient, all conspire to make fog. And after looking at historical climate data for the region, Johnstone found that the fog is diminishing.

Dr. JOHNSTONE: When you look at the evolution of that land temperature pattern, it strongly suggests that there's been maybe a 30 percent decline in fog frequency.

JOYCE: That can amount to three hours less fog a day.

Now, a lot of that decline happened before the late 1990s; the all-time low was 1997. Since then, the fog frequency has returned almost to what it was in the 1950s, when direct observations of fog began. Over the century, though, records suggest the long-term trend is a fog decline - and Johnstone doesn't know why.

And that worries people who care about California's redwood trees.

Mr. RUSKIN HARTLEY (Save the Redwoods): These forests take up to maybe 30, 40 percent of their water requirements in a year from fog, here in what is essentially a Mediterranean climate.

JOYCE: That's Ruskin Hartley. He runs a 90-year-old organization called Save the Redwoods, which helped fund the fog research.

Mr. HARTLEY: The fog that's caught sustains a rich web of life - everything from the 300-foot-high tree to the salamanders, to the ferns and the (unintelligible) that really wouldn't be there if it wasn't for the influence of the fog. So, what happens to the fog is critical to the future of these forests.

JOYCE: Johnstone's research appears in the Proceedings of the National Academy of Sciences. Most scientists predict changes in temperature, rainfall, and even ocean currents as the planet's climate warms. Johnstone says what ultimately happens to California's fog machine will depend a lot on how global climate change affects California.

Christopher Joyce, NPR News.

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