California's Vanishing Fog

by Bradford Plumer | February 16, 2010

The northern California coast tends to get smothered with fog during the summer—it just sits there and won't go away. (I'm pretty sure I complained about it constantly when I lived in San Francisco.) But now it turns out that the fog has actually been dwindling over the decades, which could spell bad news for the region's redwood trees:

A surprising new study finds that during the past century the frequency of fog along California's coast has declined by approximately three hours a day. Published in the Proceedings of the National Academy of Sciences, the researchers are concerned that this decrease in fog threatens California's giant redwoods and the unique ecosystem they inhabit.

"As fog decreases, the mature redwoods along the coast are not likely to die outright, but there may be less recruitment of new trees; they will look elsewhere for water, high humidity and cooler temperatures," explains coauthor Todd E. Dawson, professor of integrative biology and University of California, Berkeley professor of integrative biology with the Department of Environmental Science, Policy and Management (ESPM).

It's worth spelling out here that the redwoods rely very heavily on moist air hitting their needles and dripping down onto the ground; this fog drip provides anywhere from 25 percent to 40 percent of the trees' water. Indeed, that's a big reason why redwood roots are relatively shallow but often extend out over one hundred feet from the base—so they can collect the dripping fog.

Now, the reason for the fog decline seems to be that the temperature difference between the coast and the California interior has been narrowing. The researchers stressed that they're not certain whether the vanishing fog is part of a natural cycle or due to broader climate-change trends—to do that, they'll have to look more closely at redwood tree-ring data to reconstruct the region's climate over the past century, as well as analyze fog patterns elsewhere in the world. This study's mainly notable because the redwoods have enough problems as is (only about 5 percent of the original forests survive today), and they certainly don't need a dry spell on top of it all.

(Flickr photo credit: Lena M. Photography)

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