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Fog is Down in Calif.'s Farm Belt, Boding ill for Crops

By Debra Kahn, E&E reporter
5/27/2014 12:24:16 PM

The thick fog that rolls into California's Central Valley every winter is on the decline, spelling trouble for the state's agriculture industry.

To achieve high yields, fruit and nut trees need a chilly period to rest, which in the case of California's Central Valley is created by dense fog that blocks the sunlight and lowers temperatures.

That phenomenon, known as "tule fog," has been declining over the past 32 years, according to a new study from researchers at the University of California, Berkeley, published this month in the American Geophysical Union's Geophysical Research Letters.

Environmental scientists Dennis Baldocchi and Eric Waller, found that the number of winter fog events in the Central Valley has decreased 46 percent on average from 1981-2014. The region is home to almonds, pistachios, cherries, apricots and peaches that in some cases represent 95 percent of national production.

Fog covers the Golden Gate Bridge. Credit: Flickr/Patrick Smith)

"This effect can have detrimental effects on yields of fruit and nut trees -- crops that have multibillion dollar values to our economy," the paper says.

The lower half of the San Joaquin Valley is foggy up to half of the winter. But the trend observed over the last 30 years means that the region could see 400 fewer hours of winter chill per year. That could increase plants' internal temperature up to 4 degrees Celsius above air temperature, the study estimates. On foggy days, in contrast, plant buds stay close to air temperatures.

The study stops short of claiming the decrease is solely due to global warming, as researchers observed an increase in fog between the 1940s-70s. As well, concentrations of air pollution could have contributed to the observed decrease in fog.

"At this stage, the concurrent roles of changes in air pollution, agricultural burning and climate on this fog time series remain unanswered," the study says.

The study recommends that farmers tailor their decisions more to the presence or absence of fog, rather than heeding general warming trends.

A farming industry representative said he wasn't surprised by the results. "I grew up in Stockton; I remember some days when it was very, very foggy, and I don't think we've had as many of those, just anecdotally," said Dave Kranz, spokesman for the California Farm Bureau Federation.

He couldn't say whether reduced fog has had a direct effect on crop yields so far, due to other confounding variables that have increased yields, like water efficiency and better crop management. He said that researchers are already working on developing orchard varieties that could adapt to less chill but that it would have to be a long-term adaptation tool, since trees are planted to last many years if not decades.

The California Energy Commission, which partly funded the study, said that it did so in order to examine energy demand. Agency staff said the findings suggest that less fog in the Central Valley results in warmer days but colder nights, which could shift demand for heating to the nighttime hours. As well, less-foggy days would provide more sunlight for solar panels.

Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500.

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