From the June 23, 2010 issue:
By Daniel Mintz
Press Staff Writer
In a visit to Humboldt County, the scientist whose research shows an overall decline in fog levels over the last century said that the trend is mostly seen before 1950 and is probably not connected to carbon emissions.
Dr. James Johnstone of the University of Washington described his widely reported research to local meteorologists and biologists on June 16 at the National Weather Service Office on Eureka’s Woodley Island.
Johnstone said that on average, coastal fog levels have decreased by about one-third since 1901, a conclusion that’s led to speculation about impacts to redwoods.
But the reduction trend has not been seen in the last decade and was strongest in the first half of the 20th century.
“You see most of the trend occurring before 1950,” Johnstone said in response to a reporter’s question about the potential link between fog reduction and global warming. He added that fog levels have been normal since 1998.
“If I were to have come across this data in 1997 and showed you the record from 1951 to 1997, it would look as though the fog was about to fall off the table and disappear in about another 15 years,” Johnstone said. “But starting in 1998, things restabilized at a level comparable to the early fog record.”
It appears that the trend toward reduced levels of fog is not a current one. “While we do see evidence of that long, gradual change – most of which happened in the early part of the century – at least for the last decade or so, things seem to be fairly stable,” said Johnstone.
Considering that, global warming is not suspected to be a culprit of the overall fog reduction. “There’s nothing that stands out in the data that shows me that this is connected to large-scale temperatures or carbon dioxide,” Johnstone said.
Daily record-keeping of fog levels has been very precise at airports in Humboldt County and Monterey since 1951 and in Sonoma County since 2003. Johnstone relied on temperature records to gauge fog levels prior to 1951.
Fog is important to redwoods because they conserve water when their leaves are saturated with moisture and lose it through sap transpiration during dry spells, causing drought stress. But changing climate conditions and population levels are routine for trees as long-lived as redwoods.
Johnstone said that there’s “good tree ring and sedimentary evidence” that California has “undergone major, major droughts that dwarf anything that we’ve seen in the modern record.”
“The 1,000-year-old trees that are out here have survived that sort of thing,” he said, adding during glacial periods, the population of redwoods was perhaps five or ten percent of what it is today.
Fog production is encouraged by the temperature contrast between cool coastal areas and warm inland areas. Sea surface temperatures influence coastal climate and Johnstone found that fog production spiked in 1951, when ocean surface temperatures were cooler.
In that year’s summer period, there was an average of 14.8 hours of fog per day, with 13 fog-free days. In 1997, when fog production dipped to a minimum, the daily fog average was 6.4 hours, with 62 fog-free days.
Last summer, however, the daily fog average was in the normal range at a little over 10 hours.
Still, there are segments of the data record that show decreasing contrast between coastal and inland temperatures.
Johnstone found that between 1901 and 1925, the average difference was 9.6 degrees Celsius. Between 1951 and 2008, the overall difference was 6.3 degrees.
“That’s pretty substantial climate change, I would argue,” said Johnstone.