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August 27, 2009

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California humidity blamed on currents

BOUTS of extreme muggy heat lasting for days, once rare in California, are becoming more frequent and intense due to ocean patterns altered by climate change, scientists said in a study released on Tuesday.

Research meteorologists at the University of California's Scripps Institution of Oceanography reached the conclusion after examining a severe 2006 summer heat wave that was blamed for the deaths of 600 people and 25,000 cattle in California.

That heat wave, like others before it, combined high humidity and heat, keeping temperatures elevated at night, rather than allowing for cooling that normally occurs in the dry conditions more typical for California.

Hot, sticky nights in turn lead to hot humid days, helping to perpetuate the heat wave, "and the cycle feeds on itself until the winds change," said Alexander Gershunov, who led the Scripps' team.

It's a cycle familiar elsewhere in the United States, especially in the traditionally hot and humid Deep South.

In California, this pattern is caused by a feedback loop in ocean currents off Mexico's Baja Peninsula, where a warming trend fueled by climate change produces higher temperatures, along with greater humidity.

"It turns out that warming of the ocean is partially responsible for heat waves in California changing their character," Gershunov said.

This highlights the importance of water vapor in climate change - accounting for more than 80 percent of the atmosphere's heat-trapping "greenhouse" effect, compared to 12 percent for carbon dioxide.

Although hot, muggy nights have become more common in California since the early 1990s, they remain relatively rare in the state, Gershunov said, while adding that it was crucial to track changing weather patterns.

"We care about them because people and animals and the environment here are just not used to it," he said. "And it impacts health, agriculture and energy use in a huge way."


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