At the Lake: Study shows warming trend at Tahoe

Lake Tahoe may seem chilly to many, but a new University of California report says there are troubling signs that the scenic Sierra Nevada lake is warming up.

The UC Davis report says the warming trend "could have profound implications" for features that make Tahoe so popular: snowfall in the winter and a cobalt-blue lake in the summer.

The report says the average July surface water temperature has increased almost five degrees, from 62.9 degrees to 67.8 degrees, since 1999. The lake's surface waters were the warmest on record on July 26, 2006, at 78 degrees.

The study doesn't include 2007 data, and John Reuter of the UC Davis Tahoe Environmental Research Center said Wednesday it's possible that the record could be surpassed as a result of this year's warm weather.

"Regardless of what happens this year, as you recognize in this report there were some fairly significant changes through the end of 2006," Reuter added. "There's a lot to be thinking about with the 2006 information."

The report also shows that nights are warmer at Tahoe, with nighttime low temperatures rising more than 4 degrees since 1911; and the number of average air temperatures below freezing dropping from 79 days to 52 days in the same period.

Also, less precipitation falls as snow, the document states. The percentage of snow in the total precipitation has dropped from 52 percent to 34 percent since 1911.

Geoff Schladow of the Environmental Research Center said types of algae in the lake are changing and are showing up earlier in the year as Tahoe becomes "more hospitable" to invasive plants and fish. He said warm-water species such as bass and carp are increasingly common.

The report also shows that in 2006 the lake's clarity declined. Researchers measured the waters as clear to an average depth of 67.7 feet, which is 4 1/2 feet less than the 2005 measurement.

The all-time low was 64 feet in 1997. When measurements began in 1968, the lake was clear to an average depth of 102.4 feet.

Reuter noted that this year's spring runoff into the lake, which is a big factor in its clarity, was less because of a light winter, and that could improve the 2007 average clarity depth. The 2007 information won't be available until next year.