Lake Tahoe temperatures rising

SOUTH LAKE TAHOE, Calif. -- A recent NASA study showed Lake Tahoe's water is warming twice as quickly as regional air temperature, lending weight to predictions of warming lake temperatures made by UC Davis researchers in 2008.

The study, published in November, shows Tahoe's surface waters warmed a mean 0.23 of a degree Fahrenheit a year from 1992 to 2008 for a total increase of 3.7 degrees.

Air temperatures recorded in Tahoe City increased 0.1 degree annually during the same time.

Researchers from the Tahoe Environmental Research Center, UC Davis and NASA's Jet Propulsion Laboratory in Pasadena, Calif., used 18 years of temperature data from satellite sensors. They studied Tahoe, Clear Lake, Lake Almanor and Mono Lake in California and Pyramid and Walker Lake in Nevada.

"This isn't just Tahoe. Other lakes in the region are experiencing similar rates of warming, said Geoffrey Schladow, director of the Tahoe research center.

Lake Tahoe was used to calibrate the data coming from the satellite. Satellite measurements look at a very thin layer of the lake and can be influenced by the atmosphere, Schladow said.

Scientists were able to compare the satellite readings with on-the-ground recordings of Lake Tahoe's temperatures and create a method to determine the lake's real temperature.

The rate of warming is similar to what Schladow and other researchers found in a March 2008 study that showed warmer lake temperatures decreased the rate of deep-water mixing.

The study combined 40 years of weather data in the Tahoe basin with mathematical models of climate change to create scenarios of future lake conditions.

The team then took those scenarios and used a lake physics model to see how combinations of air temperatures, cloudiness and wind speed would affect the mixing of water layers.

The lake circulates water from bottom to top, with the deepest mixing in February. Deep-water mixing circulates oxygen through the lake, supports aquatic life and brings nutrients from the lake's bottom.

Less water mixing would mean less oxygen for deeper parts of the lake, which could result in phosphorous releasing to the surface to fuel algae growth.

Warmer lake temperatures make it more difficult for the lake to mix and change its biological makeup, Schladow said. "It makes it more habitable for any species where the conditions didn't fit before."

While Lake Tahoe was previously thought to be uninhabitable for invasive species such as quagga and zebra mussels, an August study by the University of Nevada, Reno and UC Davis showed quagga mussels can survive and reproduce in Tahoe's water.

Lake Tahoe has also seen a drastic increase in Asian Clam populations in the past decade.

Tahoe Regional Planning Agency spokesman Dennis Oliver said the agency will study possible local impacts.

Within two years of adopting a new regional plan, the agency expects to complete a carbon emissions inventory for the basin, Oliver said.

Schladow said many of the policies around the lake -- including the lake's regional plan and the goal of reducing small particulate matter entering the lake -- could help ease the effects of less frequent lake mixing.

Organic matter at the bottom of the lake depletes oxygen available there, Schladow said.

If the amount of small particles and nutrients entering the lake can be reduced, that would also reduce the demand for oxygen, and the lake could withstand longer periods without deep mixing, Schladow said.