NASA study confirms warmer Tahoe water temps

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 shows that from 1992 to 2008, Tahoe's surface waters warmed a mean .23 degrees Fahrenheit a year from 1992 to 2008 - a 3.7 degree raise in total. Air temperatures recorded in Tahoe City increased .1 degrees annually during the same time period.

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 experiencing similar rates of warming," said TERC Director Geoffrey Schladow.

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 Tahoe's temperatures and create a method to reach the real temperature of the lake. That method was used on the other bodies of water in the study.

The rate of warming is similar to what Schladow and other researchers used 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 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.

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

Warmer lake temperatures make it more difficult for the lake to mix and change its biological makeup, Schladow said.

While Lake Tahoe was previously thought to be uninhabitable for invasive species like 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. Tahoe has also seen a drastic increase in Asian Clam populations in the past decade.

While lake warming is the result of a global issue, 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.

"Once you have a real inventory you can try to address it," he 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 mitigate the effects of less frequent lake mixing.

Organic matter at the bottom of the lake depletes the oxygen available there, Schladow said. If the TMDL is able to reduce the amount of organic matter at the bottom of the lake, that would also reduce the demand for oxygen, and the lake could withstand longer periods without deep mixing, Schladow said.