The waters of Lake Tahoe are warming up at almost twice the rate of the world's oceans, probably as a consequence of global climate change, according to a new study released by UC Davis scientists on Dec. 20. The higher warming rate, reported for the first time in the new study, could have major implications for public plans to keep the lake blue.
It is not known if a warmer Lake Tahoe would be bluer and clearer or greener and murkier. But warmer water temperatures almost certainly will mean less mixing of lake waters, which could have two effects: Reduced mixing could mean less dilution of particle concentrations in the surface water each winter, keeping the water cloudy. Or it could mean fewer nutrients get carried from deep waters to shallow waters to stimulate algal growth, making the water clearer.
"It's not immediately obvious what the potential effects of climate change will be to the lake's clarity," said lakes expert Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center. "UC Davis and other research teams in the Tahoe Basin have been figuring out how this lake works for 40-plus years. We've had to factor in the emerging impacts of growth and development in the basin.
"Now we will factor in a warming climate, with all the changes that could bring to the dynamics of the lake's water system, including changes in rainfall and snowfall amounts, the amount and seasonal timing of rainwater and snowmelt runoff, and lake mixing."
Other researchers have previously reported that winter air temperatures in the Sierra are rising, and spring snowmelt and runoff are occurring earlier. "These are worrisome findings," Schladow said. "Weather-pattern changes in the northern Sierra are likely to affect water supplies for cities and agriculture in much of California. They have the potential to impact the entire operation of the state's system of reservoirs and rivers, and the ecological systems they support."
While important local implications of the new findings have to do with Lake Tahoe clarity, they have broader significance as evidence of climate change in the northern Sierra Nevada.
For the new study, UC Davis research ecologist Robert Coats analyzed a 33-year data set of more than 7,300 measurements of lake-water temperature collected by UC Davis scientists. Coats also is a consulting hydrologist and owner of the firm Hydroikos Ltd. in San Rafael. He found that from 1969 to 2002, the lake's water temperature increased, on average, 0.027 degrees Fahrenheit (0.015 degrees Celsius) per year.
Over the 33-year period, the temperature increased about 0.88 degrees Fahrenheit (0.5 degrees Celsius). This is similar to warming reported in other big lakes around the world, including the Great Lakes of North America; Lake Zurich, Switzerland; and Lake Tanganyika, Africa.
It is about twice the warming reported, on average, for the world's oceans, according to reports by scientists measuring indications of global climate change (such as shifts in air and water temperatures, precipitation patterns, cloud and ice coverage and plant distribution).
Coats also compared the Tahoe water-temperature records to Tahoe air-temperature records. He found that nighttime air temperatures rose 3.6 degrees Fahrenheit (2 degrees Celsius) in the past 90 years -- sufficient to account for most of the water warm-up.
"Also, in looking at the precipitation data for Tahoe City over the same period, we found a significant shift from snowfall to rain," he said.
Coats has four UC Davis co-authors on the new study. Joaquim Perez-Losada, a post-doctoral researcher, ran a computer model to confirm that the observed changes in lake temperature and internal heat storage were consistent with lake weather records. Schladow contributed to the analysis, as did pioneering Tahoe researcher Charles Goldman. And staff research associate Robert Richards measured lake temperature and other parameters of the lake ecosystem from 1969 until his retirement in October.
Coats presented the new findings at the Biennial Pacific Climate Workshop in Asilomar last March, and at a December meeting of the American Geophysical Union in San Francisco. He has submitted the study for publication in the peer-reviewed journal Climatic Change.
Goldman, a limnologist, or expert in the science of lakes, began scientific studies of Lake Tahoe in 1959. He introduced novel techniques for measuring the ecological health of the lake and was the first scientist to warn that Tahoe's famous cobalt blue color was threatened.
Academic institutions and public agencies are working together to restore and preserve the Tahoe Basin ecosystem. Some of the most active research programs are at UC Davis, the University of Nevada, Reno, the Desert Research Institute, NASA, the California Regional Water Quality Control Board (which helped fund the water-temperature study), the Nevada Department of Environmental Protection, the California Air Resources Board, the U.S. Forest Service, and the Tahoe Regional Planning Agency.
All those groups are currently engaged in producing an unprecedented set of environmental management plans for the basin, called Pathway 2007.
To further understand the lake, UC Davis is building the Tahoe Environmental Research Center, a $13 million research and education center, to be located near the lake, on the campus of Sierra Nevada College in Incline Village, Nev.