Lake Tahoe Warmer, Study Finds
Lake Tahoe is getting warmer and the likely cause is global warming, making it the largest lake in North America where rising temperatures have been linked to climate change, according to a study by UC Davis researchers.
Although the water temperature has increased by less than 1 degree Fahrenheit in three decades, scientists said, it was enough to significantly affect lake currents and to complicate the billion-dollar effort to preserve the clarity of the lake’s cobalt-blue waters.
The findings suggest that by altering the currents, the warming trend may promote algae formation near Tahoe’s surface and help make those waters more opaque -- a condition long blamed on a combination of air and water pollution from runoff, chimney smoke and auto emissions.
Analyzing 33 years of monitoring data, a team of UC Davis scientists found that the warming trend was evident in surface-level water as well as more than 400 meters beneath the surface.
They determined that the change probably is stemming from a slight uptick in temperatures around the world, ruling out other factors such as increased geothermal heat from the earth beneath the lakebed.
“Tahoe is so large, it takes a lot to change it,” said Geoffrey Schladow, the director of the UC Davis Tahoe Environmental Research Center and one of the authors of the study, which has been submitted for publication in the scientific journal Climatic Change. “The temperature at the bottom has changed by nearly 1 degree Fahrenheit, and that is greater than at any other time in recorded history.”
The researchers cautioned that it was premature to draw definitive conclusions about whether the change would help or hurt the lake’s ecology, the subject of more than 100 scientific studies over the last 25 years.
But the temperature spike appears to have altered the lake’s complex water currents. Warmer waters near the surface are not mixing as frequently with the cooler waters of the lake’s depths, they said.
In a lake like Tahoe, the result of that lack of mixing is more distinct layers of different temperatures.
Consequently, the researchers concluded that the water closer to the surface of Lake Tahoe may become a more hospitable place for algae and that sediment may linger longer at the top of the lake, hurting water clarity.
If the lake’s top and bottom waters mix less frequently, the scientists said, more phosphorus, a leading cause of algae, could collect at the bottom of the lake. When the currents do shift, the phosphorus could be driven to the surface, causing a huge algae bloom that could turn the surface of the lake virtually green overnight. However, the researchers cautioned that that was an unlikely, worst-case scenario.
“Algae respond to temperature. For every 5 degrees change in Celsius, their growth rates double,” Schladow said. “So we would expect some measurable effect.”
Until now, the efforts to retain Tahoe’s deep-blue color have focused on limiting runoff by reducing traffic and construction and by restoring wetlands, stream beds and meadows on Tahoe’s perimeter to help filter contaminants that otherwise might flow into the lake.
Aided by $1 billion in government funds, local officials are devising a series of environmental management plans to protect the lake’s water quality and keep it as pristine as possible. The warming of the lake, a previously unknown factor, adds another layer of complexity to that effort.
But the researchers also said the temperature increase could bring about more positive changes.
Zooplankton levels, which were decimated when opossum shrimp were introduced into the lake in the 1960s to help a flagging Mackinaw trout population, could see a rebirth with warmer surface waters, said Robert Coats, a researcher who was part of the study and who has been analyzing Lake Tahoe since his student days at Berkeley more than 30 years ago.
Moreover, the same warming trends that are heating Lake Tahoe also may help bring stream runoff into the lake earlier in the year. That would allow sediment to mix with deeper currents more easily before the lake’s water becomes more stratified, helping clarity.
The researchers did not attempt to determine the cause of the climate changes that they believe are affecting the lake, and whether they are due to human activities such as the burning of fossil fuels. But they said officials who were working to improve the lake’s water quality would have to take future climate-change predictions into account when devising possible solutions.
UC Davis is part of a project called Pathway 2007, which is working to alleviate the lake’s problems through a series of environmental management plans for the Tahoe basin. The university has been studying the lake for decades. Limnology professor Charles Goldman began measuring the ecological health of the lake in 1959, and was the first to warn that its cobalt color was fading.
Monday morning, UC Davis scientists boarded their research vessel, the John LeConte, and took journalists on a tour of water monitoring sites from the Tahoe City Marina on the lake’s north shore.
The ship is named for the scientist who first began recording the lake’s temperature more than a century ago.
Temperatures on shore were moderate, hovering in the mid-40s Fahrenheit. But on the lake they were considerably cooler. After a 30-minute ride to the center of the lake, the boat arrived at a large yellow buoy that monitors weather and water temperature at the surface of the lake, and provides comparison data that are used to calibrate NASA satellites that hover over the lake and provide similar measurements four times a day.
With help from a small crane, the scientists also dropped a large metal monitor into the water that records temperatures hundreds of meters deep and sends a beam of light that can measure clarity for 20 meters.
Scientists consider lake temperature an important barometer of long-term climate change, because the waters in deep lakes tend to change only slowly, filtering out so-called “static” from minor fluctuations in weather.
In addition to Lake Tahoe, scientists have drawn a link between climate change and higher temperatures at Lake Zurich in Europe, Lake Mendota in Wisconsin and Lake Tanganyika in Africa, among others.
Scientists also have documented climate change-induced warming in a bay in Lake Huron, which is larger than Lake Tahoe, but have not determined whether the changes are occurring throughout its waters.
“The news that Lake Tahoe is getting warmer should be a wake-up call to everyone,” Coats said. “Lake Tahoe is known around the world, but we’re finding similar effects in many other places.”
