UC Davis global warming researcher paints dire picture for Lake Tahoe

Over four decades, Goldman has documented a steady decline in the lake's fabled clarity – from 102 feet in 1968 to 68 feet in 2006. Urban runoff, soil erosion and algae blooms are among the culprits. Now – if Schladow is correct – another challenge looms: global warming.

Tahoe Regional Planning Agency spokesman Dennis Oliver said staff is working on ways to reduce carbon emissions in the basin. But clearly global climate change transcends Tahoe.

Schadlow's research, which he presented for the first time this week, looks into the future by applying a widely used climate change model.

He found that an air temperature increase of 3.6 degrees Fahrenheit didn't have much impact, but when combined with a 10 percent rise in long-wave solar radiation trapped in the atmosphere by moisture and greenhouse gases, the lake's deep mixing would stop in 2019.

Mixing of lake water is critical because it transports oxygen-bearing water to the bottom and recirculates nitrogen and other nutrients closer to the surface, where they become food for algae, the building blocks of the food chain.

Normally, the 1,645-foot-deep Lake Tahoe, the second-deepest lake in the nation, mixes fully to its bottom about once every four years. Since this was a cold and snowy winter, it mixed to its depths.

Should such deep mixing stop, Schladow said, Tahoe would lose "one more aspect of its uniqueness: the fact that it was always oxygen-rich."

No one knows exactly what will happen if the lake warms and stops deep mixing. But Schladow has given the matter more thought than most.

With no oxygen at the bottom, an inky world of opossum shrimp, worms, zooplankton and mackinaw may morph into a "dead zone," he said. Then, phosphorus trapped in the sediment could be swept to the surface, fueling algae blooms and fouling the lake's clarity.

New kinds of algae could flourish, too, including species that impart taste and odors or clog filters. That would mean costly improvements for communities that rely on the lake for drinking water, Schladow said.

Improving clarity and decreasing algae growth long have been goals of Lake Tahoe's tough environmental regulations, which seek to limit the flow of nitrogen, phosphorus and other sediment into the lake. Schladow's possible future for the lake complicates that effort.

"Let's say we're getting years of phosphorus being brought up (from the bottom) while people are spending hundreds of millions of dollars trying to reduce phosphorus coming in," he said. "It cuts across efforts that people are undertaking, with the best of intentions, and with good advice. It could force a re-evaluation of how the lake is managed."

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