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Great Lakes Article:

Warming of Lake Tahoe fits in with global trend
By Lisa M. Krieger
Mercury News
Published December 21st, 2004

The frigid waters of Lake Tahoe are warming -- a trend so subtle it's unlikely to be felt by swimmers but substantial enough to worry scientists.

A study by a team of researchers at the University of California-Davis reveals that the lake's average water temperature is increasing an average of 0.027 degrees a year. It has increased almost one degree since 1970, from approximately 41 degrees to 42 degrees.

What troubles scientists is that the warming could alter the lake's legendary color and clarity, or, in a worst-case scenario, reduce the cyclic churning needed to circulate air and food.

``It doesn't sound like very much,'' said UC-Davis research ecologist Robert Coats. But indirectly, ``It may have a profound effect on the plants and animals.''

The effect on the water is caused by warming air within the Tahoe Basin, the deep V-shaped valley that holds the lake. Previous research has found that spring snowmelt and runoff are occurring earlier. Scientists say this isn't a problem unique to the Tahoe Basin, but part of the much larger trend of global warming.

Other lakes around the world are undergoing a long-term warming trend, including the Great Lakes, Lake Zurich in Switzerland and Lake Tanganyika in Africa.

Statistical analyses of these warming waters show that they fit a trend seen in larger studies of global warming caused by "greenhouse gases.''

Greenhouse gases, such as carbon dioxide, that are produced mostly by burning coal, gasoline and other fossil fuels, blanket the Earth and trap heat.

Lake Tahoe owes its chilly temperatures to snowfall and the mountain streams that fill the basin. On the surface, the water rarely climbs above 68 degrees and can drop as low as 41 degrees Fahrenheit -- a fact that forces lakeside hotels to install heated swimming pools.

Its deepest waters, about one-third of a mile down, hover at 39 degrees.

These very deep waters are a good place to look for signals of climate change. While the surface and near-surface temperatures reflect daily and seasonal temperature swings, deep-water temperature responds on a time scale of years, even decades. Deep water has, in the words of scientists, ``a climatic memory.''

For the new study, Coats analyzed 33 years worth of data of more than 7,300 measurements of lake-water temperature, measured over the depth of the lake. The only big dips in temperature happened during times of volcanic eruptions, cooling the Earth.

It is not known if a warmer Lake Tahoe would be bluer and clearer -- or greener and murkier, the scientists said.

But warmer water temperatures almost certainly will mean less mixing of lake waters.

The waters in Lake Tahoe have a pattern of periodic cycling, where the warm and cold layers of water trade places. Warmer lakes don't tend to cycle as much as cold lakes.

The reduced mixing could have two effects. It could mean less dilution of particle concentrations in the surface water each winter, keeping the water cloudy. Or, alternatively, it could mean fewer nutrients get carried from deep waters to shallow waters to stimulate algal growth, making the water clearer.

And while warmer water may be good for some species of plants and animals, it could be bad for others. No one yet knows the answer.

``If it were just the lake itself, it would be cause for concern,'' said Coats. ``But the bigger picture, the global context of what is happening elsewhere -- that's cause for considerable alarm.''

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