Colin Sullivan, ClimateWire reporter
08/14/2008

SAN FRANCISCO — Global warming has brought hotter summer nights, less snow and fewer freezing days to the Lake Tahoe ecosystem in northern California and Nevada, according to an annual report out this week on the state of the lake.

Scientists at the Tahoe Environmental Research Center at the University of California, Davis, said temperatures have increased by more than 4 degrees Fahrenheit since 1910 in Tahoe City, Calif. Adding to that data point is a finding that days with air temperatures below freezing have dipped by 30 days in the same time frame.

Moreover, the percentage of precipitation falling as snow in the region has dropped from 52 percent in 1910 to 34 percent in 2007, the report says.

Just as significant, according to the scientists, are data that show peak snowmelt shifting dramatically. Since 1961, peak snowmelt in the Lake Tahoe region — which is home to ski resorts like Squaw Valley USA and Heavenly Mountain Resort — has shifted earlier by two and a half weeks.

“This shift is statistically significant and is one effect of climate change on Lake Tahoe,” the report says. The report, “Tahoe: State of the Lake Report 2008,” was compiled by UC Davis researchers who have spent the last 40 years monitoring the lake’s clarity levels and ecological health. The region has been cited by many as a barometer for how climate change is quickly transforming entire ecosystems.

Clearer water, but impacts from algae and fire

The clarity of the lake, famous for its transparent blue waters, actually increased somewhat from 2006 to 2007, according to the center’s research. The clarity level is linked closely to housing and highway development around the lake, which has been addressed aggressively in recent years following a campaign to “Keep Tahoe Blue.”

As for the effects of warming, the chief of the research center, Geoffrey Schladow, indicated that the report reflects a pattern that has been influenced by human behavior. His remarks attached to the report attribute the lake’s shifting status to both “natural forces and human actions that operate over time scales ranging from days to decades.”

The report also details the effects of the Angora forest fire, which scorched homes and campgrounds around the lake last summer. UC Davis scientists said the deposition of nitrogen and phosphorus from the fire was two to seven times greater than in a normal summer, but still had a “negligible” effect on clarity and algal biomass.

At the same time, algal growth, considered an indicator of warming’s acceleration, has increased at a rapid clip since 1968. Algal concentrations on the shoreline were above average in 2007, and the researchers said the rate of growth is now five times greater than in 1959.

Also, the lake’s average surface water temperature has increased by 41.7 degrees Fahrenheit to 42.6 degrees since 1970, the report says.