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Thursday, 20 December 2012 16:01

Weather Window: Lake Tahoe and Climate Change

As the year winds down, it's an appropriate time to review the current health and physical state of Lake Tahoe and its regional environment. On Dec. 13, Dr. Geoffrey Schladow gave a presentation on the 2012 State of the Lake Report at Sierra Nevada College in Incline Village.

Dr. Schladow is Director of the Tahoe Environmental Research Center, based at the University of California-Davis. Since 2007 TERC has released this annual report, which provides a snapshot of important environmental aspects of the Tahoe Basin, including clarity, erosion and sedimentation, air quality, climate change and more.

Similar to a struggling student, this environmental report card indicates the strengths and weaknesses of "better management practices" that have been designed to protect Big Blue from stress factors associated with development and human activity. Metrics regarding water quality, clarity, and the battle against invasive species offer some bright spots, but included in the report are issues related to climate change that depict troubling trends that may be beyond the scope of mitigation efforts.

Tahoe is one of the worlds' most studied lakes and holds a unique position among large lakes. In fact, when NASA began gathering temperature information on 167 large lakes worldwide, Tahoe's in-lake measurements were used to calibrate the satellite data for this global assessment. Dr. Schladow indicated that Tahoe's singular characteristics make it an ideal study site.

"Because Lake Tahoe is still very clean and relatively pristine, it has a high signal to noise ratio," he said. "We can often see things at Tahoe that are happening in other lakes but are just harder to tease out there."

For 25 years Tahoe researchers have watched the lake's water temperature warm more rapidly than surrounding air temperatures due to climate change. NASA has confirmed the same trend in play on virtually all observed lakes around the world. Warming water temperatures impact lakes in a variety of ways, including biological productivity and creating a more hospitable environment for invasive species. Have you ever noticed that some summers Big Blue is warmer than others?

The most obvious answer kicked around by locals is that after a big winter the snowpack melts later and dumps cold water into the lake well into spring. The main culprit, however, is the mixing of Tahoe's waters at great depth. Consider that in July 2011, Tahoe's surface water was the coldest in a decade. Makes sense right? Winter 2011 was epic with skiing into July. According to measurements taken at the Central Sierra Snow Laboratory (CSSL), more snow fell during the winter of 2006 than 2011, but Tahoe's surface temperature in July 2006 was significantly warmer than 2011, and one of the balmiest since 1968.

Like most lakes located in temperate latitudes, water in Tahoe occasionally mixes, bringing oxygen-starved water up from depth while infusing the deeper zones with oxygenated surface water. The extent of the mixing varies between relatively shallow zones or a complete turnover of water — top to bottom.

In 2011 Tahoe's liquid volume mixed to the bottom (1,645 feet — 11th deepest in world), which pushed cold water up from the depths to make for invigorating water sports that summer. There was only a shallow mixing in 2006.

Dr. Schladow has written: "Past studies suggest that climate change will reduce the extent of lake mixing, and in particular the mixing of oxygen, down to deep water. How true this really is remains an open question at Tahoe and lakes world-wide." Lake Tahoe mixes when the water column reaches the same temperature and density. Surface winds trigger the turn-over. The worry for the future is what happens if a buoyant surface cap of warm water prevents this hydrological flip and what would the impact be on Lake Tahoe? One concern is that oxygen depletion at depth will release phosphorus and nitrogen from bottom sediments, nutrients that are known drivers for algae production. Algae are a major cause of diminishing water clarity.

Next year TERC researchers are installing equipment to measure these mixing events in real-time so that they will be able to monitor the evolution of the process as it happens and learn more about it.

Other climate trends reveal that since 1961, the lake level snowpack is melting about two weeks earlier in spring. However, at the CSSL at 6,900 feet elevation, there has been no meaningful change in spring melt timing since measurements began in 1946. It's important to remember that although scientists are quantifying climate change, the impacts vary depending on location.

Due to space constraints, this article provides only a cursory glance at two components in this year's TERC report, but the entire document can be viewed online at terc.ucdavis.edu/stateofthelake/. I've cherry-picked select topics from the report and plan on discussing other climate and meteorological aspects of this important study in a future column. Stay tuned.

— Tahoe historian Mark McLaughlin is a nationally published author and professional speaker. His award-winning books are available at local stores or at www.thestormking.com. You can reach him at mark@thestormking.com. Check out Mark's new blog at www.tahoenuggets.com.

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