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Dr. David Herbst of the Sierra Nevada Aquatic Research Laboratory says that the NASA results “are consistent with elevated summer minimum air temperatures that have previously been documented.” He also found that Martis Creek, a creek north of Lake Tahoe with the best temperature record in the Sierra, has warmed 2 degrees C since the mid-1970s (in June, July, August). Nearby Sagehen Creek is groundwater-dominated, and there the summer minimum temperature was unchanged. Herbst thinks groundwater flows could be important for keeping temperatures cool in streams.
Herbst also notes that a paper by R. Coats has already shown Lake Tahoe to have warmed over the last 50 years, and surveys of benthic midges from shallow Sierra lakes have shown warm-tolerant taxa increasing while cool-tolerant taxa have been decreasing. He notes that for Mono Lake the implications are faster alkali fly growth, especially when combined with reduced salinity. He wishes we had better monitoring data.
While one of the big questions we have is why is NASA showing surface warming during summer, data from Dr. Robert Jellison of UC Santa Barbara shows no warming in springtime water temperatures 2 meters deep during the last 28 years. In addition, he says that Mono Lake is generally mixed to over 8 meters deep during summer, and “the upper meter is notoriously sensitive to diurnal wind, solar, and temperature variation.”
Jellison has additional data that will shed light on this question and we can all look forward to seeing that in the future. In the meantime, I came up with a hypothesis that reduced wind speeds are allowing the warm water to sit on the surface. This is premature to get into too much without further study, but Lee Vining average summer wind speed shows a strong and consistent decrease over the last decade, from about 4 mph to about 2.5 mph. Cain Ranch average summer wind speed, on the other hand, shows little trend during the period, but the 6 years that overlap with Lee Vining correlate strongly. The limited data gives us limited ability to draw conclusions, but does generate additional questions. Are there fewer summer thunderstorms? Is the growth of trees in Lee Vining blocking the wind at our weather station? As the data comes in over the next few months, we’ll post updates here.
One theme keeps coming up: additional study is needed.