Significant Changes to Twentieth-Century Temperature and Precipitation in the Sierra Nevada near Lake Tahoe: The Possible Anthropogenic Effect

A study of the weather records from six stations in the Sierra Nevada at and near Lake Tahoe, plus one station in the Diablo range of central California, reveals the following:1. Since 1915, the date of the first major autumnal snowstorm has become progressively later at Tahoe City by 0.24 days/year (d/yr).2. The change in the snowfall date results from an increase in the air temperature (T) at Tahoe City between 1909 and 2004, during the months of Septemberthrough December, of T′max = +0.011 °C/yr, T′min = +0.020 °C/yr, T′mean = +0.016 °C/yr. These values agree substantially with those published by Coats et al. (2006).3. On average, the other five stations show a much smaller increase, amounting toT′max = +0.001 °C/yr, T′min = +0.012 °C/yr, T′mean = +0.007 °C/yr. Thismean value agrees well with the increase of T′mean = +0.007 – +0.008 °C/yr for the North American continent due to global warming (Jones and Moberg, 2003; Karoly et al., 2003).4. The rate of temperature increase at Tahoe City is thus greater than would be expected from global warming and reflects a change in the microclimate of the Tahoe Basin, possibly due to human activity.5. No overall change in precipitation with time is observed at any of the seven stations. However, statistically significant increases in precipitation with time occur, in the month of November, at the three Sierran stations located west of the Sierra crest, as well as at Tahoe City east of the crest. These increases coincide with the onset of the first substantial precipitation at these stations in the fall, and could be the result of “cloud seeding” by atmospheric pollutants, the precipitation becoming greater with time as the amount of pollution increases, and diminishing in December after these pollutants are washed out of the atmosphere by the first rains or snows.Coats, R., J. Perez-Losada, G. Schladow, R. Richards, and C. Goldman: 2006, ‘The Warming of Lake Tahoe’. Climatic Change 76, 121–148.Jones, P. D. and A. Moberg: 2003, ‘Hemispheric and Large-Scale Surface Air Temperature Variations: an Extensive Revision and an Update to 2001’. Journal of Climate 16, 206–223.Karoly, D. J., K. Braganza, P. A. Stott, J. M. Arblaster, G. A. Meehl, A. J. Broccoli, and K. W. Dixon: 2003, ‘Detection of a Human Influence on North American Climate’. Science 302, 1200–1203