Climate change in the Tahoe basin: regional trends, impacts and drivers

Robert Coats 0 0 R. Coats Department of Environmental Science and Policy, University of California , Davis, CA 95616, USA The purpose of this study was to quantify the decadal-scale time trends in air temperature, precipitation phase and intensity, spring snowmelt timing, and lake temperature in the Tahoe basin, and to relate the trends to large-scale regional climatic trends in the western USA. Temperature data for six long-term weather stations in the Tahoe region were analyzed for trends in annual and monthly means of maximum and minimum daily temperature. Precipitation data at Tahoe City were analyzed for trends in phase (rain versus snow), decadal standard deviation, and intensity of rainfall. Daily streamflow data for nine gaging stations in and around the Tahoe basin were examined for trends in snowmelt timing, by two methods, and an existing record for the temperature of Lake Tahoe was updated. The results for the Tahoe basin, which contrast somewhat with the surrounding region, indicate strong upward trends in air temperature, a shift from snow to rain, a shift in snowmelt timing to earlier dates, increased rainfall intensity, increased interannual variability, and continued increase in the temperature of Lake Tahoe. Two hypotheses are suggested that may explain why the basin could be warming faster than surrounding regions. Continued warming in the Tahoe basin has important implications for efforts to manage biodiversity and maintain clarity of the lake. On a global scale, the general pattern of climate change is by now well documented, and is no longer scientifically controversial (Oreskes 2004). At the regional and local - 39o20L'0a"Nke Spaulding scales, however, there is considerable variation in the rates of climate change and its hydrologic and ecological impacts (Cohen 1990). Lake Tahoe is a large ultra-oligotrophic lake lying at an elevation of 1,898 m in the central Sierra Nevada on the CaliforniaNevada border (Fig. 1). The lake is renowned for its deep blue color and clarity. Due to concerns about progressive eutrophication and loss of clarity, the lake has been studied intensively since the mid TAHOE BASIN WATERSHED Temperature & total annual snowfall Temperature, precipitation & total annual snowfall Total annual snowfall Fig. 1 Map of the Tahoe region, showing locations of weather stations used in this study 1960s, and has been the focus of major efforts to halt the trends in clarity and trophic status. Previous work on the effects of climate change on the lake (Coats et al. 2006) has shown (1) that the lake is warming at an average rate of about 0.015C year1; (2) the warming trend in the lake is driven primarily by increasing air temperature, and secondarily by increased downward long-wave radiation; (3) the warming trend on monthly and annual time scales is correlated with the Pacific decadal oscillation (PDO) and (to a lesser extent) with El Niosouthern oscillation (ENSO); (4) the warming of the lake is modifying its thermal structure, and increasing its resistance to deep mixing. Recent work on climate change impacts in the western USA has focused attention on the shift in snowmelt timing toward earlier dates (Aguado et al. 1992; Dettinger et al. 2004; Cayan et al. 2001; Dettinger and Cayan 1995; Johnson et al. 1999; Stewart et al. 2005), the shift from snow to rain (Knowles et al. 2006; Regonda et al. 2005), the earlier onset of spring (Cayan et al. 2001); and the effect that these changes will have on water supply in California and throughout the western USA (Hamlet et al. 2005; Barnett et al. 2008; Mote et al. 2005). Pierce et al. (2008) showed that about half of the observed decline in western USA springtime snowpack (19501999) results from climate changes forced by anthropogenic greenhouse gases (GHGs), ozone and aerosols. In 2007, the catastrophic Angora Fire in the Tahoe basin showed how legacy vegetation changes can interact with climate change to increase fire hazard, and provided a stunning illustration of the increasing risk of wildfire in the western USA (Westerling et al. 2006; Running 2006; Brown et al. 2004). Although these trends in climate are largely attributable to increasing atmospheric concentrations of GHGs (Bonfils et al. 2008a), recent modeling work has drawn attention to the role of soot (which is mostly black carbon, or BC) in modifying climate by reducing snow albedo. Hansen and Nazarenk (2004) showed that soot may reduce snow and ice albedo in Northern Hemisphere land areas by as much as 3%, resulting in a climate forcing of +0.3 W m2. They found that due to positive feedbacks, the efficacy (change in air temperature per unit forcing) of soot is about twice that of CO2. Flanner et al. (2007), using a different model, found that the efficacy of BC/soot forcing is more than three times that of CO2, since the maximum forcing (due to aging of the snowpack and concentration of soot near the snow surface) coincides with the onset of snowmelt. The purpose of this study is to document the long-term changes in temperature and precipitation in the Tahoe basin, place those changes in a regional context, and show how climate change is affecting watershed hydrology and the lake itself. A wide variety of issues related to water quality management, including the potential for invasion by exotic species, design of long-term and costly stormwater planning strategies, and restoration of stream environment zones all depend on a more complete understanding of the impact of climate change in the basin. 2 Data sources and methods Analysis of climatic and hydrologic data in this study includes (a) annual average temperature trends at 6 stations, for the available periods of record; (b) trends in monthly averages of maximum (Tmax) and minimum (Tmin) daily temperature at 6 stations, 19562005; (c) trends in snowmelt timing at nine streamflow gages in and around the Tahoe basin; (d) the trend in the fraction of precipitation falling as snow at Tahoe City, 19102007; (e) the trend in exceedances of the 95th percentile daily rainfall amount at Tahoe City; (f) trends in the average annual and deseasonalized average daily temperature of Lake Tahoe; (g) the statistical relationships between local climatic variables and the PDO and ENSO. The climatic and hydrologic variables were selected for their likely importance to the future condition of Lake Tahoe. 2.1 Air temperature Daily temperature (maximum, minimum and average), and precipitation data were obtained from the Western Regional Climate Center (WRCC 2008) for 16 stations within 35 km of Lake Tahoe, and screened for length and completeness of record. Of these, six stations (shown in Table 1) were selected for detailed analysis. Figure 1 shows the weather station locations, and the type of data used from each. Three of the selected stationsReno, Tahoe and Lake Spauldingare part of the US Historical Climatology Network (HCN), with records through 2005 available online (Williams et (...truncated)