Climate change impacts in the Tahoe region: past and projected future trends

Analysis of 20th century air temperature, snowfall, stream discharge and lake temperature records in the Tahoe basin indicates 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 of precipitation, and warming of the lake. Regional rates of change in snowmelt timing and monthly air temperature suggest that the Tahoe basin may be warming faster than the surrounding region. Two factors may enhance the rate of warming in the Tahoe basin. First, the lake's low albedo and high heat capacity may produce higher outgoing long-wave radiation than the region's land surface, and second, the relatively high black carbon emissions in the basin may locally reduce the snow albedo and thus accelerate the rate of snowmelt.

To project future impacts of climate change in the Tahoe basin, we developed the 21st century down-scaled temperature and precipitation daily output from two General Circulation Models (the Geophysical Fluid Dynamics Laboratory, or GFDL CM2.1, and the Parallel Climate Model, or PCM1) and two emissions scenarios (A2 and B1), and corrected the precipitation data for bias by quantile mapping. For the GFDL, we also had wind (bias corrected), radiation and relative humidity data. The daily data were disaggregated to hourly values and used to drive a distributed hydrologic model (Load Simulation Program in C++) for the Tahoe basin. Output from this watershed model, along with the meteorological data, was then used as input to a 1-d hydrodynamic and water quality model of the lake (DLM-WQ, developed at UC Davis), and to calculate stream-flow statistics for the Upper Truckee River (UTR) and trends in the Palmer Drought Severity Index (PDSI) for selected sites. The results indicate that 1) recent trends in basin climate and hydrology will continue, with a possible 5 deg. C increase in average annual air temperature by 2100; 2) precipitation will continue to shift from snow to rain, and annual amounts are projected to decline in the latter half of this century; 3) the timing of snowmelt and the hydrograph centroid are likely to shift toward earlier dates; 4) the magnitude of the estimated 100-yr flood of the UTR is likely to vary greatly over the course of this century but eventually decline in response to warming and drying; 5) summer low-flow is projected to decline; 6) drought, as measured by the PDSI, is projected to increase, especially in the latter third of this century, and most strongly on the eastern (drier) side of the basin; 7) the lake may be expected to continue warming, and the resulting increasing thermal stability will likely limit deep mixing and deep ventilation, with impacts on dissolved oxygen, internal nutrient loading and water quality; and 8) the annual frequency of episodes of no-lake-outflow is likely to increase, especially toward the end of this century.

The environmental and economic impacts of these climate trends in the Tahoe basin are likely to include 1) increased tree mortality, and increased fuel loads; 2) increased wildfire frequency and intensity; 3) increased channel and bank erosion in basin streams; 4) increased infrastructure damage from flooding; 5) reduced stream habitat due to a decrease in summer low-flow; 6) a decrease in available water supply for users in the Truckee River Basin; 7) formation of anoxia in the deep water of Lake Tahoe, with subsequent release of nutrients and stimulation of algal growth; 8) changes in the theoretical climax vegetation; and 9) economic damage to the Basin's winter recreational industry. Adaptation to these climate change impacts may include aggressive fuel load reduction, establishment of plant communities and species adapted to lower elevations, redoubled efforts (including BMP implementation) to control nutrient and sediment flux to the lake, and increased water conservation efforts in the Truckee River basin.