Pacific Southwest Research Station

Research Partnerships

Tahoe Science Projects supported by SNPLMA

Final products:

Figure 1: Models used in research: 1) Two Global Climate Change models: Parallel Climate Model and Geophysical Fluid Dynamics Laboratory Model (Princeton), 2) Tahoe Watershed Model (LSPC), 3) Pollutant Load Reduction Model, and 4) Lake Clarity Model.

Final Report [pdf]

UCD Press Release, November 2010

Project Summary

The 21st Century global climate is expected to experience long-term changes in response to anthropogenic greenhouse gas emissions. Discussions on the potential impacts of climate change on water resources in the Lake Tahoe Basin have only recently begun and our scientific understanding to date has focused on identifying existing impacts and trends in the historic data. Water resource managers need to know the potential effects of changing meteorologic conditions on a variety of topics such as expected future air temperature, amount and type of precipitation, stream discharge, sediment and nutrient loading characteristics, BMP performance, lake mixing and water quality response. In this study we examined all these topics using existing water resource models already developed for the Lake Tahoe Total Maximum Daily Load (TMDL). A sophisticated statistical downscaling methodology was applied to the model outputs of the of the Geophysical Fluid Dynamics Laboratory Model (GFDL) and the Parallel Climate Model (PCM) to produce simulated data records at a 12 km grid scale in the Tahoe Basin for the 21st Century (2000-2099). This methodology was applied given two emission scenarios from the 4th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC): A2 (“Business as Usual”), with accelerating GHG emissions, and the more optimistic B1, in which GHG emissions level off by 2100 (2007; see http://www.ipcc.ch/).

The meteorologic and geographic conditions in the Tahoe Basin combine to create a vulnerable ecosystem. Temperatures in the Basin are increasing faster than in the surrounding region. This may be due to the influence of the lake and its heat (energy) budget on local air temperature, although a decrease in the albedo of the snowpack from deposition of soot (black carbon) may also play a role. The results from this study indicate that the most significant impacts of a future, modeled climate change at Lake Tahoe are:

1. Changes in hydrologic conditions

   Hydrology output from the downscaled climate modeling suggests a significant reduction in the amount of precipitation falling as snow in the Tahoe basin. This could have consequences for water supply as well as winter recreational sports. On occasion, the lake historically has fallen below its natural outlet elevation during prolonged dry years. Lake level modeling in our study suggests that under some greenhouse gas emission scenarios, outflow from Lake Tahoe could cease by the end of the 21st Century.

   
2. Reduced frequency of complete vertical mixing of the lake

   Under historic and current conditions the lake mixes to the bottom on the average of only once every four years. Continued warming will increase the lake‘s thermal stability, and likely shut down its vertical mixing altogether. Should the lake‘s deep mixing be restricted to the extent the models suggest, internal loading of nutrients from the sediments will be very significant and will drive a fundamental change in the biological productivity status of both the pelagic and littoral regions of the lake. These nutrients, particularly phosphorus, will be available to drive algal growth. Reducing the load of external nutrients entering the lake in the coming decades may be the only possible mitigation measure to reduce the impact of climate change on lake clarity and trophic status.

   
   
   
   Figure 6-18. Close view of the bottom 45m (r50 m to 495m) simulated soluble reactive phosphorus release for (a) GFDLA2 (b) GFDLB1 scenario.