Keep Tahoe Oxic!

Climate change could put Lake Tahoe at risk of slowly suffocating, with increasing warmth setting off a chain reaction that depletes oxygen from its deepest waters and eventually robs the entire lake of its famous deep blue color, according to a new study by a team of UC Davis scientists working with Scripps Institution of Oceanography, UC San Diego climate researcher Mike Dettinger.

The lake could potentially stop mixing completely after about 2070, leaving the bottom layers devoid of oxygen, or anoxic. It would also increase nutrient loadings from lakefloor sediment. These self-perpetuating feedbacks would have a disastrous impact on the lake’s famous deep blue clarity.

A paper published in Climatic Change led by Goloka Sahoo of UC Davis and others, including Dettinger, presents the findings of the team of scientists who used several models to examine the effects of climate change on Lake Tahoe. Using downscaled climatic data and a numerical model, they found that even under a scenario in which greenhouse gas emissions are reduced, deep mixing will occur only four times during 2061 to 2098. If current use of fossil fuels and other contributors to the planet’s greenhouse warming effect continue unabated, and in a business-as-usual scenario, deep mixing stops completely after 2060 in the model simulations. This deep mixing now occurs once every three to four years. It is critical to the lake’s ecosystem as it supplies dissolved oxygen from the surface to the lower layers, and many native species that live in these lower layers need oxygen to survive.

Lake Tahoe is known for its beauty and clarity, qualities that locals have fought to preserve for decades. The “Keep Tahoe Blue” campaign traces its origins to efforts to control development starting in 1960. Lake visitors can often peer to depths of 20 meters (65 feet). The lake’s deep blue color is among the draws that bring an estimated three million people to the lake each year.

As tourism and development around the lake have increased, a number of anthropogenic threats such as runoff and associated pollutants, airborne pollution, and invasion of non-native species have threatened the health and natural clarity of the lake. Researchers say, however, that these threats pale in comparison to the effects of climate change on the lake predicted by this study. There is already evidence that warming is occurring in the Sierra Nevada and is occurring at an accelerated rate in the Tahoe Basin.

“Air pollution and invasives are unlikely to ever disrupt things as much as the complete shutdowns of deep mixing that we are projecting with climate change,” said Dettinger, who also has an appointment with the U.S. Geological Survey.

Mixing of the lake’s deep water is normally driven by surface winds during times of the year when surface water is cooled enough to have the same density as deep water. As the surface water warms, however, the difference in density between surface water and underlying water becomes too great for winds to overcome in model simulations. This difference in density makes it much harder for the more buoyant surface water to be “pushed” down by the winds.

According to the model, in the coming decades, the mixing would slow or stop completely, and eventually dissolved oxygen in the deep waters would disappear. These conditions would stimulate the release of nutrients from sediment, causing increased algal growth, especially in surface waters that receive sunlight. Lake Tahoe already suffers from declining clarity because of sediments and nutrient runoff that enter the lake from developments in the mountain basins that drain to it. The levels of dissolved oxygen could also become too low for native salmon, trout, and other fish to live below 200 meters (650 feet), even before the bottom layers develop anoxia.

The analysis is a striking example of how researchers have been able to fine-tune computer climate models initially programmed to simulate climate over large regions and scale them down to examine potential impacts on small areas or important geographic features such as Lake Tahoe. In this case, earlier work by Dettinger enabled this analysis of specific climate effects on one of Northern California and Nevada’s key tourism destinations.

The application of climate models to a local system, such as Lake Tahoe, requires special methods. Global climate models have very coarse “grids,” meaning they only simulate climatic processes on scales of several hundred miles or larger. Dettinger used a technique called statistical downscaling to transform global-model climate projections into high-resolution simulations over the Tahoe basin under various climate change scenarios. Statistical downscaling depends on using historical weather patterns that can be described in sufficient detail to represent specifics of future weather patterns. Using the downscaling methods developed at Scripps, the UC Davis team corrected bias in the downscaled data and generated detailed variables such as temperature and precipitation for the Lake Tahoe area. Model outputs of temperature, precipitation, winds, and sunlight were then input into a lake clarity model (LCM) of the lake to predict how its clarity could be affected by climate change.

Using high-resolution historical data, Dettinger was able to provide predictions of weather variables for the Lake Tahoe area under two different IPCC climate change scenarios; a “business as usual” emissions case in which fossil fuel emissions continue to increase with the global population and as developing economies industrialize, and a lower emissions case in which the global population peaks midcentury and then declines, with more sustainable technologies and energy sources being employed.

“This is a fairly standard first step in assessing a system’s sensitivity to climate change,” Dettinger said, explaining that some systems are not very sensitive to a few degrees’ temperature increase. “For example, in some alpine systems at very high altitude, even if the temperature goes up by four degrees, there won’t be huge consequences, as they are already so high and cold. Possibly the seasons will come a little earlier. Running the ‘high’ and ‘low’ emission scenarios is a good diagnostic of the magnitude of impact climate change could have on a system.

“What we see when we do this for Lake Tahoe is that, in the ‘extreme’ scenario, it basically goes off a cliff and takes a nosedive. Even under the lower emission scenario, there are significant consequences.”

– Mallory Pickett is a first-year masters student in the lab of chemical oceanographer Andreas Andersson at Scripps Institution of Oceanography