STOCHASTIC CLIMATE FORCING OF PLEISTOCENE LAKE BONNEVILLE

Pleistocene Lake Bonneville was a large, closed-basin lake that fluctuated in size throughout much of its known history. This lake-level history is often interpreted as an important record of Pleistocene climate variability in the North American Great Basin. Indeed, lake-level changes have proven to be important indicators of climate change, providing particular insight into the nature of hydrological balances. Whereas modern lake-level changes over the past century are widely understood to be a combination of both climate variability and climate trends, past lake-level changes are almost always interpreted only in terms of climate change. Greater confidence in paleoclimate reconstructions from past lake-level changes requires a more sophisticated understanding of the drivers of past lake-level variations.

Two fundamental properties of geophysical systems are that there is some source of inertia, and there is stochastic climate forcing. The effect of the inertia is to integrate the short time-scale variability and produce persistent anomalies on longer timescales, even in a constant climate. It is in the context of this natural variability that climate change must be evaluated. In the case of a lake, the volume of water plays the role of the inertia, and the random stochastic forcing is from interannual variations in evaporation and precipitation.

We have developed a simple lake-level model (LLM) based on mass conservation, catchment area, surface evaporation, and lake-reservoir geometry, from which one can determine the dynamic response time of a lake. The LLM predicts lake-level changes in response to climate changes, and lake-level variability in response to stochastic, natural climate variability.

We applied the LLM to Pleistocene Lake Bonneville to quantify the lake-level variability expected from interannual climate variability. Results show that large and persistent lake-level anomalies can arise from stochastic climate variability, even in the absence of a trend or shift in mean climate, and that the magnitude and duration of these lake-level anomalies compare well with the magnitude and duration of lake-level anomalies of paleo Lake Bonneville.