Long Term Changes in Thermal Stratification in a Deep Lake: Measurements From the 20th Century and Simulations For the 21st Century

Abstract:

Since the late 1960s, temperature profile data have been collected every 10-12 days at Lake Tahoe, USA. Past analyses have shown an overall warming of the lake at all depths, attributable to measured climate warming. The annual average, volume-weighted lake water temperature of the upper 400 m has increased at a rate of 0.011 deg. C/year since 1969 – 2014. There has been a concurrent evolution in the nature of the thermal stratification. While the maximum value of lake stability has not increased in the last 45 years, the length of time the lake remains stratified has been increased by 22 days, mostly occurring as a result of an earlier onset of spring stratification Using statistically downscaled GCM outputs for two different carbon emission scenarios, 1-D numerical simulations of the lake thermal structure over a 100-year period were performed. The results suggest a continuation and intensification of the already observed trends. Depending on the carbon emission scenario assumed, the length of the stratified season will increase by between 65 and 52 days by 2098 over the 1968 value. Most of the future increase will take place at the end of the season with a delay in the onset of winter. The observed impacts of stratification change on past ecosystem and water quality conditions will be presented, and possible future changes will be discussed. Plausible future states could range from climatic eutrophication to climatic oligotrophication.