Global warming is an indisputable fact. It is causing the average temperature in the atmosphere and ocean to rise, the perpetual snow and polar ice to melt, and the sea level to rise. The rise in water temperature in recent years has had a negative impact on the ecosystem of Lake Biwa. We applied acoustic tomography to observe water temperature and flow in the lake. In this paper, we report measurements of the speed of sound and estimation of water temperature at Lake Biwa. We examined the speed of sound at Lake Biwa in March 2008. The measurements showed that the speed of sound in the lake is faster than in pure water. The water temperature was estimated from the speed of sound, and the estimation was over 0.5°C higher than the actual water temperature. Therefore, we applied a correction method to estimate water temperature. It used the salinity conversion value in the Del Grosso equation for the speed of sound. The difference of the water temperature between the measurements and the estimates is smaller than 0.12 °C.
