LAUSR

A temperate deep lake, Lake Kuttara, Hokkaido (148 m depth at the deepest point) was completely frozen in winter in the 20th century. However, non-freezing of the lake over winter occurred four times in the 21st century, which is probably due to global warming. In order to understand how thermal regime of the lake responds to climate change, its heat storage was calculated by estimating heat budget of the lake and monitoring water temperature at the deepest point for 1 June 2014 – 31 May 2016. As a result, the temporal variation of heat storage from the heat budget was very consistent with that from the direct temperature measurement (the determination coefficient R2 = 0.903). A sensitivity analysis was conducted by numerically changing main meteorological factors (air temperature, solar radiation, wind speed, precipitation) for the heat storage obtained from the heat budget estimate. The increase in air temperature and precipitation was very effective to increase the heat storage. It is noted that, considering the increasing rate of air temperature (0.024°C/yr), the lake could be permanently unfrozen in about two decades.