Changes in heat balance characteristics of ponded shallow water in Japan under the current and future climates were investigated using a simple physical model for estimating water temperature with climatic data and the future climate scenario. The most notable results from the simulations were that the latent heat loss from the shallow water in the period June to August under the doubled CO₂-climate would become larger by about 2.0MJ m^-2day^-1, compared with that under the present climate conditions. The increase in the latent heat loss under the doubled CO₂-climate may lead to concomitant decrease in sensible heat loss from the shallow water to air under the doubled CO₂-climates. The changes in the heat balance characteristics resulted in a decrease in the Bowen ratio of the ponded shallow water under the doubled CO₂-climate conditions.