As growing attention is paid to climate change adaptation as an actual policy issue, the significant meaning of climate variability in adaptation decisions is beginning to be recognized. By using a real option framework, we shed light on how climate change and climate variability affect individuals’ (farmers’) investment decisions with regard to adaptation. As a plausible case in which the delay carries policy implications, we investigate farmers’ choices when adaptation involves the use of an open-access resource (water). The results show that uncoordinated farmers with a high risk aversion may under-adapt while farmers with a low risk aversion would over-adapt under the same conditions. Private adaptation should be supported or discouraged accordingly if farmers are not convinced about the possibilities of collective resource management in the long run.