This paper analyzes the implications of a gradual increase in the frequency of climatic disasters for public debt sustainability and sovereign default risk. I develop a simple stochastic model of sovereign default that allows for time-varying probability of climatic disasters. I show that the default ratio– the maximum debt-to-GDP ratio that a country can sustain without defaulting – is decreasing and highly nonlinear in the probability of disaster. The model emphasizes the crucial role of the perception of disaster risk, independently from the realizations of disasters. Different perceptions of disaster risk may have very different implications for sovereign default risk. A perception of disaster risk based on a constant disaster probability leads to a constant default ratio. On the other hand, a naive perception of disaster risk– creditors revising the disaster probability in each period while disregarding any future changes– leads to a time dependent default ratio, but it relatively underestimates default risk compared to a fully forward looking perception of disaster risk.