We develop a structural model for pricing a defaultable bond issued by a company subject to climate transition risk. We assume that the magnitude of the transition risk impacts depends on a transition scenario, which is initially unknown but is progressively revealed through the observation of the carbon price trajectory. The bond price, credit spread and optimal default/restructuring thresholds are then expressed as function of the firm's revenue level and the carbon price. Numerical implementation of the resulting formulas is discussed and illustrated using real data. Our results show that under transition scenario uncertainty, carbon price adjustments are more likely to trigger a default than when the true scenario is known because after each adjustment the more environmentally stringent scenario becomes more likely. We also find that faster discovery of scenario information leads to higher credit spreads since better information allows the shareholders to optimize the timing of default, increasing the value of default option and decreasing the bond price. Joint work with Theo Le Guenedal. The paper is available at: https://ssrn.com/abstract=4152325.