Climate change strains public finances, eroding sovereigns’ capacity to fund climate

policies. We build a forward-looking stress-test framework for sovereign debt that links

an integrated climate-economy assessment model to stochastic debt-sustainability

analysis using IPCC narrative scenarios. The framework pairs shared socioeconomic

pathways and warming narratives with low- and high-climate-impact functions to

calibrate aleatory scenario trees for the economic, fiscal, and financial determinants of

debt. It projects debt dynamics under optimal debt financing. Stress-testing eight

economies across climate vulnerability and fiscal space, we find adverse shifts of the

cost-risk frontier of debt financing, with material increases in expected costs and high

regional heterogeneity. Debt trajectories trend upward from mid-century, and

stabilizing them requires fiscal consolidation that exceeds a feasibility threshold for the

more vulnerable economies. Adaptation moderates the impact and breaks even when

governments finance about one-third of the cost, with the private sector covering the

rest, yet it cannot alone restore sustainability. Maintaining current public spending in

the face of climate-related damages appears impossible. Thus, climate change emerges

as a first-order risk for fiscal authorities.