We quantify the energy transition risk to sovereign debts, linking integrated assessment models (IAM) with stochastic debt sustainability analysis. We obtain forward-looking projections of the risk premia for a sample of diverse major economies and eurozone countries under orderly and disorderly transitions from an ensemble of IAMs and document adverse transition effects manifested from the mid-2030s. We compute fiscal adjustments to stabilize debts with high confidence, averaging about 1% GDP p.a. for both orderly and disorderly transitions but significant country differences. Our findings are qualitatively robust to the cross-IAM quantitative differences and muted under low interest rates. Green growth of 0.4-0.7% could offset the debt increases in the long run.