Sovereigns on thinning ice: debt sustainability, climate impacts and adaptation

A fundamental problem for sovereigns enacting climate policies is whether they can manage increasing debts as their economies suffer from adverse climate impacts. We develop stochastic debt sustainability analysis integrating a coupled climate-economy model with debt financing scenario optimisation, and stress test sovereign debt for representative countries globally under the Intergovernmental Panel on Climate Change marker narrative scenarios of climate change. The stress test combines socioeconomic and climate pathways with calibrated aleatory scenario trees of economic, fiscal and financial variables to generate forward-looking debt projections over the century. These projections incorporate climate-induced damages to economic growth, spanning the broad spectrum of impact functions from the literature. Our findings reveal significant risks to sovereign debt sustainability, particularly under high climate damages, that are large from mid-century. Expected costs increase by up to 3 percent of GDP under high climate impact in a world of regional rivalries, or 0.25 percent under low impact in a middle-of-the-road narrative, with considerable variation between countries. The long-run debts of highly impacted countries are unsustainable. We assess whether adaptation investments or fiscal consolidation can mitigate potential climate-debt crises. Public financing of reactive adaptation is a justified expenditure that breaks even but does not fully restore the debt sustainability of highly impacted high-debt countries. Maintaining public spending while ensuring debt sustainability appears infeasible under climate impacts.