Sovereign Debt at the Limit: How Advanced Economies Are Running Out of Fiscal Space

Debt Dynamics: The Mathematics of Sustainability

Debt sustainability analysis rests on a relatively simple but powerful accounting identity. The change in a government's debt-to-GDP ratio each year is determined by three variables: the primary fiscal balance (revenues minus non-interest spending), the nominal interest rate on existing debt, and the nominal growth rate of the economy. When the interest rate exceeds the growth rate — the so-called r > g condition — the debt ratio will rise automatically unless the government runs a primary surplus large enough to offset the differential. When growth exceeds the interest rate — r < g — governments can run small primary deficits while still seeing their debt ratio decline over time.

For most of the post-2008 period, the r < g condition held comfortably. Real interest rates were near or below zero while economies grew in nominal terms. This benign arithmetic meant that governments could run moderate deficits and still see debt ratios stabilise or even decline without explicit austerity. The work of economists Thomas Piketty and later Olivier Blanchard explored the macroeconomic implications of this condition, with Blanchard arguing in his 2019 AEA presidential address that the fiscal cost of public debt was lower than conventionally assumed when r < g prevailed.

The post-2022 rate environment has shifted this calculus materially. With policy rates above 4% in the US and real rates moving into clearly positive territory, the r < g assumption can no longer be taken for granted. IMF staff estimates suggest that for the US to stabilise its debt ratio at current levels, it would need to run a primary surplus of roughly 1.5% of GDP — a significant swing from a current primary deficit of over 3%. For Italy, the required adjustment is even larger given higher spreads over German bunds.

Contingent Liabilities and Hidden Fiscal Risks

Official debt figures, large as they are, likely understate the true fiscal challenge. Governments carry substantial contingent liabilities — obligations that do not appear on the balance sheet unless a specific event triggers them. These include guarantees on bank deposits (which become real liabilities in a banking crisis), government-backed mortgage institutions, state pension obligations on a present-value basis, and climate-related infrastructure commitments. The BIS and IMF have estimated that including pension liabilities and other off-balance-sheet commitments could add 50–100% of GDP to the effective fiscal burden in many advanced economies.

Public-private partnerships represent another category of contingent exposure. Governments that fund infrastructure through PPP arrangements often carry the tail risk implicitly — if a privately-operated toll road or hospital fails commercially, the government typically steps in. The UK's Private Finance Initiative (PFI) programme, for example, left the public sector with long-term contractual obligations that are only now being fully unwound, at substantial cost.

Interest Rate Sensitivity and Refinancing Risk

The vulnerability of a government's debt position to interest rate changes depends critically on the maturity structure of its outstanding debt. A government with a high proportion of short-dated debt — bills and bonds maturing within one to three years — faces immediate repricing risk when rates rise: a large fraction of the debt stock must be refinanced at current market rates within a short window. A government with a more extended maturity profile can absorb rate shocks more gradually, as only the fraction maturing each year needs to be rolled over at new rates.

The UK Debt Management Office publishes data showing that the average maturity of UK gilts in issue is around fourteen years — one of the longest in the G7. This long duration partially insulates the UK's debt service costs from short-term rate moves but also means the gilt portfolio carries significant mark-to-market risk for holders, as demonstrated in the pension fund liability-driven investment (LDI) crisis of September 2022. The US Treasury has a notably shorter average maturity profile — closer to six years — making the US more exposed to near-term refinancing cost increases.

Political Economy of Fiscal Adjustment

The technical economics of debt sustainability are well understood. The political economy is far harder. Fiscal consolidation requires either raising taxes, cutting spending, or both — all of which produce visible and immediate losers while delivering diffuse and deferred benefits in the form of lower interest costs and reduced future risk. Democratic systems are structurally biased against this trade-off. The voters who bear consolidation costs are present; the voters who benefit from avoided future crises are partly hypothetical.

Historical episodes of successful fiscal consolidation — Canada in the 1990s, Sweden in the mid-1990s, Germany in the 2000s — share certain features: clear external pressure or a credible fiscal anchor such as a currency commitment, broad political consensus or a single-party government with a working majority, and the good fortune of a period of strong nominal growth that eased the arithmetic of adjustment. Most advanced economies today lack at least one of these conditions, which helps explain why consolidation plans announced with fanfare tend to slip in practice.