The world owes itself another whole world economy. Global public debt has climbed from 68% of GDP in 2005 to 95% in 2025. History offers three paths out: austerity, inflation, or productivity. The UK brought debt-to-GDP down from ~250% after WWII to ~50% by 1980, mostly through nominal growth and inflation. Austerity is rare and painful. That leaves productivity, the option markets still underweight. If AI lifts productivity by even a point a year, the math on these debt stocks shifts from doom to manageable. The catch is timing. Productivity revolutions diffuse more slowly than markets expect. Debt service compounds immediately. Debt sustainability is really a productivity race. Whether AI-driven productivity arrives fast enough to outrun debt service may be the defining macro question of the decade.