While economists may waste lots of hot air debating this, that and the other about the future growth trajectory of the US economy, in the aftermath of Goldman's cut of US GDP to just a 1.1% annualized rate of growth. And with the fiscal cliff, debt ceiling, Europe, China, and a plethora of other unknowns up ahead, this number will certainly decline further. Now here lies the rub: as the chart below shows total US marketable debt has doubled in the past 4 years, or an annualized growth rate of just above 21%. And as Zero Hedge has shown before, total US Debt/GDP is on the verge of crossing 102%, the highest since WWII. Simply said, the divergence between the two data series will only accelerate as every incremental dollar of debt generates ever less bank for the GDP buck. And that, from a "sustainability" perspective, is what the problem is in a nutshell.