What is, and isn't, worrying about 100% debt to GDP

Data: Bureau of Economic Analysis and Treasury Department; Chart: Neil Irwin/AxiosThe United States has crossed a symbolic milestone: The national debt is now larger than its gross domestic product. But it's not the level of that ratio that is alarming — it's the trajectory.The big picture: There's nothing inherently unsustainable about a 100% debt-to-GDP level. What matters is why it got that high, the prospects for future borrowing, and the forecast for growth and borrowing costs.Across those...