The author notes how high the current US Debt is, about $15.7 trillion dollars. (That's 15,700,000,000,000). In 2011, the entire US GDP was just over $15T.
You can research and find that many feel this ratio i.e. debt to GDP is a significant factor in a country's ability to pay its debt. We are still running a deficit this year, and the author points out that given these factors, he'd expect investors in US bonds to demand a higher return for their risk.
On a lighter note, we've been watching our debt for decades. I am near 50, and remember seeing the debt clock when I was a teen. It's on a building in Manhattan on 44th street just west of 6th avenue. It's wasn't until the early 80's that our debt went over $1Trillion for the first time, and the clock itself needed to have more digits added to its structure. $999 billion was its limit.