What Does Unsustainable Mean?

These sentences hurt my head:

"By President Obama’s own optimistic projections, American deficits will not return to what are widely considered sustainable levels over the next 10 years."

Doesn't this mean that American deficits will remain at unsustainable levels for at least 10 years?How can unsustainable deficits persist for 10 or more years?

And then after remaining at unsustainable levels for 10 years, deficits "start rising again sharply" in 2019-2020.

Which I guess means that the deficits will be unsustainably small, and so naturally after 10 years they get bigger.