As noted, the Congressional Budget Office says that the federal deficit will shrink to 2.1 percent of GDP in two years and then start to grow again after 2015. The most important factor contributing to the widening budget deficit over the next 10 years, according to the CBO, is not Social Security, or even Medicare, but a predicted rise in interest payments on the debt, as you can see here:
The net interest projection is based on assumptions about what policy decisions the Federal Reserve will make in the future; in this case, the Fed is assumed to raise interest rates substantially. The deficit tops out at 3.5 percent of GDP in 2023 in the latest CBO forecast (which is just above the 40-year average of 3.1 percent of GDP), but it continues to climb outside of the 10-year window, and this is what has many people concerned.
Although much of the discussion of the long-term budget has been focused on “entitlements” and healthcare costs in particular, rising interest payments also play a key role in the CBO’s long-term forecast. In fact, James Galbraith has argued that they play the key role in terms of arguments about the “sustainability” of the debt:
The CBO’s assumption, which is that the United States must offer a real interest rate on the public debt higher than the real growth rate, by itself creates an unsustainability that is not otherwise there. … Changing that one assumption completely alters the long-term dynamic of the public debt. By the terms of the CBO’s own model, a low interest rate erases the notion that the US debt-to-GDP ratio is on an “unsustainable path.” The prudent policy conclusion is: keep the projected interest rate down. Otherwise, stay cool.