Former U.S. Sen. Phil Gramm and Steve McMillin warn Wall Street Journal readers against thinking that a reduction in projected future federal deficits means that the national debt problem has been addressed.

President Obama has raised the national debt by nearly $6.2 trillion, the equivalent of $78,385 per family of four. It is true that projected deficits recently have been reduced. April tax filings increased 28% from 2012, but much of this was thanks to a one-time rush at the end of 2012 to report income before rates rose in January. The second largest reduction in the deficit came from Fannie Mae FNMA +11.04% taking a one-time accounting adjustment.

But unless the economy soars, or a significant budget agreement is reached, the most lasting legacy of the Obama presidency will be a $10 trillion increase in the national debt—a burden that bodes ill for the nation’s future.

Once the Federal Reserve’s easy-money policy comes to an end and interest rates return to their post-World War II norms, the cost of servicing this debt will explode. The cost will increase further as the Fed sells down its $1.85 trillion holding of government bonds, and the Social Security system runs deeper and deeper into the red. The Treasury will then have to pay interest on an ever-growing percentage of the debt.

Since the World War II era, the average maturity of outstanding federal debt has been about five years, and the average interest cost on a five-year Treasury note has been 5.9%. At this interest rate, the expected cost of the Obama debt burden will eventually approach some $590 billion per year in perpetuity, exceeding the current annual cost of any federal program except Social Security.