If you breathed a sigh of relief when the president and Congress reached a deal to resolve the “fiscal cliff” debate at the beginning of the calendar year, Columbia Business School Dean Glenn Hubbard argues you shouldn’t have. He explains why in a recent Fortune magazine column.

The $600 billion tax increase over 10 years weakened the recovery and did very little to address America’s dangerous long-term budget deficits and rising federal debt relative to GDP. To understand the source of our budget woes and determine a path forward, we must first confront four thorny truths.

The principal fiscal choice is about spending. We must ask ourselves how large we want our government to be, and what we want government to do. Politicians like to focus on discretionary spending in the budget wars, but the growth in our debt is not being driven by foreign assistance or public-broadcasting subsidies or defense. What we are facing is an entitlement crisis. It has been looming for decades, and is now approaching a catastrophic scale. Federal discretionary spending has declined as a share of GDP over the past 40 years, while entitlement spending has increased dramatically. The Congressional Budget Office projects that this pattern will continue in the coming decades.

The second truth is that, in cases in which consolidation is brought about by reducing government spending, fiscal restructurings around the world have successfully reduced and stabilized the debt-to- GDP ratio. Such examples lie in contrast to the U.S., where there is so little emphasis on spending restraint.

Third, if the U.S. chooses the path of revenue increases, today’s soak-the-rich approach will prove inadequate. Wealth redistribution is a small contributor, as the fiscal cliff resolution makes clear — tax increases on the well-off have a minimal effect on the deficit; the income tax is poorly equipped to raise much additional revenue. A less progressive tax system, with greater reliance on payroll and consumption taxes — as in other industrial economies with large public sectors — would be needed to do so.

Hubbard’s fourth truth? The concept of entitlements, including Social Security and Medicare, must change if advocates want to preserve a social safety net “without breaking the fiscal bank.”