You either raise the payroll taxes or decrease benefits or some combination thereof. Here is what the Trustees said in their 2012 report:
Social Security’s expenditures exceeded non-interest income in 2010 and 2011, the first such occurrences since 1983, and the Trustees estimate that these expenditures will remain greater than non-interest income throughout the 75-year projection period. The deficit of non-interest income relative to expenditures was about $49 billion in 2010 and $45 billion in 2011, and the Trustees project that it will average about $66 billion between 2012 and 2018 before rising steeply as the economy slows after the recovery is complete and the number of beneficiaries continues to grow at a substantially faster rate than the number of covered workers. Redemption of trust fund assets from the General Fund of the Treasury will provide the resources needed to offset the annual cash-flow deficits. Since these redemptions will be less than interest earnings through 2020, nominal trust fund balances will continue to grow. The trust fund ratio, which indicates the number of years of program cost that could be financed solely with current trust fund reserves, peaked in 2008, declined through 2011, and is expected to decline further in future years. After 2020, Treasury will redeem trust fund assets in amounts that exceed interest earnings until exhaustion of trust fund reserves in 2033, three years earlier than projected last year. Thereafter, tax income would be sufficient to pay only about three-quarters of scheduled benefits through 2086.
A temporary reduction in the Social Security payroll tax rate reduced payroll tax revenues by $103 billion in 2011 and by a projected $112 billion in 2012. The legislation establishing the payroll tax reduction also provided for transfers of revenues from the general fund to the trust funds in order to "replicate to the extent possible" payments that would have occurred if the payroll tax reduction had not been enacted. Those general fund reimbursements comprise about 15 percent of the program's non-interest income in 2011 and 2012.
While the combined OASDI program continues to fail the long-range test of close actuarial balance, it does satisfy the test for short-range financial adequacy. The Trustees project that the combined trust fund assets will exceed one year’s projected cost for more than ten years, through 2027.
However, the Disability Insurance (DI) program satisfies neither the long-range test nor the short-range test. DI costs have exceeded non-interest income since 2005, and the Trustees project trust fund exhaustion in 2016, two years earlier than projected last year. The DI program faces the most immediate financing shortfall of any of the separate trust funds; thus lawmakers need to act soon to avoid reduced payments to DI beneficiaries four years from now.