http://www.socialsecurity.gov/history/pdf/tr09summary.pdf
Social Security’s annual surpluses
of tax income over expenditures are expected to fall sharply this
year and to stay about constant in 2010 because of the economic recession,
and to rise only briefly before declining and turning to cash flow
deficits beginning in 2016 that grow as the baby-boom generation retires.
The combined difference grows each year, so that by 2016, net revenue
flows from the general fund would total $369 billion (1.8 percent of
GDP). The positive amounts that begin in 2016 for OASDI, and started in
2008 for HI, initially represent payments the Treasury must make to the
trust funds when assets are depleted to help pay benefits in years prior to
exhaustion of the funds.
Neither the redemption of trust fund bonds, nor
interest paid on those bonds, provides any new net income to the Treasury,
which must finance redemptions and interest payments through
some combination of increased taxation, reductions in other government
spending, or additional borrowing from the public.