$1.6 trillion (not billion) is annual federal spending on such programs, including spending that might be financed by redeeming T-bills from the SS trust funds.
$33 trillion of the unfunded liabilities represents the shortfall over the next 75 years, and the rest is the shortfall for all the years beyond that, discounted appropriately.
See http://www.ncpa.org/pub/ba/ba490/ for a good overview.
