Picking up on Pete's post about the 2008 Trustees Report, I always go first to this table, Table IV.B7, which shows the present value of Social Security's unfunded obligations over an infinite horizon.  The number is $13.6 trillion, or 3.2% of taxable payroll or 1.1% of gross domestic product over the same horizon. 

I've blogged extensively about these summary numbers over at Vox Baby.  For the present post, I'd like to make two quick points.

1) I tend to focus on the middle number--3.2% of taxable payroll--when describing what needs to be done to Social Security to remove its projected shortfall.  The number itself means that if we increased payroll taxes by 3.2 percentage points, from 12.4 to 15.6%, and invested the near-term surpluses at the rate of return projected on Treasuries, there would be enough funds available to pay all projected benefits in perpetuity.  That doesn't mean we have to follow that strategy, but it does indicate the size of the projected shortfall.  Since the deficits come in future years, I don't see any good reason why we don't change the rules for future contributions and benefits to remove them.

2) Pete points out, as others like CBO Director Peter Orszag have, that the projected increases in per-capita medical expenditures in Medicare and Medicaid become a much larger fiscal challenge than the demographic-driven changes in both Social Security and Medicare expenditures.  This is well captured by this chart, available in the Trustees Report summary:

Social Security's costs increase to 6% of GDP as the Baby Boomers move from the workforce to retirement.  That shift in costs is permanent, even as the Baby Boomers expire, because of longer term trends toward longer lives and fewer children.  That projected increase is swamped by the impact of projected medical expenditure increases. And unlike Social Security, there is very little in the way of projected revenue sources that aren't general revenues, as shown in the next chart:

Everything above that purple sliver is money that will come from future taxpayers beyond the tax rates that current workers are paying through the HI payroll tax.

Given pictures like these, I think it is important to discuss comprehensive reform.  The sooner, the better.  On Social Security, I've put my name on a plan that could serve as a compromise.  On Medicare, I think the best option is to raise the age of full eligibility for future beneficiaries, allowing younger retirees to pay their way in.  But that's a blog for another day.